Tropical Storm Isaias

Hurricane Isaias Track 1100 Hours August 1 2020
Hurricane Isaias Track 1100 Hours August 1 2020

Hurricane IsaiasHurricane Isaias Wind FieldNWS National Hurricane Center Miami FL 1100 AM EDT Sat Aug 01, 2020 (see Saturday video below)

Data from an Air Force Reserve reconnaissance aircraft around 1410 UTC indicated that the center of the eye of Isaias was located along the southern coast of northern Andros Island about 15 nmi south-southwest of Andros Town. The eye appearance in Bahamas and aircraft radar data, along with the visual reports from the flight crew, has gone from nearly closed a few hours ago to open in the southwestern quadrant more recently. However, the diameter of the eye has been has been holding steady between 20-22 nmi, an indication that Isaias has been able to fight off some modest southwesterly wind shear. The initial intensity has been lowered to 70 kt based on maximum 700-mb flight-level winds of 77 kt, which equals about 70-kt surface wind speed, and the central pressure fluctuating between 987-990 mb.

The initial motion estimate remains 315/10 kt. The latest NHC model guidance is now tightly packed about the previous 18-h worth of official track forecasts, and as a result, no significant changes were made to the previous advisory track. The global and regional models have come into much better agreement compared to 24 hours ago in taking Isaias northwestward slowly for the next 36 h or so, and moving the center near or keeping it just offshore the east-central Florida coast. By 48 hours, the hurricane is expected to turn northward around the western portion of the Bermuda-Azores ridge that will slowly be eroded by an approaching mid-level shortwave trough currently situated over the central United States. By 60 h and beyond, the aforementioned trough is expected to gradually accelerate Isaias northeastward near or along the coast from South Carolina to New England. The new NHC track forecast is basically an extension of the previous one, and lies very close to an average of the simple consensus model TVCA, and the corrected- consensus models NOAA-HCCA and FSSE.

In the near term, Isaias could weaken a little bit this afternoon while passing over northern Andros Island. However, the still impressive vertical structure of the cyclone should allow for some re-strengthening after the center moves back over the warm Gulf Stream by this evening. The current SHIPS analyzed westerly vertical wind shear of 25 kt could be too high due to the model incorporating some of the storm’s outflow. The models forecast the shear to weaken somewhat over the next 36 h while Isaias is moving over the Gulf Stream, and the 06Z UKMET shows Isaias moving underneath a 200-mb anticyclone, which would normally favor some strengthening. Given all of these factors, the official forecast maintains a steady intensity through Monday night. Slow weakening is forecast when Isais encounters more significant southwesterly vertical wind shear ahead of a strong upper-level trough that will be approaching the U.S. east coast on days 3-5.

Key Messages:

  • 1. Hurricane conditions and dangerous storm surge will continue in portions of the northwest Bahamas today and tonight.
  • 2. Hurricane conditions are expected along portions of the Florida east coast by late tonight and Sunday. Preparations to protect life and property should be rushed to completion.
  • 3. Dangerous storm surge is possible along the Florida east coast from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4 feet above ground level are possible along the immediate coastline and adjacent waterways. Residents there should follow advice given by local emergency officials.
  • 4. Isaias will produce heavy rains and potentially life-threatening flash flooding in the Bahamas, and flash urban flooding, especially in low-lying and poorly drained areas, in eastern Florida, and from the Carolinas to the mid-Atlantic and Northeast U.S. Minor river flooding is possible across portions of the Carolinas and Virginia early next week.
  • 5. Tropical storm watches and warnings are in effect for portions of the northeast Florida and southern Georgia coasts. Additional watches and warnings will likely be issued later today and Sunday along the U.S. east coast as the risk of wind, heavy rainfall, and storm surge impacts continues to increase.

NWS National Hurricane Center Miami FL 1100 PM EDT Fri Jul 31, 2020 

Isaias has a somewhat asymmetrical appearance on satellite images, with lots of deep convection over the eastern semicircle and not as much over the western part of the circulation. Reports from an Air Force Hurricane Hunter aircraft indicate that the central pressure has dropped a few mb, and flight-level and SFMR-observed surface wind observations from the aircraft indicate an intensity of near 70 kt.

