Tropical Storm Grace

Post Tropical Grace Track 1600 Hours August 21 2021

Tropical Storm Grace Wind Speed FieldTropical Storm GraceNWS National Hurricane Center Miami FL 400 PM CDT Sat Aug 21, 2021 (see 4:00 video below)

The mountainous terrain of Mexico has taken its toll on Grace. Surface observations and high-resolution visible satellite images indicate that the system no longer has a definite surface circulation, and Grace has degenerated into a trough to the west of Mexico City. Therefore, this is the last advisory on this system.

Although the surface center has dissipated, the mid-tropospheric remnants of Grace are expected to continue westward, and to emerge into the eastern Pacific Ocean by late Sunday. There is high likelihood that this will lead to the formation of a new tropical cyclone over that basin by early next week.

Key Messages:

1. Through tonight, lingering heavy rainfall from the remnants of Grace may result in additional areas of flash and urban flooding, along with mudslides, over central Mexico.

FORECAST POSITIONS AND MAX WINDS

INIT  21/2100Z 19.6N 100.1W   20 KT  25 MPH - Low (ESE Purísima Jaltepec,MX )
 12H  22/0600Z...DISSIPATED

NWS National Hurricane Center Miami FL 1000 AM CDT Sat Aug 21, 2021 

Grace has continued to move over east-central Mexico since making landfall around 0600 UTC. Visible satellite images show that the system’s organization is degrading, but there is still some deep convection near the center with a few showers and thunderstorms near the coast. Assuming a slightly faster rate of weakening than given by the Decay-SHIPS output to account for the mountainous terrain, the advisory intensity is set to 60 kt. Additional rapid weakening should occur while the cyclone moves over the mountains of central and west-central Mexico today and tonight. Grace is likely to weaken below tropical storm strength tonight and dissipate tomorrow morning.

After making a southwestward turn just after landfall, the current motion is just a little south of west, or 250/11 kt. A mid-level ridge to the north of Grace should cause a generally westward motion until dissipation on Sunday.

The mid-level remnants of Grace are expected to continue westward after the surface center dissipates, and to emerge into the eastern Pacific Ocean by late Sunday. There is a good chance that this will lead to the formation of a new tropical cyclone over that basin by early next week.

Key Messages:

  • 1. Through the weekend, heavy rainfall across Veracruz, Puebla, Tlaxcala, Hidalgo, Queretaro, and eastern San Luis Potosi will lead to significant flash and urban flooding, along with the likelihood of mudslides.
FORECAST POSITIONS AND MAX WINDS

INIT  21/1500Z 19.7N  98.9W   60 KT  70 MPH - Tropical Storm (ENE San Andrés,MX)
 12H  22/0000Z 19.4N 100.6W   40 KT  45 MPH - Tropical Storm (ENE Jungapeo,MX) 
 24H  22/1200Z 19.0N 102.5W   25 KT  30 MPH - Post Tropical(WSW Capire,MX)
 36H  23/0000Z...DISSIPATED

NWS National Hurricane Center Miami FL 1000 PM CDT Fri Aug 20, 2021 

Hurricane Grace has rapidly intensified this evening. Deep cold convection has been wrapping around the center, with some evidence of mesovorticies rotating within the eyewall following the GOES-16 GLM lightning data. The Air Force Reserve Hurricane Hunters arrived in Grace around 0000 UTC and found that the storm had intensified into Category 2 hurricane with the pressure dropping down to 967 mb, which is a pronounced deepening rate of 2 mb per hour compared to the previous advisory. More recently, the aircraft was able to pass through the northeastern eyewall, and recently found flight level winds up to 115 kt with SFMR winds of 105 kt. These observations support Grace being upgraded to a major hurricane this advisory, with maximum sustained winds of 105 kt.

Some additional strengthening is possible while Grace remains over the very warm waters in the Bay of Campeche, though the hurricane should be making landfall tonight within the next 3-6 hours just south of Tuxpan, Mexico. By tomorrow morning, the storm should be well inland, and rapid weakening will likely be underway over the very mountainous terrain of mainland Mexico. The latest NHC intensity forecast now had Grace dissipating over Mexico in about 36 hours. However, as discussed in previous advisories, while the low-level circulation is expected to dissipate, the mid-level vortex is forecast to survive the passage of Mexico, and this feature is likely to lead to the development of a new tropical cyclone in the eastern Pacific basin later this weekend or early next week.

Center fixes from the aircraft indicate that Grace has stayed on a mostly due westward heading, at 270/9 kt. This general motion, though with a bit more southward component should continue through landfall and dissipation. This southward deflection over often occurs with strong hurricanes in this region, due to the topographical effects of the wind field to the north ascending over the higher terrain. The official NHC track is very similar to the previous advisory, and remains near the middle of the guidance consensus.

Key Messages:

  • 1. Hurricane conditions and dangerous storm surge are likely along portions of the coast of eastern mainland Mexico beginning tonight and tomorrow morning within the Hurricane Warning area from Puerto Veracruz northward to Cabo Rojo.
  • 2. A dangerous storm surge is likely near and to the north of the where the center of Grace crosses the coast of Mexico.
  • 3. Through the weekend, heavy rainfall across Veracruz, Puebla, Tlaxcala, Hidalgo, northern Queretaro, and eastern San Luis Potosi will lead to significant flash and urban flooding, along with the likelihood of mudslides.
FORECAST POSITIONS AND MAX WINDS

INIT  21/0300Z 20.7N  96.3W  105 KT 120 MPH - Category 3 (WSW Tecolutla,MX)
 12H  21/1200Z 20.3N  97.9W   75 KT  85 MPH - Tropical Storm (WSW Los Arroyos,MX)
 24H  22/0000Z 20.0N 100.9W   45 KT  50 MPH - Tropical Storm (WNW Las Pilas,MX)
 36H  22/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 400 PM CDT Fri Aug 20, 2021

The satellite presentation of Grace has greatly improved over the past several hours. Deep convection has become more concentrated over the center, and the central dense overcast now appears more symmetric. Dropsonde winds and peak 850-mb flight level wind measured by the Air Force Hurricane Hunters around midday were consistent with an intensity of around 75 kt. However, the surface pressure has been gradually falling throughout the day, and recent satellite data suggest the hurricane is becoming better organized. Therefore, the initial intensity is raised to 80 kt for this advisory, which is supported by the latest UW-CIMSS objective ADT estimate.

Given the high oceanic heat content within the southwestern Gulf of Mexico, additional strengthening is expected through Grace’s landfall tonight. The 10-15 kt of northerly vertical wind shear over Grace does not appear to be having much of a negative effect on the cyclone at this time. Thus, the official NHC intensity forecast remains on the high end of the guidance envelope, and is closest to the statistical-dynamical models DSHP and LGEM. The current forecast shows Grace making landfall within the next 12-18 h. Once the center of Grace moves inland over the mountains of central Mexico, the cyclone should rapidly weaken and then dissipate by 48 h. However, its remnants appear likely to move into the eastern Pacific and lead to the development of a new tropical cyclone in that basin later this weekend or early next week.

Earlier reconnaissance data indicate that Grace has slowed down a bit today, and its initial motion is estimated to be 265/9 kt. This general motion should continue through landfall as Grace is steered by a mid-level ridge over the northern Gulf of Mexico. The official NHC track forecast is slightly slower, but otherwise very similar to the previous one and lies near the center of the guidance envelope.

Key Messages:

  • 1. Hurricane conditions and dangerous storm surge are likely along portions of the coast of eastern mainland Mexico beginning this evening within the Hurricane Warning area from Puerto Veracruz northward to Cabo Rojo.
  • 2. Through the weekend, heavy rainfall across Veracruz, Puebla, Tlaxcala, Hidalgo, northern Queretaro, and eastern San Luis Potosi will lead to significant flash and urban flooding, along with the likelihood of mudslides.
FORECAST POSITIONS AND MAX WINDS

INIT  20/2100Z 20.5N  95.1W   80 KT  90 MPH - Category 1 (ESE Tecolutla,MX)
 12H  21/0600Z 20.4N  96.6W   90 KT 105 MPH - Category 2 (ESE Lázaro Cárdenas,MX)
 24H  21/1800Z 19.9N  98.9W   45 KT  50 MPH - Tropical Storm (WNW Las Pintas,MX)
 36H  22/0600Z 19.5N 101.9W   30 KT  35 MPH - Depression (WNW Tingambato,MX)
 48H  22/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 1000 PM CDT Thu Aug 19, 2021

During the past few hours, the center of Grace has emerged over the southwestern Gulf of Mexico. The associated convection is currently poorly organized and mainly occuring in a band to the east of the center. However, data from Air Force Reserve and NOAA Hurricane hunter aircraft indicate that 55 kt surface winds are occurring about 20 n mi northeast of the center, and that will be the initial intensity for this advisory. The aircraft also reported that the minimum central pressure was near 994 mb.

Conditions appear favorable for re-intensification once the convection becomes better organized near the center, a process that could take several more hours. The new intensity forecast calls for a peak intensity of 75 kt in 24 h, just before the center makes landfall on the coast of mainland Mexico. The intensity for this part of the forecast follows the trend of the intensity guidance and is near the intensity consensus. After landfall, Grace is expected to weaken rapidly, with the circulation dissipating over the mountains of Mexico just after 48 h. The remnants of Grace are expected to subsequently move into the Pacific and develop into a new tropical cyclone there.

The cyclone has nudged a little northward since the last advisory, and the initial motion is 280/14 kt. Other than that, there is little change to either the forecast philosophy or the forecast track. Strong mid-level ridging should steer Grace westward for the next 12 h or so, followed by a west-southwestward motion through 48 h. This motion should take the center across the southwestern Gulf of Mexico to a landfall in mainland Mexico in just over 24 h. After landfall, the cyclone should continue west-southwestward into the mountains of Mexico until dissipation. The new forecast track is close to the consensus models and has only minor adjustments since the previous forecast.

Key Messages:

  • 1. Hurricane conditions and dangerous storm surge are likely along portions of eastern mainland Mexico beginning late Friday. A Hurricane Warning is in effect from Puerto Veracruz northward to Cabo Rojo.
  • 2. Through the weekend, heavy rainfall across Veracruz, Puebla, Tlaxcala, Hidalgo, northern Queretaro, and eastern San Luis Potosi will lead to flash and urban flooding, along with the likelihood of mudslides.
FORECAST POSITIONS AND MAX WINDS

INIT  20/0300Z 20.8N  91.8W   55 KT  65 MPH - Tropical Storm (WSW Celestún,MX)
 12H  20/1200Z 20.8N  93.7W   65 KT  75 MPH - Category 1 (ENE Poza Rica,MX)
 24H  21/0000Z 20.7N  95.9W   75 KT  85 MPH - Category 1 (ENE Tecolutla,MX)
 36H  21/1200Z 20.1N  98.2W   45 KT  50 MPH - Tropical Storm (WSW Coyametepec,MX)
 48H  22/0000Z 19.6N 100.5W   25 KT  30 MPH - Post Tropical(WSW El Llano, MX)
 60H  22/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 400 PM CDT Thu Aug 19, 2021

Grace has now been inland for almost 12 hours and the inner core structure has been gradually decaying. In fact, the low-level center is now partially exposed to the north of the mid-level vortex which still has the majority of the deep convection. The initial intensity has been lowered to 45 kt this advisory, assuming a bit more weakening of the low-level center has occurred since 1800 UTC.