The hurricane is experiencing some westerly shear, and this is likely to limit intensification. However, some strengthening is still possible on Saturday. When Isais moves farther north in a couple of days, the shear should cause a gradual weakening trend to begin. The official intensity forecast is about the same as the previous one, and a little above the model consensus. The hurricane continues moving northwestward at a bit slower pace, or 310/13 kt. The general track forecast philosophy has not changed. Isais should continue to move northwestward on the southwest side of a mid-level ridge through Saturday and begin to turn north-northwestward through a weakness in the ridge on Sunday. Thereafter, a turn toward the north-northeast and northeast with an increase in forward speed is likely during days 3 to 5. The official track forecast is close to the previous one and also very close to the latest dynamical model consensus. The hurricane warning has been extended northward along the Florida east coast.

Key Messages:

  • 1. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas through Saturday, and Hurricane Warnings are in effect.
  • 2. Hurricane conditions are expected along portions of the Florida east coast late Saturday and Saturday night, and a Hurricane Warning has been issued. Preparations to protect life and property should be rushed to completion.
  • 3. Dangerous storm surge is possible along the Florida east coast from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4 feet above ground level are possible along the immediate coastline and adjacent waterways. Residents in these areas should follow advice given by local emergency officials.
  • 4. Isaias will produce heavy rains and potentially life-threatening flash and urban flooding, especially in low-lying and poorly drained areas, across South to east-Central Florida, and across the Carolinas to the mid Atlantic. Isolated minor river flooding is possible across the eastern Carolinas and into Virginia early next week. 5. There is a risk of impacts from winds, heavy rainfall, and storm surge spreading along much of the the U.S. east coast through early next week, and interests there should monitor the progress of Isaias and updates to the forecast.

NWS National Hurricane Center Miami FL 500 PM EDT Fri Jul 31, 2020

Deep convection, with occasional overshooting cloud tops of -85C to -90C just north of the center, has continued to develop during the normal diurnal convective minimum period, which is quite impressive. The most recent Air Force Reserve recon flight-level wind data, along with ASCAT surface wind data, indicate that the inner-core and outer wind field have both contracted in size. Furthermore, radar data from the Bahamas and an 1810Z AMSR-2 microwave pass also indicate that a small 10-nmi-wide mid-level eye is forming. The last recon central pressure was 991 mb and the 700 mb height had decreased by 30 meters since the earlier maximum height around 1230Z. These data indicate that Isaias is getting better organized. The initial intensity remains 65 kt based on an earlier 700-mb flight-level wind speed of 72 kt, which reduces to a 65-kt surface wind speed using a 90-percent adjustment factor.

The initial motion remains northwestward or 305/12 kt. The 12Z global models have once again made a westward shift due to the ridge to the north of Isaias not weakening as quickly as expected. This is partly due to the ridge being stronger than expected and a shortwave trough over the central United States moving a little slower into the southeastern U.S. than previously indicated. The UKMET and ECMWF explicitly show Isaias making landfall in 36-48 hours along the southeast Florida coast, but appear to weaken the system below hurricane strength. The GFS similarly brings the cyclone close to the southeast and east-central Florida coasts, but also as a somewhat weaker system.

In the 48 to 60-hour period, the cyclone is forecast to move slowly north-northwestward and northward through a break in the subtropical ridge extending westward from the Atlantic across Florida and into the northern Gulf of Mexico. By that time, however, Isaias is expected to weaken below hurricane strength due to the combination of strong southwesterly vertical wind shear and interaction the Florida peninsula. Around 72 hours, the cyclone should accelerate northeastward and possibly strengthen some before passing over eastern North Carolina on day 4, and across eastern New England on day 5. The NHC track forecast lies close to a blend of the consensus models TVCA and NOAA-HCCA and is east of the UKMET and ECMWF with the system forecast to be stronger than those models indicate. Due to the westward shift in the NHC forecast track, a Hurricane Warning and Storm Surge Watch have been issued for portions of the Florida east coast.