Environmental conditions appear favorable for Grace to restrengthen after the storm moves offshore this evening. However, it will take a bit of time for the inner-core to reorganize before a faster rate of intensification can occur. After that process occurs, significant intensification is possible, and the latest intensity forecast makes Grace a hurricane again by 24 hours and is near the upper end of the intensity guidance by 36 hours when Grace will be just inland over Mexico. However, it remains possible that the storm could intensify more than indicated between the 24 and 36 hour points, when both the global and regional high-res hurricane model guidance suggests Grace will be near peak intensity.

Grace is still moving a bit north of due west at 280/13 kt. A continued westward to west-northwestward motion is expected for the next 6-12 hours and Grace should be emerging offshore of the western Yucatan Peninsula later this evening. Thereafter, a strong mid-level ridge oriented southwest-to-northeast ahead of Grace is likely to steer the cyclone towards the west, and then west-southwest as it nears mainland Mexico. The track guidance remains in good agreement, and the latest track forecast is quite similar to the previous one, staying close to the multi-model consensus aids. Although Grace is likely to dissipate by 60 hours over the high terrain of Central Mexico, the mid-level circulation should survive and emerge into the East Pacific. This feature could contribute to the formation of a new tropical cyclone in that basin.

Key Messages:

  • 1. Tropical storm conditions will continue this afternoon and evening across the northern Yucatan Peninsula but should subside later tonight.
  • 2. Hurricane conditions and dangerous storm surge are likely along portions of eastern mainland Mexico beginning late Friday. A Hurricane Warning is in effect from Puerto Veracruz northward to Cabo Rojo.
  • 3. Through the weekend, heavy rainfall across central and northern portions of the Yucatan Peninsula and Veracruz State should lead to flash and urban flooding. In addition, the heavy rainfall from Grace will be capable of producing mudslides in Veracruz.
FORECAST POSITIONS AND MAX WINDS

INIT  19/2100Z 20.6N  90.2W   45 KT  50 MPH - Tropical Storm (ENE San Mateo,MX)
 12H  20/0600Z 20.8N  92.5W   55 KT  65 MPH - Tropical Storm (WSW Celestún,MX)
 24H  20/1800Z 20.7N  94.9W   70 KT  80 MPH - Category 1 (ESE Tecolutla,MX)
 36H  21/0600Z 20.4N  97.0W   75 KT  85 MPH - Category 1 (WNW El Encanto,MX )
 48H  21/1800Z 19.6N  99.4W   35 KT  40 MPH - Tropical Storm (ESE Quinto Barrio,MX)
 60H  22/0600Z...DISSIPATED

NWS National Hurricane Center Miami FL 1000 AM CDT Thu Aug 19 2021 

Grace made landfall as a hurricane near Tulum, Mexico around 0945 UTC (445am CDT). A storm chaser in Tulum reported a minimum pressure of 983 mb at the time of landfall. Since then, the cyclone has moved further inland where observations are far more sparse, and we have no recent in situ observations to assist the intensity analysis. The intensity is therefore set at 55 kt, based in part on the inland decay wind model built into the SHIPS model. The central pressure estimate of 995 mb is likewise uncertain.

Some additional weakening is likely this afternoon while Grace continues to cross the Yucatan peninsula. The tropical storm should emerge over the southern Gulf of Mexico this evening, and re-strengthening is anticipated shortly thereafter. Environmental conditions are expected to be quite conducive for strengthening, but it will likely take Grace at least a little time to redevelop its inner-core after disruption by land. That could act to limit the rate at which the cyclone will intensify at first. The HWRF and HMON both suggest that the most significant reintensification could occur in the last 12 hours before final landfall occurs in mainland Mexico, and that scenario seems likely. It would not be surprising if Grace strengthened slightly more between the 36 h forecast point over water and the 48 h point inland, particularly if the hurricane moves slightly slower, giving it more time over the warm waters of the southern Gulf of Mexico. After landfall, Grace should quickly weaken and dissipate over the mountainous terrain of Mexico. Overall, the NHC intensify forecast is very similar to the previous advisory.

Grace continues to move to the west near 16 kt. A westward or west-northwestward motion is likely for the next 12 hours or so, after which a strong-deep layer ridge extending well over the northern Gulf of Mexico and southern U.S. will steer Grace westward until landfall. Only minor adjustments were made to the official track forecast, which remains near the multi-model consensus. Although the tropical cyclone will quickly dissipate after it moves inland, the ridge will likely steer its remnants further westward toward the eastern North Pacific, where it could contribute to the formation of a new tropical cyclone there.

Key Messages:

  • 1. Tropical storm conditions will continue today across the northern Yucatan Peninsula.
  • 2. Hurricane conditions and dangerous storm surge is likely along portions of eastern mainland Mexico beginning late Friday. A Hurricane Warning is in effect from Puerto Veracruz northward to Cabo Rojo.
  • 3. Through the weekend, heavy rainfall across central and northern portions of the Yucatan Peninsula and Veracruz State should lead to flash and urban flooding. In addition, the heavy rainfall from Grace will be capable of producing mudslides in Veracruz.
FORECAST POSITIONS AND MAX WINDS

INIT  19/1500Z 20.2N  88.8W   55 KT  65 MPH - Tropical Storm (WNW San Bernabé,MX)
 12H  20/0000Z 20.6N  91.2W   55 KT  65 MPH - Tropical Storm (WSW Celestún,MX)
 24H  20/1200Z 20.6N  94.0W   65 KT  75 MPH - Category 1 (ENE Poza Rica,MX)
 36H  21/0000Z 20.4N  96.3W   75 KT  85 MPH - Category 1 (ESE Tecolutla,MX)
 48H  21/1200Z 20.0N  98.7W   40 KT  45 MPH - Tropical Storm (WSW Epazoyucan,MX)
 60H  22/0000Z 19.6N 101.7W   25 KT  30 MPH - Post Tropical(ESE Erongarícuaro,MX)
 72H  22/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 1000 PM CDT Wed Aug 18, 2021

Grace has become a bit better organized during the past several hours. Cuban radar data indicates that the eye and eyewall have become better defined, and reports from Air Force Reserve and NOAA Hurricane Hunter aircraft show that the inner core circulation has become better defined. There are also occasional attempts at eye formation in satellite imagery. However, these changes have not yet resulted in significant intensification, with the maximum winds remaining near 70 kt and the central pressure hovering near 988 mb. One possible restraint on development is a dry slot that is seen wrapping around the central core in both satellite imagery and Cuban radar data.

The center has jogged a little to the left during the past few hours, and the initial motion is now 280/16. The hurricane should continue to move generally westward to west-northwestward for the next 24-36 h, followed by a westward to west-southwestward motion from 36-48 h. This motion should bring the center over the Yucatan Peninsula of Mexico during the next 12 h, followed by passage across the southwestern Gulf of Mexico or the Bay of Campeche to a second landfall in mainland Mexico between 48-60 h. After that, the cyclone should continue moving into the mountains of Mexico until it dissipates. The new forecast track is nudged a little south of the previous forecast, and it lies near the various consensus models.

Except for the aforementioned dry slot, conditions appear favorable for intensification before landfall on the Yucatan peninsula. While not explicitly show in the intensity forecast, Grace could reach an intensity of 75-80 kt before it reaches Yucatan. The cyclone should weaken some as it crosses the peninsula, then re-intensify over the Gulf of Mexico until it reaches mainland Mexico. After final landfall in Mexico, Grace is expected to quickly weaken and dissipate over the mountains of Mexico just after 72 h. The remnants of Grace are likely to emerge in the Pacific and possibly re-develop there, but the uncertainty of whether this will be the original center or a new center precludes forecast points over the Pacific at this time.

A Hurricane Watch and a Tropical Storm Watch have been issued for portions of mainland Mexico.

Key Messages:

  • 1. Hurricane conditions and a dangerous storm surge are expected in portions of the Hurricane Warning area in the eastern Yucatan Peninsula of Mexico beginning during the next several hours. Preparations to protect life and property should be rushed to completion.
  • 2. There is an increasing risk of hurricane conditions and dangerous storm surge in portions of eastern mainland Mexico beginning late Friday, and a Hurricane Watch has been issued for part of this area.
  • 3. Through the weekend, heavy rainfall across central and northern portions of the Yucatan Peninsula and Veracruz State should lead to flash and urban flooding. In addition, the heavy rainfall from Grace will be capable of producing mudslides in Veracruz.
FORECAST POSITIONS AND MAX WINDS

INIT  19/0300Z 19.8N  85.6W   70 KT  80 MPH - Category 1 (ESE Tulum,MX)
 12H  19/1200Z 20.3N  88.1W   70 KT  80 MPH - Category 1 (WNW Villahermosa,MX)
 24H  20/0000Z 20.6N  91.4W   60 KT  70 MPH - Tropical Storm (WSW Celestún,MX)
 36H  20/1200Z 20.7N  94.0W   65 KT  75 MPH - Category 1 (ENE Poza Rica,MX)
 48H  21/0000Z 20.5N  96.1W   75 KT  85 MPH - Category 1 (ESE Papantla,MX)
 60H  21/1200Z 20.0N  98.8W   35 KT  40 MPH - Tropical Storm (WSW La Higa,MX)
 72H  22/0000Z 19.7N 101.6W   20 KT  25 MPH - Post Tropical (WNW El Gallito,MX)
 96H  23/0000Z...DISSIPATED

NWS National Hurricane Center Miami FL 500 PM EDT Wed Aug 18, 2021 

A ragged eye appeared for a while on visible satellite imagery, but it has recently been obscured by cumulonimbus tops. The overall cloud pattern has been maintaining its organization with some convective banding features and very cold cloud tops, mainly over the eastern portion of the circulation. The current intensity estimate is set at 70 kt, which is a blend of Dvorak estimates from TAFB and SAB. Another Air Force Reserve Hurricane Hunter aircraft is scheduled to investigate the hurricane in a few hours to provide another intensity estimate.

Grace will continue to move over waters of very high oceanic heat content prior to reaching the east coast of the Yucatan Peninsula, which of course favors intensification. Aside from moderate northwesterly shear, a possible impediment to strengthening is a northeast to southwest-oriented shear axis located just to the northwest of the hurricane. Nonetheless, some intensification is anticipated before landfall in Yucatan, with restrengthening over the Bay of Campeche. The official intensity forecast continues to be in general agreement with the multi-model consensus predictions, IVCN and HCCA.

Grace is moving west-northwestward, or about 285/14 kt. There has been essentially no change to the track forecast philosophy. Grace should maintain a west-northwestward to westward motion, on the southern periphery of a strong subtropical ridge, for the next several days. The official track forecast is very similar to the previous one and about on top of the latest multi-model consensus, TVCA.

A Hurricane Watch will likely be required for a portion of the coast of eastern mainland Mexico tonight.

Key Messages:

  • 1. Hurricane conditions and a dangerous storm surge are expected in portions of the Hurricane Warning area in the eastern Yucatan Peninsula of Mexico beginning late tonight or early Thursday. Preparations to protect life and property should be rushed to completion.
  • 2. There is an increasing risk of hurricane conditions and dangerous storm surge in portions of eastern mainland Mexico beginning late Friday, and a Hurricane Watch will likely be issued for part of this area tonight.
  • 3. Through the weekend, heavy rainfall across the Cayman Islands as well as portions of the Yucatan Peninsula and Veracruz State should lead to flash and urban flooding. In addition, the heavy rainfall from Grace will be capable of producing mudslides in Veracruz.
FORECAST POSITIONS AND MAX WINDS

INIT  18/2100Z 19.7N  83.7W   70 KT  80 MPH - Category 1 (WNW Tulum,MX)
 12H  19/0600Z 20.2N  86.1W   75 KT  85 MPH - Category 1 (ESE Cozumel,MX)
 24H  19/1800Z 20.7N  89.3W   55 KT  65 MPH - Tropical  Storm (WSW Homún,MX)
 36H  20/0600Z 20.9N  92.2W   60 KT  70 MPH - Tropical Storm (WNW Celestún,MX)
 48H  20/1800Z 20.8N  94.6W   70 KT  80 MPH - Category 1 (ENE Poza Rica,MX)
 60H  21/0600Z 20.4N  97.3W   75 KT  85 MPH - Category 1 (ESE Papantla,MX)
 72H  21/1800Z 20.0N  99.9W   30 KT  35 MPH - Tropical Storm (WSW El Ermitaño,MX)
 96H  22/1800Z 19.5N 104.0W   20 KT  25 MPH - Post Tropical (WSW Tolimán,MX)
120H  23/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 1100 AM EDT Wed Aug 18, 2021 

Grace is becoming better organized on satellite images with a comma-shaped cloud pattern, and the upper-level outflow remains well defined. Wind gusts well into the hurricane-force range were measured on Grand Cayman earlier this morning, along with some damage on the island. Recent flight-level and SFMR observations from both the Air Force and NOAA Hurricane Hunter aircraft support sustained 65-kt surface winds, so the system is being upgraded to a hurricane on this advisory.

Although Grace is situated over waters of very high oceanic heat content, moderate northwesterly shear and only marginally moist mid-level air could slightly impede intensification. However, some additional strengthening is expected before landfall tonight or early Thursday. Weakening will occur due to Grace’s passage over Yucatan, although that land mass has relatively low terrain. Reintensification is anticipated over the Bay of Campeche on Friday, and the system should regain hurricane strength before reaching the east coast of mainland Mexico. The official intensity forecast is similar to the numerical model consensus.

Grace has been moving west-northwestward near 13 kt. A prominent mid-tropospheric ridge is likely to remain in place to the north of the cyclone for the next several days. This pattern should keep steering Grace on a west-northwestward to westward track for 72-96 hours. The track guidance models are in excellent agreement and little change has been made to the NHC track forecast compared to the previous few packages.

A Hurricane Watch will likely be required for a portion of the coast of eastern mainland Mexico later today.

Key Messages:

  • 1. Hurricane conditions and a dangerous storm surge are expected in portions of the Hurricane Warning area in the eastern Yucatan Peninsula of Mexico beginning late tonight or early Thursday. Preparations to protect life and property should be rushed to completion.
  • 2. There is an increasing risk of hurricane conditions and dangerous storm surge in portions of eastern mainland Mexico beginning late Friday, and a Hurricane Watch will likely be issued for part of this area later today.
  • 3. Over the next few days, heavy rainfall across the Cayman Islands as well as portions of the Yucatan Peninsula and Veracruz State should lead to flash and urban flooding, with mudslides possible in the Mexican state of Veracruz.
FORECAST POSITIONS AND MAX WINDS

INIT  18/1500Z 19.4N  82.2W   65 KT  75 MPH - Category 1 (WSW West Bay,Grand Cayman)
 12H  19/0000Z 19.7N  84.5W   70 KT  80 MPH - Category 1 (ESE Coxumel,MX)
 24H  19/1200Z 20.3N  87.6W   75 KT  85 MPH - Category 1 (WNW Tulum,MX)
 36H  20/0000Z 20.6N  90.5W   50 KT  60 MPH - Tropical Storm (WSW Celestún,MX)
 48H  20/1200Z 20.7N  93.2W   65 KT  75 MPH - Category 1 (WNW Campeche,MX)
 60H  21/0000Z 20.7N  95.3W   75 KT  85 MPH - Category 1 (ENE Tecolutla,MX)
 72H  21/1200Z 20.5N  97.7W   60 KT  70 MPH - Tropical Storm (WSW El Crucero,MX)
 96H  22/1200Z 20.0N 103.0W   20 KT  25 MPH - Post Tropical (ENE Mazamitla,MX)
120H  23/1200Z...DISSIPATED

WS National Hurricane Center Miami FL 1100 PM EDT Tue Aug 17, 2021

NOAA and Air Force Hurricane Hunter aircraft that have been investigating Grace this evening have found that the center moved off the western tip of Jamaica shortly before 0000 UTC. Radar and conventional satellite imagery, as well as earlier microwave data, show that convective banding has increased with a band that wraps around the eastern and northern portions of the circulation. The Air Force reconnaissance aircraft measured a peak 850-mb flight-level wind of 64 kt, and both aircraft have measured believable SFMR winds of 50-52 kt. Based on those data the maximum winds were increased to 50 kt on the 0000 UTC intermediate advisory, and the initial intensity remains unchanged for this advisory.

Grace will be moving over the high ocean heat content waters of the northwestern Caribbean Sea during the next day or so. That, along with generally low to moderate vertical wind shear conditions, should allow for steady strengthening until Grace reaches the coast of the Yucatan Peninsula in just under 36 hours. The NHC intensity forecast call for Grace to become a hurricane on Wednesday, and the storm could peak near category 2 intensity before it reaches the coast of Mexico late Wednesday night or early Thursday. By the 36- hour forecast, some weakening is likely to have occurred as that point is now inland. Some weakening will likely occur while Grace moves over land, but restrengthening is predicted when the storm moves over the Bay of Campeche. The NHC intensity forecast is in best agreement with the HCCA and IVCN consensus aids.

Grace has been moving slightly north of due west, or 280/12 kt. A strong deep-layer ridge the extends from the western Atlantic across the eastern and central Gulf of Mexico should continue to steer the tropical storm westward to west-northwestward for the next several days. While the dynamical models agree on this overall forecast scenario, there remains some north-south spread with the HWRF, HMON, and GFS along the northern side of the guidance envelope, taking Grace near the northeastern portion of the Yucatan Peninsula. On the other hand, the UKMET and its ensemble mean depict a more westward track toward the southern portion of the Yucatan Peninsula of Mexico. The NHC forecast splits these differences and lies near the various consensus aids, which were just a tad farther south this cycle.

Key Messages:

  • 1. Hurricane conditions and a dangerous storm surge are expected over portions of the Hurricane Warning area in the eastern Yucatan Peninsula of Mexico beginning late Wednesday night.
  • 2. Heavy rainfall across Jamaica, the Cayman Islands, and the Yucatan Peninsula is likely to lead to flash and urban flooding. Mudslides are possible in Jamaica.
  • 3. Tropical storm conditions are expected over portions of Jamaica for a few more hours, and over the Cayman Islands later tonight and Wednesday. Hurricane conditions are possible in the Cayman Islands by Wednesday morning. Tropical storm conditions are expected to spread westward from portions of the southern coast of Cuba in the warning area to, possibly, other portions of the southern coast of Cuba in the watch area tonight through Wednesday.
FORECAST POSITIONS AND MAX WINDS

INIT  18/0300Z 18.4N  79.2W   50 KT  60 MPH - Tropical Storm (WSW Negril,Jamaica)
 12H  18/1200Z 18.9N  81.5W   60 KT  70 MPH - Tropical Storm (WSW West Bay, Grand Cayman)
 24H  19/0000Z 19.5N  84.7W   75 KT  85 MPH - Category 1 (ESE Punta Allen,MX)
 36H  19/1200Z 20.0N  87.8W   80 KT  90 MPH - Category 1 (ESE Chumpón,MX)
 48H  20/0000Z 20.4N  90.7W   50 KT  60 MPH - Tropical Storm (WNW Balantanche,MX)
 60H  20/1200Z 20.7N  93.2W   65 KT  75 MPH - Category 1 (WSW Celestún,MX)
 72H  21/0000Z 20.8N  95.6W   75 KT  85 MPH - Category 1 (ENE Tecolutla,MX)
 96H  22/0000Z 20.7N 100.1W   35 KT  40 MPH - Tropical Storm (WNW Palmas,MX)
120H  23/0000Z 20.5N 104.5W   20 KT  25 MPH - Post Tropical (ESE Cuyutlán,MX)

NWS National Hurricane Center Miami FL 500 PM EDT Tue Aug 17, 2021 

Radar imagery and earlier Hurricane Hunter aircraft and ASCAT observations indicate that the center of Grace has been moving over Jamaica. The aircraft and scatterometer data indicate that the maximum winds are near 45 kt. Cirrus-level outflow from the system remains quite pronounced, suggestive of weak vertical shear at this time.

Grace will be moving over the waters of the northwestern Caribbean Sea beginning later tonight. These waters are of very high oceanic heat content. Although mid-level environmental humidities are shown by the SHIPS diagnoses to be only marginally moist for the next couple of days, other factors should be conducive for strengthening. The official intensity forecast, which shows Grace becoming a hurricane tomorrow, is in reasonable agreement with the latest multi-model consensus. Some slight weakening should occur due to passage over the Yucatan Peninsula, followed by re-intensification over the Bay of Campeche. There is, however, significant uncertainty in the intensity prediction for this latter part of the forecast period.

The storm continues on a heading just north of west, or 280/13 kt. A strong 500-mb ridge should be maintained to the north of Grace for essentially the entire forecast period. There is good agreement among the track models on a continued generally westward to west-northwestward motion for the next 5 days. Little change has been made to the official track forecast in comparison to the previous one. The new NHC track is, again, very close to the dynamical model consensus.

Key Messages:

  • 1. Hurricane conditions and a dangerous storm surge are expected over portions of the Hurricane Warning area in the eastern Yucatan Peninsula of Mexico late Wednesday night.
  • 2. Heavy rainfall across Haiti, Cuba, Jamaica, the Cayman Islands, and the Yucatan Peninsula is likely to cause to flash, urban, and small stream flooding, with the potential for mudslides highest in Haiti.
  • 3. Tropical storm conditions are expected over portions of Jamaica through this evening, and over the Cayman Islands tonight and early Wednesday. Tropical storm conditions are expected to spread westward from portions of the southern coast of Cuba in the warning area to, possibly, other portions of the southern coast of Cuba in the watch area tonight through Wednesday.
FORECAST POSITIONS AND MAX WINDS

INIT  17/2100Z 18.4N  77.9W   45 KT  50 MPH - Tropical Storm (ESE Montego Bay,Jamaica)
 12H  18/0600Z 18.9N  80.3W   60 KT  70 MPH - Tropical Storm (WSW West Bay,Grand Cayman)
 24H  18/1800Z 19.4N  83.4W   70 KT  80 MPH - Category 1 (WSW West Bay,Grand Cayman)
 36H  19/0600Z 20.0N  86.7W   75 KT  85 MPH - Category 1 (WSW Cozumel,MX)
 48H  19/1800Z 20.5N  89.7W   60 KT  70 MPH - Category 1 (WSW Muná,MX)
 60H  20/0600Z 20.9N  92.2W   65 KT  75 MPH - Category 1 (WNW Celestún,MX)
 72H  20/1800Z 21.2N  94.8W   75 KT  85 MPH - Category 1 (ESE Tampico,MX)
 96H  21/1800Z 21.0N  99.5W   40 KT  45 MPH - Tropical Storm (WNW La Polvareda,MX)
120H  22/1800Z 21.0N 103.0W   20 KT  25 MPH - Post Tropical (ESE San Gabriel,MX)

NWS National Hurricane Center Miami FL 1100 AM EDT Tue Aug 17, 2021

The cloud pattern of Grace is gradually becoming better organized on satellite images, but the system still lacks distinct banding features over the eastern semicircle. Cirrus-level outflow from the system appears to be well defined. Flight-level and SFMR-observed surface winds, as well as dropsonde measurements, support an intensity of 45 kt for this advisory.