The center of Isaias is now located in the center of an expanding CDO feature. The improved inner-core wind field and aforementioned convective structure, along with very warm SSTs near 30C, should support some strengthening overnight and early Saturday morning. However, increasing southwesterly vertical wind shear is expected to cause a gradual decrease in intensity by Sunday and continue into early next week. The new official intensity forecast is a little lower than the previous advisory and is near the higher end of the intensity guidance.

Key Messages:

1. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas through Saturday, and Hurricane Warnings are in effect.

2. Hurricane conditions are expected along portions of the Florida east coast late Saturday and Saturday night, and a Hurricane Warning has been issued. Preparations to protect life and property should be rushed to completion.

3. Dangerous storm surge is possible along the Florida east coast from Jupiter Inlet to Ponte Vedra Beach where water rises of 2 to 4 feet above ground level are possible along the immediate coastline and adjacent waterways. Residents in these areas should follow advice given by local emergency officials.

4. Isaias will produce heavy rains and potentially life-threatening flash and urban flooding, especially in low-lying and poorly drained areas across south to east-central Florida, and across the Carolinas to the mid Atlantic. Isolated minor river flooding is possible across the eastern Carolinas and into Virginia early next week.

5. There is a risk of impacts from winds, heavy rainfall, and storm surge spreading along much of the the U.S. east coast through early next week, and interests there should monitor the progress of Isaias and updates to the forecast.

NWS National Hurricane Center Miami FL 1100 AM EDT Fri Jul 31, 2020 

After a brief convective hiatus earlier this morning, a small burst of deep convection with cloud tops as cold as -80C to -85C have developed over the previously exposed low-level center. Dry downslope air coming off of Hispaniola wrapped into the center, eroding the inner-core convection somewhat. However, a surge of moist southwesterly inflow moving through the Windward Passage now appears to be fueling the recent increase in convection. The upper-level outflow has improved somewhat in the western semicircle but still remains restricted due to some modest southwesterly vertical wind shear. The initial intensity has been lowered to 65 kt based on 700-mb flight-level winds of near 70 kt and an increase in the central pressure of 992 mb, which is based on a dropsonde report of 995 mb with 26-kt surface winds. Isaias is continuing on a steady northwestward motion of 305/14 kt.

This motion is expected to continue for the next 36 hours or so as the hurricane rounds the southwestern periphery of the Bermuda-Azores High. A gradual turn to the north-northwest and north is expected by 48-60 hours due to a break in the ridge developing in response to a central U.S. shortwave trough digging into the southeastern United States. The timing and strength of this trough will determine how far west Isaias moves before the hurricane turns northward. The 06Z UKMET and and GFS have shifted westward, closer to the Florida coast, which is similar to the westward shift seen in the 00Z ECMWF model fields. By 72 hours, the hurricane is forecast to begin accelerating northeastward, possibly passing over eastern North Carolina by day 4 and across eastern New England on day 5. As a result of the westward shift in the latest model guidance, the new NHC forecast track has also been shifted farther west closer to the southeastern U.S. coast, and lies a little to the west of the consensus models TVCA and NOAA-HCCA. The westward shift in the track forecast has required the issuance of a Hurricane Watch for portions of the Florida east coast.

Although some slight weakening has occurred, radar data from the aircraft and the Bahamas radar indicate about a 60-percent eyewall has formed in the northeastern semicircle, which is an indication that the cyclone is trying to reorganize. As a result, strengthening is still expected during the next day or so, especially tonight and Saturday morning during the convective maximum period when the hurricane will be moving over the Gulf Stream where SSTs are 30C or warmer and while the vertical remains reasonably low. Increasing southwesterly shear could cause a gradual decrease in intensity over the weekend. The new official intensity forecast is similar to the previous advisory, and is a little above the available model guidance out of respect for continuity.