Not much change in intensity is likely through this afternoon while the circulation of Grace interacts with the island of Jamaica. Thereafter, since the cyclone will be over the very high heat content waters of the northwestern Caribbean Sea with modestly low vertical shear tonight and Wednesday, strengthening is anticipated. Grace will likely become a hurricane tomorrow and continue to intensify before moving over the Yucatan Peninsula. The interaction with that land mass is expected to temporarily interrupt strengthening, with some reintensification over the Bay of Campeche in 3-4 days. The official intensity forecast is near the model consensus for the first half of the forecast period, but below the consensus for the latter part of the period owing to the inherent uncertainties for that extended time frame.

Grace continues to move just a little north of due west, or at about 280/13 kt. A well-defined 500 mb ridge should remain entrenched to the north of the tropical cyclone for the next several days. Therefore, a continued west to west-northwestward track is likely to continue for most, if not all, of the forecast period. The official forecast is close to the previous one through 72 hours and just slightly south of it at 4-5 days. This is very similar to the latest multi-model consensus prediction.

Key Messages:

  • 1. Heavy rainfall across Haiti, Cuba, Jamaica, the Cayman Islands, and the northern Yucatan Peninsula may lead to flash, urban, and small stream flooding, with the potential for mudslides highest in Haiti and Jamaica.
  • 2. Tropical storm conditions are expected over portions of Jamaica today, and over the Cayman Islands later today and tonight. Tropical storm conditions are expected over portions of the southern coast of Cuba today in the warning area, spreading westward to possibly other portions of the southern coast of Cuba this evening through Wednesday morning.
  • 3. There is an increasing risk of wind, rainfall, and storm surge impacts on the Yucatan Peninsula of Mexico Wednesday night and Thursday, and a Hurricane Warning will likely be needed for that area later today.
FORECAST POSITIONS AND MAX WINDS

INIT  17/1500Z 18.3N  76.8W   45 KT  50 MPH - Tropical Storm (ENE Water Valley,Jamaica)
 12H  18/0000Z 18.8N  79.0W   55 KT  65 MPH - Tropical Storm (WNW Montego Bay,Jamaica)
 24H  18/1200Z 19.2N  82.2W   65 KT  75 MPH - Category 1 (WSW West Bay,Grand Cayman)
 36H  19/0000Z 19.7N  85.3W   70 KT  80 MPH - Category 1 (ESE Cozumel,MX)
 48H  19/1200Z 20.4N  88.3W   65 KT  75 MPH - Category 1 (WNW Xchac-Hua,MX)
 60H  20/0000Z 20.9N  91.3W   60 KT  70 MPH - Category 1 (WNW Celestún,MX)
 72H  20/1200Z 21.1N  94.0W   65 KT  75 MPH - Category 1 (ESE Tampico,MX)
 96H  21/1200Z 21.5N  98.5W   65 KT  75 MPH - Category 1 (ENE Pochuco,MX)
120H  22/1200Z 21.5N 103.0W   30 KT  35 MPH - Post Tropical (ENE Palma Cuata,MX)

NWS National Hurricane Center Miami FL 1100 PM EDT Mon Aug 16, 2021 

IR and microwave imagery during the past few hours indicate that Grace’s convective organization has improved a little since this afternoon. However, it is currently unknown if that has translated to a better-defined surface wind field. Dvorak intensity estimates from TAFB and SAB at 00Z supported tropical storm strength, and it is possible Grace has redeveloped sustained winds of that magnitude. A hurricane hunter aircraft is scheduled to investigate Grace in a few hours, so it seems prudent to avoid making a big change before the plane gets there. Grace is therefore conservatively maintained as a 30 kt depression for this advisory. Regardless of Grace’s exact intensity right now, the immediate threat is still torrential rainfall across Hispaniola overnight, which will likely cause severe flooding in some locations.

The initial motion estimate is 280/12 kt. Confidence in the track forecast has increased considerably during the past 24 h and the track model spread is low. A mid-level ridge is forecast to build over the Gulf of Mexico during the next several days, which should keep Grace on a generally westward or west-northwestward track through day 5. Once Grace moves away from Haiti, it should remain over water until it reaches the Yucatan peninsula late Wednesday or early Thursday. Very little change was made to the official track forecast, which is based primarily on the multi-model consensus TVCN.

Now that Grace is forecast to avoid all major land masses for the next couple of days, all of the intensity guidance calls for some strengthening to occur. That said, with the exception of the COAMPS-TC model, the intensity guidance as a whole shows a slower rate of strengthening than it did 6 h ago. The NHC forecast is now very near the IVCN intensity consensus throughout the forecast, including over the Gulf of Mexico where it continues to show Grace reaching hurricane strength. Despite the slightly lower guidance for this forecast, it still can not be ruled out that Grace will reach hurricane strength over the western Caribbean. When the hurricane hunter aircraft reaches Grace in a couple hours and provides more information about the current organization of cyclone, we should have a better feel for how quickly it could intensify prior to reaching the coast of Mexico.

Key Messages:

  • 1. Heavy rainfall across the Dominican Republic, Haiti, Cuba, Jamaica, the Cayman Islands, and the northern Yucatan Peninsula may lead to flash, urban, and small stream flooding, with the potential for mudslides highest in Haiti and the Dominican Republic.
  • 2. Tropical storm conditions are possible over portions of Hispaniola overnight, and over Jamaica on Tuesday. Tropical storm conditions are expected over portions of the southern coast of Cuba on Tuesday, spreading westward to the Cayman Islands and possibly other portions of the southern coast of Cuba Tuesday evening through Wednesday morning.
  • 3. There is a increasing risk of wind, rainfall, and storm surge impacts on the Yucatan Peninsula of Mexico Wednesday night and Thursday. Interests there areas should monitor the progress of Grace and updates to the forecast.
FORECAST POSITIONS AND MAX WINDS

INIT  17/0300Z 18.2N  73.8W   30 KT  35 MPH - Depression (WSW Guilerne,Haiti)
 12H  17/1200Z 18.6N  75.7W   35 KT  40 MPH - Tropical Storm (ENE Port Antonio,Jamaica)
 24H  18/0000Z 19.1N  78.8W   40 KT  45 MPH - Tropical Storm (WNW Montego Bay,Jamaica)
 36H  18/1200Z 19.7N  82.0W   45 KT  50 MPH - Tropical Storm (WSW West Bay,Grand Cayman)
 48H  19/0000Z 20.3N  85.1W   50 KT  60 MPH - Tropical Storm (WSW Cozumel,MX)
 60H  19/1200Z 21.0N  88.2W   50 KT  60 MPH - Tropical Storm (WNW California,MX)
 72H  20/0000Z 21.6N  91.3W   55 KT  65 MPH - Tropical Storm (WNW Celestún,MX)
 96H  21/0000Z 22.2N  95.9W   65 KT  75 MPH - Category 1 (ESE Tampico,MX)
120H  22/0000Z 22.5N 100.0W   40 KT  45 MPH - Tropical Storm (ESE El Gavilán,MX)

NWS National Hurricane Center Miami FL 500 PM EDT Mon Aug 16, 2021

Grace may have regained tropical storm status just before the center moved across the Barahona Peninsula of the Dominican Republic around midday. The Air Force Reserve Hurricane Hunters measured peak 925-mb winds of 45 kt very near the coast, which would equate to an intensity right at the threshold of tropical storm. Since then, however, land interaction has likely weakened these winds, and Grace is still estimated to be a 30-kt depression. The center appears to have moved back over water just to the south of the Haitian coast.

Grace has been moving a little slower today, possibly due to interaction with the terrain of Hispaniola, and the initial motion is estimated to be 285/11 kt. A mid-level high centered over the western Atlantic is forecast to build westward over the southeastern United States and northern Gulf of Mexico in the coming days. This pattern should keep Grace on a westward to west-northwestward trajectory for the entire 5-day forecast period, moving across the northwestern Caribbean Sea, across the northern Yucatan Peninsula, and into the southwestern Gulf of Mexico. The most significant model trends are on days 4 and 5, when many of the track models are indicating a slight west-southwest bend as the cyclone approaches mainland Mexico. The NHC track forecast has been nudged southward at most forecast times, but that may be within the noise level of typical model run-to-run variability.

Once the center of Grace moves away from Hispaniola tomorrow, the deep, warm waters of the northwestern Caribbean Sea and relatively light deep-layer shear should be conducive for strengthening. Model data suggest that some mid-level shear could come into play at times, so it’s not a sure bet that conditions will be ideal for significant strengthening. Partly for that reason, the NHC intensity forecast is near or just below the intensity consensus. However, even this new forecast is a little higher than the previous forecast, and many of the models indicate that Grace could be near or at hurricane strength when it approaches the Yucatan Peninsula in about 60 hours. After passing the Yucatan Peninsula, additional strengthening is likely, and the new forecast now explicitly shows Grace reaching hurricane intensity over the southwestern Gulf of Mexico.

Key Messages:

  • 1. Heavy rainfall across the Dominican Republic, Haiti, Cuba, Jamaica, and the Cayman Islands may lead to flash, urban, and small stream flooding, with the potential for mudslides highest in Haiti and the Dominican Republic.
  • 2. Tropical storm conditions are possible over portions of Hispaniola this evening and tonight, and over Jamaica on Tuesday. Tropical storm conditions are expected over portions of the southern coast of Cuba on Tuesday, spreading westward to the Cayman Islands and possibly other portions of the southern coast of Cuba Tuesday evening through Wednesday morning.
  • 3. There is a increasing risk of wind, rainfall, and storm surge impacts on the Yucatan Peninsula of Mexico Wednesday night and Thursday. Interests there areas should monitor the progress of Grace and updates to the forecast.
FORECAST POSITIONS AND MAX WINDS

INIT  16/2100Z 17.9N  72.4W   30 KT  35 MPH - Depression (WSW Jacmel,Haiti)
 12H  17/0600Z 18.3N  74.4W   30 KT  35 MPH - Depression (WSW Tiburon,Haiti)
 24H  17/1800Z 18.9N  77.2W   35 KT  40 MPH - Tropical Storm (WNW Ocho Rios,Jamaica)
 36H  18/0600Z 19.6N  80.4W   40 KT  45 MPH - Tropical Storm (WSW Little Cayman)
 48H  18/1800Z 20.2N  83.5W   50 KT  60 MPH - Tropical Storm (WSW Nueva Gerona,Cuba)
 60H  19/0600Z 20.9N  86.7W   60 KT  70 MPH - Tropical Storm (WNW Puerto Morelos,MX)
 72H  19/1800Z 21.6N  89.9W   60 KT  70 MPH - Tropical Storm (WNW Progreso,MX)
 96H  20/1800Z 22.4N  95.1W   65 KT  75 MPH - Category 1 (ENE Tampico,MX)
120H  21/1800Z 22.4N  98.8W   65 KT  75 MPH - Category 1 (ENE González Ortega,MX

NWS National Hurricane Center Miami FL 1100 AM EDT Mon Aug 16, 2021 

Air Force Reserve and NOAA Hurricane Hunters were both able to locate a center for Grace–probably the most well-defined center observed over the past few days. That center now appears to be moving onshore along the Barahona Peninsula of the Dominican Republic as we speak. The planes measured several possible areas of tropical-storm-force winds from the SFMR, however these observations have not been supported by the more reliable 925-mb flight-level winds for weaker systems, which were only as high as 38 kt, and warrant maintaining the 30-kt initial intensity. Dropsonde data indicate that the central pressure has fallen to 1007 mb.