Key Messages:

  • 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Dominican Republic, northern Haiti, Turks and Caicos, and the Bahamas. Heavy rains associated with Isaias may begin to affect south and east-central Florida late Friday night, and the eastern Carolinas by early next week, potentially resulting in isolated flash and urban flooding, especially in low-lying and poorly drained areas. Isolated minor river flooding is possible in the Carolinas early next week.
  • 2. Hurricane conditions and dangerous storm surge are expected in portions of the Bahamas today and Saturday, and Hurricane Warnings are in effect. Preparations to protect life and property should be rushed to completion.
  • 3. Tropical storm conditions are expected with hurricane conditions possible along portions of the Florida east coast beginning Saturday, and a Tropical Storm Warning and Hurricane Watch are in effect. Storm surge watches or warnings could be needed for part of this area this afternoon.
  • 4. There is a risk of impacts from winds, heavy rainfall, and storm surge beginning late this weekend along the northeastern Florida coast and spreading northward along the remainder of the U.S. east coast through early next week. Interests along the entire U.S. east coast should monitor the progress of Isaias and updates to the forecast.

NWS National Hurricane Center Miami FL 1100 PM EDT Thu Jul 30, 2020 

Isaias is beginning to form a small Central Dense Overcast, but much of the deep convection is situated over the eastern portion of the circulation. Water vapor imagery suggests that upper-level outflow is modest to the northwest of the cyclone. The southern portion of the system is still interacting with Hispaniola at this time. Dvorak intensity estimates are 45 kt from SAB and 55 kt from TAFB so the current maximum wind speed is 50 kt for this advisory.

An Air Force Hurricane Hunter aircraft is headed for Isaias and should soon provide a more precise intensity estimate. Although southwesterly shear may limit strengthening of the system during the next couple of days, the environment should still be conducive enough for Isaias to become a hurricane in 24 to 36 hours. The official intensity forecast is similar to the previous one and a little above the model consensus.

The initial motion is a somewhat uncertain 305/16 kt. There are no basic changes to the track forecast or reasoning from the previous advisory. Isaias should continue to move generally northwestward on the southern and southwestern side of a subtropical high pressure area for the next day or so. Then, as an approaching mid-tropospheric trough erodes the high, a gradual turn toward the north-northwest and north should occur. In 3 to 5 days, the trough should cause Isaias to turn toward the north-northeast and northeast and accelerate. The official track forecast is close to the previous one and follows the latest simple and corrected dynamical model consensus tracks.

Key Messages:

  • 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Dominican Republic, northern Haiti, Turks and Caicos, and the Bahamas.
  • 2. Hurricane or tropical storm conditions and dangerous storm surge are expected in portions of the northwestern and central Bahamas late Friday and Saturday, and Hurricane and Tropical Storm Warnings, respectively, are in effect for these areas. Preparations to protect life and property should be rushed to completion. Tropical storm conditions will continue to spread across portions of the Dominican Republic, Haiti, the Turks and Caicos and the southeastern Bahamas tonight through Friday, and Tropical Storm Warnings are in effect for these areas.
  • 3. Tropical storm conditions are possible along portions of the Florida east coast beginning Saturday, and a Tropical Storm Watch is in effect. While storm surge watches are not currently needed for this area, they may be required on Friday if the forecast track shifts closer to the coast. Heavy rains associated with Isaias may begin to affect South Florida and east-Central Florida beginning late Friday night, potentially resulting in isolated flash and urban flooding, especially in low-lying and poorly drained areas.
  • 4. There is a risk of impacts from winds, heavy rainfall, and storm surge late this weekend from the northeastern Florida coast and spreading northward along the remainder of the U.S. east coast through early next week. The details of the track and intensity forecast remain uncertain, and it is too soon to determine the magnitude and location of these potential impacts, but interests along the entire U.S. east coast should monitor the progress of Isaias and updates to the forecast.

NWS National Hurricane Center Miami FL 500 PM EDT Thu Jul 30, 2020 

Surface observations in the Dominican Republic indicate that the poorly defined center of Isaias moved over the southeastern portion of Hispaniola around 1600 UTC. There is currently a significant concentration of convection near the mid-level center that is located along the northern coast of the island, and recent surface observations suggest that a new center may be forming near the area of mid-level rotation seen in satellite imagery. The advisory position has been placed between the previous estimated center location and the mid-level center until it becomes more clear that reformation has occurred. Earlier ASCAT data that arrived shortly after the previous advisory and reconnaissance aircraft data from this morning supported an intensity of 45-50 kt, and since there has no significant degradation the overall organization since that time, the initial intensity remains 50 kt. The central pressure of 999 mb is based on a surface observation of 1001 mb and 25 kt of wind at Puerto Plata in the Dominican Republic.

The initial motion estimate is a somewhat uncertain 310/17 kt. The track forecast reasoning has not changed since the previous advisory. The tropical storm should continue on a general northwest heading during the next 24 to 36 hours along the southern side of a subtropical ridge. After that time, a trough moving into the east-central United States should cause Isaias to turn north-northwestward and northward as the western portion of the ridge erodes. By early next week, Isaias is expected to turn northeastward and accelerate ahead of the aforementioned trough. The 1200 UTC dynamical model guidance has shifted eastward and now that Isaias is expected to become a stronger and deeper cyclone, that also favors a more eastward track. The updated official forecast is a blend of the various consensus aids. Since the new center is still in its formative stage some additional shifts in the track may occur. In addition, strong winds and heavy rainfall will extend far from the center.

Visible imagery has shown significant banding near the mid-level circulation. Assuming that the center reforms near that feature and moves away from Hispaniola tonight, strengthening is expected during the next 24-36 hours and Isaias is now forecast to become a hurricane. After that time, there are mixed signals regarding the amount of southwesterly vertical wind shear and the models generally do not show much additional strengthening. Therefore, the NHC intensity forecast shows a leveling off in intensity after 48 h. It still needs to be stressed that that there is a higher than usual amount of uncertainty in the intensity forecast.

Key Messages:

  • 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across Puerto Rico, the Dominican Republic, northern Haiti, Turks and Caicos and the Bahamas.
  • 2. Hurricane conditions and dangerous storm surge are expected in portions of the central and northwest Bahamas late Friday and Saturday. Preparations to protect life and property should be rushed to completion. Tropical storm conditions will continue to spread across portions of the Dominican Republic, Haiti, the Turks and Caicos and the southeast Bahamas tonight through Friday, and Tropical Storm Warnings are in effect for these areas.
  • 3. Tropical storm conditions are possible along portions of the Florida east coast beginning Saturday, and a Tropical Storm Watch has been issued. While storm surge watches are not currently needed for this area, they may be required tonight or early tomorrow if the forecast track shifts closer to the coast. Heavy rains associated with Isaias may begin to affect eastern Florida this weekend, potentially resulting in isolated flash and urban flooding.
  • 4. There is a risk of impacts from winds, heavy rainfall, and storm surge this weekend along the Florida east coast and spreading northward along the remainder of the U.S. east coast through early next week. The details of the track and intensity forecast remain uncertain and it is too soon to determine the magnitude and location of these potential impacts, but interests along the entire U.S. east coast should monitor the progress of Isaias and updates to the forecast.

NWS National Hurricane Center Miami FL 1100 AM AST Thu Jul 30, 2020 

The satellite presentation of Isaias has improved over the past 24 hours with a concentrated area of deep convection occurring near and to the northeast of the low-level center. Radar imagery from Puerto Rico has shown a mid-level circulation that moved over the Mona passage and is now along the northern coast of Hispaniola. Surface observations and data from an Air Force Reserve reconnaissance aircraft, however, continue to place the center near the southeastern coast of Hispaniola. It is possible that a new center will re-form along the northern coast of Hispaniola as the system interacts with high terrain of that island later today or tonight. The initial intensity remains 50 kt, and is based on the earlier Doppler radar data and observations along the southern coast of Puerto Rico.