The aircraft fixes confirm that Grace is moving westward, or 280/13 kt. High pressure over the western Atlantic is forecast to slide westward over the southeastern United States during the next several days, which should keep Grace on a westward to west- northwestward trajectory for the entire 5-day forecast period. This scenario is agreed upon by all of the available track models, and the new NHC track forecast has only been nudged slightly southward from the previous forecast based on the latest consensus aids.

Grace’s intensity forecast remains complicated by interaction with land and the possibility of some westerly shear during the forecast period. However, the southern shift in the forecast track takes the center of Grace more definitively over very warm 30 degrees Celsius waters in the northwestern Caribbean Sea. Therefore, gradual strengthening is anticipated while Grace approaches the Yucatan coast of Mexico. Once the system reaches the Gulf of Mexico, the shear appears to decrease, and conditions there will likely be conducive for additional strengthening. In fact, many of the models, including the consensus aids, bring Grace to hurricane intensity, and the NHC intensity forecast has therefore been bumped upward, bringing Grace very near hurricane strength by the end of the forecast period.

Key Messages:

  • 1. Heavy rainfall across the Dominican Republic, Haiti, Cuba, Jamaica, and the Cayman Islands may lead to flash, urban, and small stream flooding, with the potential for mudslides highest in Haiti and the Dominican Republic.
  • 2. Tropical storm conditions are possible over portions of Hispaniola today and tonight, and over Jamaica on Tuesday. Tropical storm conditions are expected over portions of the southern coast of Cuba on Tuesday, spreading westward to the Cayman Islands and other portions of the southern coast of Cuba Tuesday evening through Wednesday morning.
  • 3. There is a increasing risk of wind and rainfall impacts over the Yucatan Peninsula of Mexico Wednesday night and Thursday. Interests there areas should monitor the progress of Grace and updates to the forecast.
FORECAST POSITIONS AND MAX WINDS

INIT  16/1500Z 17.7N  71.4W   30 KT  35 MPH - Depression (ESE Oviedo,DR)
 12H  17/0000Z 18.2N  73.4W   30 KT  35 MPH - Depression (ESE Zanglais,Haiti)
 24H  17/1200Z 18.8N  76.2W   35 KT  40 MPH - Tropical Storm (ESE Port Antonio,Jamaica)
 36H  18/0000Z 19.4N  79.0W   40 KT  45 MPH - Tropical Storm (WSW Pilón,Cuba)
 48H  18/1200Z 20.1N  82.1W   45 KT  50 MPH - Tropical Storm (ESE Nueva Gerona,Cuba)
 60H  19/0000Z 20.9N  85.2W   50 KT  60 MPH - Tropical Storm (WSW Cancún,MX)
 72H  19/1200Z 21.6N  88.4W   50 KT  60 MPH - Tropical Storm (WNW San Felipe,MX)
 96H  20/1200Z 22.5N  94.0W   60 KT  70 MPH - Tropical Storm (WNW Progreso,MX)
120H  21/1200Z 23.0N  98.0W   60 KT  70 MPH - Tropical Storm (WNW Morón,MX)

NWS National Hurricane Center Miami FL 1100 PM AST Sun Aug 15, 2021 

An Air Force Reserve Hurricane Hunter Aircraft investigated Grace earlier this evening and found maximum flight level winds just less than 40 kt at 925 mb, which supports Grace’s estimated 30 kt intensity. The plane also found that the depression still has a slightly elongated but closed surface circulation. During the past few hours, satellite imagery has shown a slight increase in convective banding features and associated heavy rain associated with Grace. That activity highlights the primary threat from Grace during the next 24 hours: prolonged heavy rainfall that could lead to flash and urban flooding along with the potential for mudslides over Puerto Rico and Hispaniola.

The forecast for Grace is incredibly challenging. Imminent interactions with the high terrain of Hispaniola and Cuba could cause Grace to dissipate as soon as Monday evening. However, a track south of Cuba, as shown by recent runs of the GFS and COAMPS-TC, may allow Grace to maintain its tropical cyclone status and possibly even intensify. The HWRF even shows it becoming a hurricane over the western Caribbean, with the caveat that the model has produced several poor forecasts for Grace thus far. Although it is not explicitly forecast, slight intensification is still possible tonight or tomorrow morning before the center of Grace moves inland. After that time, the NHC forecast assumes Grace will continue as a tropical depression through 72 h. Once/if Grace makes it to the western Caribbean Sea and Gulf of Mexico in 3-4 days, it could have an opportunity to reorganize and intensify, and this is again shown in the official intensity forecast. That said, users are encouraged to not focus on the exact track or intensity forecasts at days 4 and 5.

The track guidance has shifted south for this advisory, and generally calls for Grace to move westward to west-northwestward through the forecast period. The official track forecast has been shifted a little south once again, but is north of the most recent multi-model consensus.

Key Messages:

  • 1. Heavy rainfall across Puerto Rico, the Dominican Republic, and Haiti may lead to flash, urban, and small stream flooding, along with the potential for mudslides.
  • 2. Tropical storm conditions are possible over portions of Hispaniola Monday and Monday night.
  • 3. There is a risk of some wind and rainfall impacts across Cuba beginning Tuesday morning, but forecast uncertainty is much higher than usual. Interests there areas should monitor the progress of Grace and updates to the forecast.
FORECAST POSITIONS AND MAX WINDS

INIT  16/0300Z 17.3N  68.6W   30 KT  35 MPH - Depression (ESE Santo Domingo,DR)
 12H  16/1200Z 17.7N  70.3W   30 KT  35 MPH - Depression (WSW Santo Domingo,DR)
 24H  17/0000Z 18.7N  72.8W   30 KT  35 MPH - Depression (ESE Gonave Island,Haiti)
 36H  17/1200Z 19.5N  75.3W   30 KT  35 MPH - Depression (WSW Guantánamo,Cuba)
 48H  18/0000Z 20.4N  78.1W   30 KT  35 MPH - Depression (WNW Manzanillo,MX)
 60H  18/1200Z 21.2N  81.0W   30 KT  35 MPH - Depression (ESE Playa Giron,Cuba)
 72H  19/0000Z 22.2N  84.0W   30 KT  35 MPH - Depression (ENE El Sabalo,Cuba)
 96H  20/0000Z 23.5N  89.0W   35 KT  40 MPH - Tropical Storm (ESE Mérida,MX)
120H  21/0000Z 24.5N  93.5W   45 KT  50 MPH - Tropical Storm (ESE S Padre Beach,TX)

NWS National Hurricane Center Miami FL 500 PM AST Sun Aug 15, 2021 

An earlier ASCAT pass over Grace suggested that the maximum winds were 25-30 kt and this has been confirmed by observations from a NOAA Hurricane Hunter aircraft. Aircraft and scatterometer data also suggest that the circulation is elongated and disorganized. Based on these data, the system is being downgraded to a 30-kt depression at this time. Since the system is below storm strength and is passing by Puerto Rico, the Tropical Storm Warnings for Puerto Rico and the U.S. Virgin Islands have been discontinued.

The island of Hispaniola is kept under a Tropical Storm Watch given the possibility that the system could restrengthen tonight or tomorrow morning before moving over land. Most of the reliable guidance shows little change in strength over the next 2-3 days. This seems reasonable since the circulation will be interacting with the mountainous land mass of Hispaniola and Cuba for the next 72 hours. The official intensity forecast is near or a little above the DSHIPS and LGEM guidance during that time. Some restrengthening is possible later in the forecast period when the system is expected to move over the Gulf of Mexico. However, the global models are not very bullish on intensification in 4-5 days, perhaps due to drier air.

The system has moved mainly westward today, but the track models are generally in agreement on a west-northwestward motion over the forecast period. This is consistent with a well-defined mid-level ridge staying in place over the southwestern Atlantic and across Florida during this week. The official track forecast has been shifted a little south of the previous one, following the multi-model consensus.

Key Messages:

  • 1. Heavy rainfall could lead to flash and urban flooding over the Virgin Islands. Across Puerto Rico, the Dominican Republic and Haiti, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides.
  • 2. Tropical storm conditions are possible over portions of Hispaniola Monday and Monday night.
  • 3. There is a risk of some wind and rainfall impacts across the Turks and Caicos Islands, the Bahamas, Cuba, and Florida, but forecast uncertainty is much higher than usual. Interests in those areas should monitor the progress of Grace and updates to the forecast.
FORECAST POSITIONS AND MAX WINDS

INIT  15/2100Z 17.0N  67.3W   30 KT  35 MPH - Depression (WSW Cabo Rojo,Puerto Rico)
 12H  16/0600Z 17.6N  68.8W   30 KT  35 MPH - Depression (WSW Saona Island,DR)
 24H  16/1800Z 18.2N  71.0W   30 KT  35 MPH - Depression (WSW Barahona,DR)
 36H  17/0600Z 19.2N  73.2W   30 KT  35 MPH - Depression (WSW Grande-Saline,Haiti)
 48H  17/1800Z 20.2N  75.8W   30 KT  35 MPH - Depression (WNW Dos Caminos,Cuba)
 60H  18/0600Z 21.0N  78.7W   30 KT  35 MPH - Depression (ESE Cayo Algodón Grande,Cuba)
 72H  18/1800Z 22.1N  81.8W   30 KT  35 MPH - Depression (WNW Playa Girón,Cuba)
 96H  19/1800Z 24.0N  87.0W   35 KT  40 MPH - Tropical Storm (WSW Key West,FL)
120H  20/1800Z 25.0N  91.0W   45 KT  50 MPH - Tropical Storm (ESE S Padre Island,TX)

NWS National Hurricane Center Miami FL 1100 AM AST Sun Aug 15, 2021

Grace remains a disorganized system, with only slight evidence of curved bands of deep convection on satellite imagery. WSR-88D radar data from San Juan show broad rotation of the precipitation echoes, but no definite center. The advisory intensity is kept at 35 kt in agreement with Dvorak T-numbers from TAFB. We are waiting for additional observations in the system from an upcoming NOAA Hurricane Hunter mission into Grace in a few hours.

Grace is in an environment of moderate shear and reasonably moist mid-level environmental air. Therefore some strengthening is expected before the system reaches Hispaniola tomorrow morning. Thereafter, the future intensity of Grace is dependent on how much the circulation interacts with the islands of the Greater Antilles. Weakening is likely due to the expected passage over the mountainous terrain of Hispaniola, and some restrengthening could occur if the center emerges over water near the Windward Passage. Grace is forecast to maintain minimal tropical storm strength while moving near the northern coast of Cuba. There is, however, considerable uncertainty in the intensity forecast in 2-5 days.