Isaias is moving northwestward or 310/16 kt. A high pressure ridge over the western Atlantic should steer Isaias on a west- northwestward to northwestward heading over the next couple of days, but the likelihood of a center re-formation during the next 12-18 hours means that some adjustments to the track and motion are possible. By late Friday, a mid-latitude trough moving into the east-central United States is expected to weaken the western portion of the ridge. This pattern should cause the cyclone to turn northwestward to north-northwestward on Saturday when it is near the northwestern Bahamas and South Florida. As the trough slides eastward over the United States, this should steer Isaias northward and northeastward early next week. Although the bulk of the track guidance agrees on this overall scenario, the confidence in the track forecast remains lower than usual due to the expected land interaction and possible center reformation in the short term. The new NHC track forecast is a blend of the HFIP corrected consensus and the TCVA multi-model consensus, and is similar to the previous advisory.

The intensity forecast remains challenging. The structure of the storm is likely to be disrupted by its passage near or over Hispaniola today, and some weakening is likely. Once the system moves away from the Greater Antilles gradual strengthening is anticipated. The global models and the SHIPS guidance suggest that Isaias will encounter an area of moderate southwesterly shear over the weekend, and the NHC intensity forecast is again leveled off at that time. There are models that continue to suggest Isaias could become a hurricane when it is near the U.S., but given the continued uncertainty, the NHC intensity forecast remains near the intensity consensus.

Key Messages:

  • 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across Puerto Rico, the Dominican Republic, northern Haiti, the Turks and Caicos, and the Bahamas. Heavy rains associated with Isaias may begin to affect South Florida Saturday morning, potentially resulting in isolated flash and urban flooding.
  • 2. Tropical storm conditions will continue across portions of the Virgin Islands and Puerto Rico through early afternoon and will spread across portions of the Dominican Republic, Haiti, the Turks and Caicos and the Bahamas later today through Friday, and Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system.
  • 3. There is a risk of impacts from winds, heavy rainfall, and storm surge along portions of the U.S. east coast beginning this weekend in Florida and spreading northward to the Carolinas and southern mid-Atlantic states early next week. Due to Isaias interacting with Hispaniola, the details of the track and intensity forecast remain uncertain and it is too soon to determine the magnitude and location of these potential impacts. Interests in these areas should monitor the progress of Isaias and updates to the forecast. Tropical storm or hurricane watches could be issued for portions of South Florida later today.

NWS National Hurricane Center Miami FL 1100 PM AST Wed Jul 29, 2020 

Observations from recent scatterometer passes over the system show that it now has a sufficiently well-defined center to be designated as a tropical cyclone. The current intensity is estimated to be 45 kt, but these winds are currently occurring over the Atlantic waters well to the north and northeast of the center. Since the cyclone is expected to move over Hispaniola on Thursday some weakening is likely within the next 24 hours. However since Isaias has such a broad wind field, the weakening will probably not be as significant as in a typical tropical cyclone with a small radius of maximum winds. Also, a re-formation of the center to the north of Hispaniola may occur. Later in the forecast period some strengthening is likely, although this may be offset by southwesterly wind shear on the order of 20 kt in 2-3 days. The official forecast is close to the intensity model consensus but well below the latest LGEM guidance.

The scatterometer data show that the center of the system is south of the previously estimated track, so there is a lot of uncertainty in the initial motion estimate of 285/17 kt. Isaias should move on a west-northwestward to northwestward track on the southern and southwestern side of a mid-tropospheric ridge. In 2-3 days, the system is expected to turn north-northwestward due to a weakness in the ridge and an approaching trough. Later in the forecast period, the trough should cause Isaias to turn toward the northeast. The official track forecast is a little to the east of the previous one and a little west of the simple and corrected dynamical model consensus tracks. It should be noted that further adjustments to the forecast tracks are indeed possible, especially after Isaias moves north of Hispaniola.

Key Messages:

  • 1. Isaias will produce heavy rains and potentially life-threatening flash flooding and mudslides across the Virgin Islands, Puerto Rico, the Dominican Republic, northern Haiti, and over the southeastern Bahamas.
  • 2. Tropical storm conditions are likely across portions of the the Virgin Islands, and Puerto Rico through tonight and will spread westward to portions of the Dominican Republic, Haiti, and the southeastern Bahamas and Turks and Caicos and the Central Bahamas on Thursday and Friday. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system.
  • 3. While this system could bring some rainfall and wind impacts to portions of Cuba, the northwestern Bahamas, and Florida later this week and this weekend, it is too soon to determine the location or magnitude of those impacts. Interests there should monitor the progress of this system and updates to the forecast over the next few days.

NWS National Hurricane Center Miami FL 500 PM AST Wed Jul 29, 2020 

Deep convection has continued to i crease in association with the disturbance since the previous advisory. More recently, a band has developed over the southwestern portion of the broad circulation and it appears that the system may be closer to acquiring a well-defined center. Earlier ASCAT data that arrived shortly after the release of the previous advisory revealed a large area of 35-40 kt winds well to the north of the vorticity maximum. This supports maintaining the initial intensity of 40 kt.

The estimated motion remains a brisk 290/20 kt. A strong subtropical ridge that extends westward over the western Atlantic should continue to steer the system west-northwestward over the next couple of days, bringing the system near or over Hispaniola late tonight and Thursday. A mid-latitude trough moving into the central United States on Friday is foreast to weaken the western portion of the ridge which should cause a reduction in the forward speed of the system and a turn toward the northwest and north later in the forecast period. The latest iterations of the dynamical models have trended toward a more eastward solution, taking the system near or just east of the Florida peninsula. Since the system still lacks a well-defined center it is not surprising to see these inconsistencies in the model runs. As a result, the NHC forecast has been shifted eastward, but it remains to the west of the latest multi-model consensus aids out of respect for continuity and the continued possibility of further model shifts.

The system is still forecast to become a tropical storm before reaching Hispaniola, and some slight strengthening could occur before landfall Thursday morning. Some weakening is likely as the system moves over that island. The system is likely to take some time to recover after its passage over land, and given the forecast for at least moderate south to southwesterly shear, only gradual strengthening is indicated at that time. As mentioned above, some of the global models are now showing a track farther away from eastern Cuba and east of Florida, and if that occurs the NHC intensity forecast could be somewhat conservative. Interests in the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both the track and intensity are likely.

Key Messages:

  • 1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands, Puerto Rico, the Dominican Republic, northern Haiti, and over the Inagua Islands into the southeastern Bahamas.
  • 2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands, and Puerto Rico through tonight and will spread westward to portions of the Dominican Republic, Haiti, and the southeastern Bahamas and Turks and Caicos on Thursday and Thursday night. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system.
  • 3. The details of the long-range track and intensity forecasts remain more uncertain than usual since the system does not have a well-defined center and it is expected to move near or over portions of the Greater Antilles later this week and move near or over Florida this weekend. While this system could bring some rainfall and wind impacts to portions of Cuba, the central and northwest Bahamas, and Florida later this week and this weekend, it is too soon to determine the location or magnitude of those impacts. Interests there should monitor the progress of this system and updates to the forecast over the next few days.

NWS National Hurricane Center Miami FL 1100 AM AST Wed Jul 29, 2020 

Surface observations from the Lesser Antilles show that the broader circulation of the disturbance has become slightly better defined but a recent Air Force Reserve reconnaissance aircraft was still unable to find a well-defined circulation. Therefore, the system has not yet become a tropical cyclone. The aircraft measured peak flight-level winds of 46 kt and several SFMR winds of around 35 kt, so the initial intensity remains 40 kt for this advisory.