The initial motion estimate is a slightly slower 285/14 kt. A well-defined mid-level high pressure system over the southwestern Atlantic should maintain a general west-northwestward motion for the next several days. The global models are in reasonable agreement on a track near/over the Greater Antilles, including Cuba, during the next several days and over the central Gulf of Mexico in the latter part of the forecast period. The official forecast has been changed little from the previous one and is very close to the latest multi-model consensus, TVCA, solution.

Key Messages:

  • 1. Heavy rainfall could lead to flash and urban flooding over the Virgin Islands. Across Puerto Rico, the Dominican Republic and Haiti, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides.
  • 2. Tropical storm conditions are expected over portions of the Virgin Islands and Puerto Rico today. Tropical storm conditions are expected over parts of the Dominican Republic tonight and Monday. Tropical storm conditions are possible over northwestern portions of the Dominican Republic and Haiti Monday and Monday night.
  • 3. There is a risk of wind and rainfall impacts across the rest of the Dominican Republic, Haiti, the Turks and Caicos Islands, the Bahamas, Cuba, and Florida, but forecast uncertainty remains higher than usual. Interests in those areas should monitor the progress of Grace and updates to the forecast.
FORECAST POSITIONS AND MAX WINDS

INIT  15/1500Z 17.2N  66.0W   35 KT  40 MPH - Tropical Storm (ESE Ponce, Puerto Rico)
 12H  16/0000Z 17.8N  67.8W   40 KT  45 MPH - Tropical Storm (WSW Guánica,PR)
 24H  16/1200Z 18.5N  69.7W   45 KT  50 MPH - Tropical Storm (ESE La Piedra,DR)
 36H  17/0000Z 19.3N  71.7W   30 KT  35 MPH - Tropical Storm (WSW Restauracion,DR)
 48H  17/1200Z 20.3N  74.0W   35 KT  40 MPH - Tropical Storm (WSw Maisí,Cuba)
 60H  18/0000Z 21.3N  76.6W   35 KT  40 MPH - Tropical Storm (WSW Playa La Boca,Cuba)
 72H  18/1200Z 22.3N  79.4W   35 KT  40 MPH - Tropical Storm (ESE El Mamey,Cuba)
 96H  19/1200Z 24.6N  84.7W   45 KT  50 MPH - Tropical Storm (WSW Key West,FL)
120H  20/1200Z 26.5N  89.0W   55 KT  65 MPH - Tropical Storm (WSW Sarasota,FL)

NWS National Hurricane Center Miami FL 1100 PM AST Sat Aug 14, 2021 

Grace is a poorly organized tropical cyclone. Trying to pinpoint the center of the storm over the last 6-12 hours has been a challenge, even with the help of earlier NOAA aircraft, surface observations, and radar data from Guadalupe. Earlier data from the NOAA aircraft reconnaissance Twin Doppler Radar showed that the mid-level center was tilted significantly further southeast relative to the poorly-defined low-level circulation. Around 0000 UTC, multiple reporting stations in Guadalupe showed a wind shift to the west, and there were some skeletal bands seen on the nearby Guadalupe radar ahead of a convective squall propagating ahead, which is currently producing the coldest cloud tops with Grace. Given all of these data, the center of Grace is estimated to have passed by just north of Guadalupe over the last few hours. Despite the higher subjective and objective Dvorak estimates, the initial intensity was kept at 35 kt for this advisory given the lackluster wind data from the earlier NOAA aircraft reconnaissance mission.

Grace appears to have reformed a bit further North tonight, and also now appears to be on a somewhat slower west-northwest heading, at 285/17 kt. There remains a large amount of spread in the guidance, even in the short-term, which is likely related to the current disorganized nature of Grace. In general, Grace is expected to maintain a west-northwestward motion and gradually slow down over the next 24-36 hours. A strong low- to mid-level ridge poleward of Grace should then maintain this west-northwest heading through the remainder of the forecast. A lot of the track uncertainty in the latter part of the forecast appears to be related to the future intensity of the storm, and both the ECMWF and GFS ensembles show a large north-to-south spread with stronger members tracking further north and weaker members tracking further south. For now, the NHC track forecast has been shifted north of the previous track, mostly related to the further north initial position, and is in close agreement with the HFIP corrected-consensus approach (HCCA) model.

Grace appears to finally be slowing down a bit this evening, but will still need to slow down a bit more in order to allow the low- to mid-level centers to become better aligned. Because of this disjointed structure, only slow intensification is anticipated. The latest intensity guidance is a bit lower, and the NHC intensity guidance follows suit, with a peak intensity of 45-kt in 24 hours. Thereafter, it appears likely that Grace will have to deal with significant land interaction over Hispaniola and weakening is indicated by 48 hours. If Grace survives, it is possible some modest intensification could occur in the latter part of the forecast period. The intensity forecast remains closely tied to the track forecast, and any deviations to the north or south could allow Grace to stay stronger than indicated in this forecast. However, it also remains distinctly possible that Grace could dissipate before the end of this 5-day forecast due to the forecasted land interaction.

As previously mentioned, the exact track of the center and the intensity of the system will likely not be as important as the heavy rainfall that is forecast to fall across the Leeward Islands and the Greater Antilles during the next few days.

Key Messages:

  • 1. Tropical storm conditions are expected over the Virgin Islands and Puerto Rico on Sunday. Tropical storm conditions are possible over eastern parts of the Dominican Republic Sunday night and Monday.
  • 2. Heavy rainfall could lead to flash and urban flooding over the Leeward and Virgin Islands, Haiti and the Dominican Republic. Across Puerto Rico, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides.
  • 3. There is a risk of wind and rainfall impacts across the rest of the Dominican Republic, Haiti, the Turks and Caicos Islands, the Bahamas, Cuba, and Florida next week, but forecast uncertainty remains higher than usual. Interests in those areas should monitor the progress of Grace and updates to the forecast.
FORECAST POSITIONS AND MAX WINDS

INIT  15/0300Z 16.8N  62.4W   35 KT  40 MPH - Tropical Storm (WSW Brades,Montserrat)
 12H  15/1200Z 17.3N  64.6W   40 KT  45 MPH - Tropical Storm (WSW Frederiksted,USVI)
 24H  16/0000Z 18.1N  67.0W   45 KT  50 MPH - Tropical Storm (ENE 
 36H  16/1200Z 18.7N  69.1W   45 KT  50 MPH - Tropical Storm (WSW El Seibo,DR)
 48H  17/0000Z 19.3N  70.9W   35 KT  40 MPH - Tropical Storm (WSW Pedregal,DR)
 60H  17/1200Z 20.2N  73.2W   35 KT  40 MPH - Tropical Storm (WSW Pointe Quest,DR)
 72H  18/0000Z 21.2N  75.8W   35 KT  40 MPH - Tropical Storm (ENE Guardalavaca,Cuba)
 96H  19/0000Z 23.7N  81.0W   40 KT  45 MPH - Tropical Storm (ENE Varadero,Cuba)
120H  20/0000Z 25.6N  84.6W   45 KT  50 MPH - Tropical Storm (WSW Marco Island,FL)

NWS National Hurricane Center Miami FL 500 PM AST Sat Aug 14, 2021 

Grace is a poorly organized tropical storm this afternoon. The NOAA Hurricane Hunters were unable to close off a circulation, at least at their flight level of 5000 feet, but dropsonde observations and reports from some of the islands in the Lesser Antilles suggest that there’s at least a broad cyclonic circulation at the surface. The various data also indicate that the center has sped up, or re-formed, and is located farther southwest than previously estimated. Based on the aircraft data and earlier ASCAT data, the initial intensity is set at 35 kt.

Both the future track and intensity of Grace have a high level of uncertainty. For the track, the forecast is likely to be complicated by the fact that the system doesn’t currently have a tight low-level circulation, and the center could always re-form at any time, especially with convection ongoing farther to the north. In addition, the storm has not yet slowed down, and in fact, the initial motion is estimated to be toward the west (275 degrees) at 23 kt. The guidance envelope has made a notable southward shift due to the adjustment of the initial position, and the models insist that Grace will primarily have a west-northwestward heading for much of the forecast period with the speed gradually decreasing during the next 48 hours or so. The NHC track forecast has been shifted southward accordingly, although any re-formations of the center could cause this track to shift again in future advisory cycles.

If Grace slows down as forecast–which is obviously not a sure thing–environmental conditions should be conducive to allow for some strengthening before the system reaches the Greater Antilles. The southward adjustment in the official forecast now takes Grace over the Greater Antilles for a longer period of time, and the official intensity forecast is therefore lowered beyond 48 hours. This is a middle-of-the-road solution, and actually lower than most of the intensity guidance. If the forecast track shifts north or south, the system could strengthen further over water. Alternatively, Grace could go the way of Fred and dissipate before the end of the 5-day period.

In the end, the exact track of the center and the intensity of the system will likely not be as important as the heavy rainfall that is forecast to fall across the Leeward Islands and the Greater Antilles during the next few days.

Key Messages:

  • 1. Tropical storm conditions are expected in portions of the Leeward Islands tonight, and the Virgin Islands and Puerto Rico on Sunday. Tropical storm conditions are possible over eastern parts of the Dominican Republic Sunday night and Monday.
  • 2. Heavy rainfall could lead to flash and urban flooding over the Leeward and Virgin Islands. Across Puerto Rico, the Dominican Republic, and Haiti, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides.
  • 3. There is a risk of wind and rainfall impacts across the rest of the Dominican Republic, Haiti, the Turks and Caicos Islands, the Bahamas, Cuba, and Florida next week, but forecast uncertainty remains higher than usual. Interests in those areas should monitor the progress of Grace and updates to the forecast.
FORECAST POSITIONS AND MAX WINDS

INIT  14/2100Z 15.9N  60.7W   35 KT  40 MPH - Tropical Storm (WSW Grand-Bourg,GP)
 12H  15/0600Z 16.5N  63.3W   40 KT  45 MPH - Tropical Storm (WSW Brades,Montserrat)
 24H  15/1800Z 17.1N  66.1W   45 KT  50 MPH - Tropical Storm (ESE Ponce,Puerto Rico)
 36H  16/0600Z 17.7N  68.5W   50 KT  60 MPH - Tropical Storm (ESE Soana Island,DR)
 48H  16/1800Z 18.3N  70.6W   50 KT  60 MPH - Tropical Storm (WSW Palmar de Ocoa,DR)
 60H  17/0600Z 19.2N  72.8W   40 KT  45 MPH - Tropical Storm (WSW Dugazon,Haiti)
 72H  17/1800Z 20.2N  75.3W   40 KT  45 MPH - Tropical Storm (WSW Sempre,Cuba)
 96H  18/1800Z 22.4N  80.1W   35 KT  40 MPH - Tropical Storm (ENE Ranchuelo,Cuba)
120H  19/1800Z 24.8N  84.3W   45 KT  50 MPH - Tropical Storm (WNW Key West,FL)

NWS National Hurricane Center Miami FL 1100 AM AST Sat Aug 14, 2021

Grace is sending mixed signals on its intensity this morning. The storm has been producing persistent convective bursting since overnight, which would suggest that some strengthening has occurred. The latest subjective data-T numbers are 2.5 (35 kt) from TAFB and SAB, while objective satellite estimates are higher, roughly between 45-50 kt. Then, an ASCAT pass from 1302 UTC showed winds between 35-40 kt. Given these data, the initial intensity is set at 40 kt. A NOAA Hurricane Hunter aircraft is scheduled to investigate Grace this afternoon to provide a better estimate of the storm’s intensity.

Grace is speeding along toward the west (280 degrees) at 19 kt. Mid-level ridging, entrenched over the western Atlantic, is expected to weaken slightly during the next couple of days. This evolution should cause Grace to slow down to 10-15 kt by Sunday night and Monday and take on a west-northwestward heading. That general trajectory should continue through the end of the forecast period. The track guidance is tightly clustered during the first 48-60 hours or so, showing Grace moving near or across the Leeward Islands, Puerto Rico, and the Dominican Republic. On days 3-5, there is a little more spread, with a few models keeping the system on a southern track across Cuba while others show tracks across the Bahamas. The NHC track forecast splits this difference and continues to show a track running between Cuba and the Bahamas, very close to the HCCA consensus solution.

Deep-layer shear is forecast to be 10 kt or less for the next 36 hours or so while Grace is approaching the islands of the Greater Antilles. The thermodynamic environment also appears conducive for strengthening, but the system’s continued fast motion is likely to be an inhibiting factor on the rate of intensification. The NHC intensity forecast remains on the conservative side, bringing Grace’s intensity up to 50 kt by the time the storm reaches the Virgin Islands and Puerto Rico, but it should be noted that this forecast is on the lower side of the guidance. Only one model, the HWRF, brings Grace to hurricane strength, but it does this by having the storm move farther north and avoid land interaction altogether. The intensity forecast is highly uncertain on days 3-5 since it depends on exactly how much Grace moves over the Greater Antilles, and there is some model signal that increasing northeasterly upper-level winds over the Bahamas and Florida could become a negative factor.

Key Messages:

  • 1. Tropical storm conditions are expected in portions of the Leeward Islands tonight and early Sunday, and the Virgin Islands and Puerto Rico on Sunday. Tropical storm conditions are possible over eastern parts of the Dominican Republic by early Monday.
  • 2. Heavy rainfall could lead to flash and urban flooding over the Leeward and Virgin Islands. Across Puerto Rico and the Dominican Republic, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides.
  • 3. There is a risk of wind and rainfall impacts across the rest of the Dominican Republic, Haiti, the Turks and Caicos Islands, the Bahamas, and Florida next week, and interests in those areas should monitor the progress of Grace and updates to the forecast.
FORECAST POSITIONS AND MAX WINDS

INIT  14/1500Z 16.2N  57.9W   40 KT  45 MPH - Tropical Storm (ENE Grand-Bourg,Guad)
 12H  15/0000Z 16.8N  60.6W   45 KT  50 MPH - Tropical Storm (ESE English Harbour,Antigua)
 24H  15/1200Z 17.5N  63.8W   50 KT  60 MPH - Tropical Storm (WNW The Bottom,Saba)
 36H  16/0000Z 18.1N  66.4W   50 KT  60 MPH - Tropical Storm (WNW Coamo,Puerto Rico)
 48H  16/1200Z 18.6N  68.6W   50 KT  60 MPH - Tropical Storm (WSW Higüey,DR)
 60H  17/0000Z 19.4N  70.7W   45 KT  50 MPH - Tropical Storm (WSW Las Charcas,DR)
 72H  17/1200Z 20.5N  72.8W   45 KT  50 MPH - Tropical Storm (ENE Tortuga,Haiti)
 96H  18/1200Z 22.8N  77.3W   45 KT  50 MPH - Tropical Storm (ENE Cayo Romano,Cuba)
120H  19/1200Z 25.4N  81.2W   45 KT  50 MPH - Tropical Storm (ESE Marco Island,FL)

NWS National Hurricane Center Miami FL 1100 PM AST Fri Aug 13, 2021

While Tropical Depression Seven has maintained a irregularly shaped cirrus canopy of deep cold cloud tops near its estimated center, the convection does not appear well organized. Several SSMIS microwave passes between 2011 UTC and 2205 UTC did not reveal much organization under the cirrus, with just a few patches of deeper convection contributing to the larger stratiform region. A helpful ASCAT-B pass at 0030 UTC showed that the center was near the southeastern end of this cirrus canopy, and found peak winds lower than earlier today at only 27 kt. The 0000 UTC subjective Dvorak estimates were T2.5/35 kt from SAB and T2.5/30 kt from TAFB and the most recent objective ADT estimate was in between at T2.2/32 kt. A blend of these data support keeping the initial intensity at 30-kt for this advisory.

The small cyclone continues to move quickly off to the west at 280/18 kt. A large low- to mid-level ridge draped across the central and western North Atlantic is expected to maintain the system on a general west-northwest heading, though with gradual deceleration as the ridge is eroded some by a mid- to upper-tropospheric trough. The latest track guidance remains tightly clustered but a bit more poleward through the first 72 hours. Afterwards, more track guidance spread becomes apparent. A quick look at the latest ECMWF ensemble guidance suggest that some of this spread is driven by the forecast intensity of the system, with stronger members taking the cyclone on a more poleward track. For now, the latest NHC track forecast is fairly close to, but a little poleward of the previous track. This track remains close to the HCCA and TVCN consensus aids, and roughly splits the difference between the deterministic GFS and ECMWF model solutions.

The intensity forecast is somewhat conflicting. Even though both the GFS & ECMWF based SHIPS guidance depict low 200-850 hPa vertical wind shear between 5-10 knots over the next 48 hours, the depression is also embedded in very dry mid-level air, with 700-500 hPa layer mean relative humidity as low as 44 percent currently in the ECMWF-SHIPS. In addition, the system is moving rapidly westward, and a continued fast motion in the short-term may result in higher westerly mid-level shear which may have a larger than normal effect to a small tropical cyclone in a very dry environment.

After 48 hours, vertical wind shear out of the northwest is expected to increase, ahead of a large upper-level trough digging southwestward, upstream of the cyclone. Moreover, land interaction with both Puerto Rico and Hispaniola remains a distinct possibility, especially if the cyclone tracks left of the current forecast track. It is worth noting the latest HWRF run continues to be a extreme outlier with a much higher intensity than the remaining guidance. In fact, much of global model guidance and COAMPS-TC regional hurricane model barely maintains a closed circulation over the next 36-48 hours. I have elected to maintain a very similar forecast to the previous advisory, with peak winds of only 45 kt in 48-60 hours. This forecast remains conservative and is still lower than the SHIPS and HCCA intensity guidance.

Key Messages:

  • 1. Tropical storm conditions are expected in portions of the Leeward Islands late Saturday or early Sunday, and are possible over the Virgin Islands and Puerto Rico on Sunday. The risk of strong winds will then spread westward to the Dominican Republic Sunday night and Monday.
  • 2. Heavy rainfall could lead to flash and urban flooding over the Leeward and Virgin Islands. Across Puerto Rico, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides.
  • 3. There is a risk of wind and rainfall impacts across Haiti, the Turks and Caicos Islands, the southeastern Bahamas, and Cuba next week, and interests in those areas should monitor the progress of this system.
FORECAST POSITIONS AND MAX WINDS

INIT  14/0300Z 15.5N  53.8W   30 KT  35 MPH - Depression (WSW La Désirade,GP)
 12H  14/1200Z 16.0N  56.8W   35 KT  40 MPH - Tropical Storm (WSW La Désirade,GP)
 24H  15/0000Z 16.7N  60.3W   35 KT  40 MPH - Tropical Storm (WSW La Désirade,GP)
 36H  15/1200Z 17.5N  63.5W   40 KT  45 MPH - Tropical Storm (ESE The Bottom,Saba)
 48H  16/0000Z 18.3N  66.2W   45 KT  50 MPH - Tropical Storm (ESE San Juan,Puerto Rico)
 60H  16/1200Z 18.9N  68.2W   45 KT  50 MPH - Tropical Storm (ESE Macao,DR)
 72H  17/0000Z 19.6N  70.4W   35 KT  40 MPH - Tropical Storm (ESE Cabarete,DR)
 96H  18/0000Z 21.7N  74.3W   40 KT  45 MPH - Tropical Storm (ENE Banes,Cuba)
120H  19/0000Z 24.3N  78.6W   40 KT  45 MPH - Tropical Storm (WSW Red Bay,Bahamas)

NWS National Hurricane Center Miami FL 500 PM AST Fri Aug 13, 2021 

Over the past few hours, the system moved just to the north of a buoy owned by the Woods Hole Oceanographic Institute. The buoy’s winds backed from northwest to west to south, indicating that the system has a closed surface circulation. In addition, Dvorak estimates from TAFB and SAB were a unanimous T2.0, and the system is therefore being designated as a tropical depression on this advisory. Maximum winds remain 30 kt based on earlier ASCAT data and the Dvorak estimates.

The circulation has closed off despite the depression moving quickly westward (275 degrees) at about 19 kt. The track guidance is tightly clustered during the first 48-60 hours, with the depression being driven quickly westward across the Leeward Islands and toward the Greater Antilles by ridging to the north. After 60 hours, there is considerably more spread, with the regional dynamical models keeping the system farther south over the Caribbean Sea, and most of the other models indicating a turn toward the west-northwest, following a track similar to Tropical Depression Fred. The NHC track forecast generally favors the latter scenario and is very close to the HCCA and TVCN consensus aids.

During the next 2 days, the depression is expected to move beneath an upper-level ridge axis, which should allow the deep-layer shear to fall below 10 kt, with the system also moving over warmer waters and through an environment of increased moisture. However, the depression’s fast motion, as well as the possible development of some mid-level westerly shear, could stunt the rate of strengthening. Due to these conflicting factors, the NHC intensity forecast remains on the conservative side and is not quite as high as the solutions shown by the SHIPS and HCCA models. The HWRF model is quite aggressive, bringing the depression to hurricane strength by day 3, but that model is an extreme outlier compared to the other guidance. After 48 hours, the current forecast takes the center of the depression over Puerto Rico and Hispaniola, a scenario that would cause weakening and suppress the system’s intensity. As is typically the case, the system could get stronger than shown in the official forecast if it ends up moving over less land, or dissipate entirely if it moves over land for too long.

Key Messages:

  • 1. Tropical storm conditions are possible in portions of the Leeward Islands late Saturday or early Sunday, and over the Virgin Islands and Puerto Rico on Sunday. The risk of strong winds will then spread westward to the Dominican Republic Sunday night and Monday.
  • 2. Heavy rainfall could lead to flash and urban flooding over the Leeward and Virgin Islands. Across Puerto Rico, heavy rainfall may lead to flash, urban and small stream flooding, along with the potential for mudslides.
  • 3. There is a risk of wind and rainfall impacts across Haiti, the Turks and Caicos Islands, the southeastern Bahamas, and Cuba next week, and interests in those areas should monitor the progress of this system.
FORECAST POSITIONS AND MAX WINDS

INIT  13/2100Z 15.4N  51.8W   30 KT  35 MPH - Depression (ENE La Désirade,Guadeloupe)
 12H  14/0600Z 15.9N  54.7W   35 KT  40 MPH - Tropical Storm (ENE La Désirade,GP)
 24H  14/1800Z 16.4N  58.5W   35 KT  40 MPH - Tropical Storm (ENE La Désirade,GP)
 36H  15/0600Z 17.0N  61.9W   40 KT  45 MPH - Tropical Storm (ESE Urlings,A&B)
 48H  15/1800Z 17.7N  65.0W   45 KT  50 MPH - Tropical Storm (ESE Rainbow Beach,USVI)
 60H  16/0600Z 18.4N  67.7W   45 KT  50 MPH - Tropical Storm (WNW Rinco,Puerto Rico)
 72H  16/1800Z 19.0N  70.3W   40 KT  45 MPH - Tropical Storm (ESE Caballero,DR)
 96H  17/1800Z 20.9N  74.7W   35 KT  40 MPH - Tropical Storm (ENE Moa,Cuba)
120H  18/1800Z 23.6N  79.1W   40 KT  45 MPH - Tropical Storm (ESE Marathon,FL)

NWS National Hurricane Center Miami FL 1100 AM AST Fri Aug 13, 2021 

A tropical wave and small area of low pressure moving quickly westward across the central tropical Atlantic has been producing bursts of deep convection near and to the west of the wave axis. Scatterometer data from last evening indicated that the system did not quite have a closed circulation, but it is close. In addition, the convective organization has increased a bit since yesterday, with TAFB and SAB providing Dvorak classification of T1.5 and T2.0. Therefore, only an additional slight increase in convective organization and the low-level circulation closing off would make the system a tropical depression, and it could be at tropical storm strength when it approaches the Leeward Islands during the next couple of days.

Therefore, advisories are being initiated on the system in order to issue Tropical Storm Watches for portions of the Leeward Islands at this time. Subtropical ridging extending across nearly the entire Atlantic is pushing the low along quickly toward the west, or 280/18 kt. In general this ridging is expected to persist into the middle part of next week. However, around the Sunday-Monday time frame, some mid-level troughing over the western Atlantic could allow the system to gain a little more latitude before it runs into additional ridging located near the east coast of the United States at the end of the forecast period. The GFS, which has perhaps one of the strongest solutions, is a northern outlier and shows the system turning farther into the weak troughing, while the ECMWF has a weaker solution and keeps the system on a westward track across the Lesser and Greater Antilles. At this time, the NHC official forecast lies close to the TVCN and HCCA consensus aids, and it is also relatively close to the ECMWF ensemble mean, which is farther north than its parent model.

The biggest negative to the system becoming a tropical cyclone and strengthening is its fast motion of 15-20 kt. However, that motion is expected to gradually decrease in 2 to 3 days. At that same time, deep-layer shear has decreased a bit, and the system will be moving over warmer waters and toward a slightly more moist environment. Therefore, gradual strengthening is anticipated, and the NHC intensity forecast closely follows the HCCA and IVCN consensus aids through 72 hours, up until the system is near Hispaniola. After that time, land interaction is likely to disrupt the circulation, and the official forecast is below the model consensus aids on days 4 and 5.

Key Messages:

  • 1. Tropical storm conditions are possible in portions of the Leeward Islands late Saturday or early Sunday, and the risk of tropical storm conditions will spread westward to the Virgin Islands and Puerto Rico late Sunday and Sunday night.
  • 2. Heavy rainfall with this system may produce areas of scattered flash flooding over the northern Leeward Islands Saturday into Sunday.
  • 3. There is a risk of wind and rainfall impacts in portions of Hispaniola, the Turks and Caicos Islands, the southeastern Bahamas, and Cuba next week, and interests in those areas should monitor the progress of this system.
FORECAST POSITIONS AND MAX WINDS

INIT  13/1500Z 15.3N  49.3W   30 KT  35 MPH - Depression (WSW St John's, A&B)
 12H  14/0000Z 15.7N  52.0W   30 KT  35 MPH - Depression (WSW St John's, A&B)
 24H  14/1200Z 16.2N  55.9W   35 KT  40 MPH - Tropical Storm (WSW St John's, A&B)
 36H  15/0000Z 16.7N  59.6W   35 KT  40 MPH - Tropical Storm (WSW St John's, A&B)
 48H  15/1200Z 17.2N  63.0W   40 KT  45 MPH - Tropical Storm (WSW Oranjestad)
 60H  16/0000Z 17.8N  66.0W   40 KT  45 MPH - Tropical Storm (WSW Arroyo,Puerto Rico)
 72H  16/1200Z 18.4N  68.7W   45 KT  50 MPH - Tropical Storm (WSW San Rafael de Yuma,DR)
 96H  17/1200Z 20.0N  73.5W   35 KT  40 MPH - Tropical Storm (WSW Tortuga,Cuba)
120H  18/1200Z 23.0N  77.0W   40 KT  45 MPH - Tropical Storm (WSW Rolletown,Bahamas)

NWS National Hurricane Center Miami FL 800 PM EDT Thu Aug 12, 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Tropical Depression Fred, located about 240 miles southeast of southern Andros Island, Bahamas.

  • 1. A small low pressure system located about 1100 miles east of the Lesser Antilles is producing disorganized shower and thunderstorm activity primarily west of the center. However, environmental conditions are expected to gradually become more conducive for additional development, and a tropical depression is likely to form by the weekend. The system is forecast to move generally westward at about 20 mph across the tropical Atlantic, reaching portions of the Leeward Islands late Saturday or early Sunday, and interests there should monitor its progress. * Formation chance through 48 hours…high…70 percent. * Formation chance through 5 days…high…70 percent.

NWS National Hurricane Center Miami FL 200 PM EDT Thu Aug 12 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Tropical Depression Fred, located about 105 miles west of Great Inagua, Bahamas.

  • 1. Satellite-derived wind data from this morning indicated that a small area of low pressure has developed along a tropical wave about 1200 miles east of the Lesser Antilles. However, the low does not quite have a closed circulation, and the associated shower and thunderstorm activity remains disorganized. Environmental conditions are expected to become more conducive for additional development, and a tropical depression is likely to form by the weekend while moving generally westward at about 20 mph across the tropical Atlantic. This system is expected to reach portions of the Leeward Islands late Saturday or early Sunday, and interests there should monitor its progress. * Formation chance through 48 hours…medium…60 percent. * Formation chance through 5 days…high…70 percent.

NWS National Hurricane Center Miami FL 800 PM EDT Wed Aug 11 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on recently downgraded Tropical Depression Fred, located inland over the central portion of Hispaniola.

  • 1. A tropical wave located over the central tropical Atlantic about 1600 miles east of the Lesser Antilles continues to produce disorganized showers and thunderstorms. Environmental conditions are expected to gradually become more conducive for development, and a tropical depression could form by early next week while the system moves generally westward at about 20 mph across the tropical Atlantic. This system could reach portions of the Leeward Islands by late Saturday. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…medium...60 percent.

NWS National Hurricane Center Miami FL 200 PM EDT Wed Aug 11, 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Tropical Storm Fred, located about 30 miles west of Santo Domingo, Dominican Republic.

  • 1. A tropical wave located over the eastern tropical Atlantic several hundred miles west-southwest of the Cabo Verde Islands continues to produce disorganized showers and thunderstorms. Environmental conditions are expected to become gradually more conducive for development of this system, and a tropical depression could form by early next week while moving generally westward across the tropical Atlantic near 20 mph. This system could reach portions of the Leeward Islands by late Saturday. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…medium...50 percent.

NWS National Hurricane Center Miami FL 200 PM EDT Tue Aug 10, 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

The National Hurricane Center is issuing advisories on Potential Tropical Cyclone Six, located over the northeastern Caribbean Sea. * Formation chance through 48 hours…high…90 percent.  * Formation chance through 5 days…high…90 percent.

  • 1. A concentrated area of showers and thunderstorms located a few hundred miles southwest of the Cabo Verde Islands is associated with a tropical wave. Some gradual development of this system is possible over the next several days while it moves westward across the tropical Atlantic at 15 to 20 mph. * Formation chance through 48 hours…low…10 percent. * Formation chance through 5 days…low...20 percent.

NWS National Hurricane Center Miami FL 800 PM EDT Sun Aug 8, 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

  • 1. A low-pressure system located about 400 miles east-southeast of the Leeward Islands is producing disorganized showers and thunderstorms. However, environmental conditions are expected to be conducive for gradual development over the next few days, and a tropical depression could form while the low moves west-northwestward at about 15 mph. The disturbance is forecast to reach portions of the Lesser Antilles by late Monday, and then move near the Virgin Islands and Puerto Rico on Tuesday, and be near Hispaniola around the middle of this week. Tropical storm watches or warnings could be required with shorter-than-normal lead times for the Leeward Islands, Virgin Islands, and Puerto Rico. In addition, heavy rains and flooding are likely for the Leeward Islands, Virgin Islands, Puerto Rico. Interests in those areas should monitor the progress of this system. * Formation chance through 48 hours…medium…50 percent. * Formation chance through 5 days…medium…60 percent.
  • 2. Disorganized showers and thunderstorms have changed little in association with an elongated low pressure system located over the tropical Atlantic about midway between the Cabo Verde Islands and the Lesser Antilles. Although environmental conditions appear to be only marginally conducive for development, this system could still become a tropical depression later this week while it moves toward the west or west-southwest at around 10 mph. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…medium…40 percent.

NWS National Hurricane Center Miami FL 800 AM EDT Sun Aug 8, 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

  • 1. An area of low pressure is producing disorganized showers and thunderstorms over the tropical Atlantic several hundred miles east of the Windward Islands. Environmental conditions are expected to be favorable to support some gradual development, and this system could become a tropical depression while it moves west- northwestward at 10 to 15 mph. The disturbance is forecast to reach portions of the Lesser Antilles late Monday and then move across the eastern Caribbean Sea and Greater Antilles through the middle of this week. Interests in those areas should monitor the progress of this system, as it could bring locally heavy rainfall and gusty winds to portions of that area. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…medium…40 percent.
  • 2. Showers and thunderstorms associated with an area of low pressure located over the tropical Atlantic about midway between the Cabo Verde Islands and the Lesser Antilles have become more concentrated since yesterday. Environmental conditions are expected to become a little more favorable for gradual development, and this system could become a tropical depression within the next few days. The system is forecast to move slowly toward the west-southwest or west during the next couple of days, and then move a little faster toward the west by mid-week. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…medium…40 percent.
  • 3. Showers and thunderstorms associated with a broad area of low pressure located about a hundred miles west of the southwesternmost Cabo Verde Islands have diminished since yesterday. Strong upper-level winds and cooler waters are expected to prevent the development of this system while it moves toward the west or west-northwest. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…low…near 0 percent.

NWS National Hurricane Center Miami FL 800 PM EDT Fri Aug 6, 2021

For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

  • 1. A broad area of low pressure located a few hundred miles south of the Cabo Verde Islands is producing a large area of disorganized cloudiness and showers. Environmental conditions are expected to be somewhat conducive for development, and a tropical depression could form by the early to middle part of next week while the system moves west-northwestward to westward across the eastern and central tropical Atlantic at 10 to 15 mph. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…medium...50 percent.
  • 2. A small but well-defined area of low pressure located over the eastern tropical Atlantic about a thousand miles west of the Cabo Verde Islands is producing limited shower activity. Development of this system, if any, is expected to be slow to occur while it drifts toward the west-southwest or west over the central tropical Atlantic through the middle of next week and potentially interacts with the broader area of low pressure to its east. * Formation chance through 48 hours…low…10 percent. * Formation chance through 5 days…low...20 percent.
  • 3. A tropical wave located over the west-central tropical Atlantic is producing limited shower activity. Significant development of this system is not expected as it moves west-northwestward across the Lesser Antilles and eastern Caribbean Sea during the early to middle part of next week. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…low…10 percent.

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