The system continues moving briskly west-northwestward at 290/20 kt. The forecast reasoning remains unchanged as a ridge to the north of the system is expected to steer the disturbance west-northwestward during the next couple of days. This motion should bring the system near or over Hispaniola on Thursday and near eastern Cuba Thursday night and Friday. The ridge is forecast to weaken by the weekend which should cause a reduction in forward speed. It still must be stressed that since the system lacks a well-defined center and remains in its formative stage, uncertainty in the specifics of the track forecast remain high in both the short and longer range. The latest NHC track foreast has been nudged slightly south of the previous advisory and lies between the latest ECMWF solution and the various consensus aids.

Deep convection has consolidated and there is some evidence of banding over the northern and western portions of the large circulation. As a result, the system is still expected to become a tropical storm later today or tonight. Some additional intensification is then possible before the system reaches Hispaniola on Thursday, but weakening is likely to occur while it interacts with land. After that time, the system’s close proximity to eastern Cuba and an expected increase in southwesterly shear are likely to inhibit significant re-strengthening. The latest NHC wind speed forecast has been lowered from the previous advisory at 72-120 h. Given the expected land interaction and less than ideal upper-level environment it is best to remain conservative at this time.

Interests in Hispaniola, the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both the track and intensity are likely.

Key Messages:

  • 1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands, Puerto Rico, and the Dominican Republic.
  • 2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands, and Puerto Rico today and will spread westward to portions of the Dominican Republic, Haiti, and the southeastern Bahamas and Turks and Caicos on Thursday and Thursday night. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system.
  • 3. The details of the long-range track and intensity forecasts remain more uncertain than usual since the system does not have a well-defined center and it is expected to move near or over portions of the Greater Antilles later this week. However, this system could bring some rainfall and wind impacts to portions of Cuba, the central and northwest Bahamas, and Florida later this week and this weekend. Interests there should monitor its progress and updates to the forecast over the next few days.

NWS National Hurricane Center Miami FL 500 PM AST Tue Jul 28, 2020 

The overall structure of the disturbance has not changed much since this morning. New clusters of convection have developed over the northern portion of the elongated circulation and the Air Force Reserve reconnaissance aircraft has been unable to find a well-defined center. The aircraft measured some SFMR winds of 30-35 kt well to the northwest of the trough axis, and these data along with the earlier ASCAT form the basis of the 35 kt initial intensity. The lastest dynamical model guidance still suggests that the system will consolidate over the next 12-18 hours and the disturbance is expected to become a tropical storm before it reaches the Leeward Islands on Wednesday.

The particulars of the track forecast remain uncertain since the system lacks a well-defined center. The initial motion estimate is a highly uncertain 285/20 kt. The overall track foreast reasoning remains the same, with the disturbance expected to move west-northwestward over the next several days to the south of a subtropical ridge that extends westward over the western Atlantic. The 12Z track guidance is in agreement on the overall scenario, but some differences are noted due primarily to the system’s strength and vertical depth. Models such as the UKMET and HWRF, which depict a stronger cyclone, are along the northern side of the guidance envelope, while the weaker solutions remain more equatorward. The updated NHC track forecast is similar to the previous advisory and lies a little south of the various consensus aids. Regardless of the exact track, the system is expected to bring locally heavy rainfall to much of the Lesser Antilles, and tropical-storm-force winds to portions of the Leeward Islands, the Virgin Islands, and Puerto Rico within the next 24-36 hours.

The global model guidance suggests that the system will consolidate overnight and should acquire a better defined center. As this occurs, the disturbance is likely to gradually strengthen within the low vertical wind shear environment that it is situated in. Between 36 and 72 hours, the strength of the system will largely be dependent on the amount of land it encounters. If the system moves over the Greater Antilles it is likely to be weaker than indicated in the official forecast, but a path north or south of Hispaniola could result in a stronger system. Later in the period, some southwesterly shear could also temper further strengthening. The GFS and ECMWF models generally weaken the system in the longer range due to these negative factors. The NHC intensity forecast is unchanged from the previous advisory, but an usually high degree of uncertainty regarding the future track and intensity of the system remains. Interests in Hispaniola, the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both track and intensity are likely.

Key Messages:

  • 1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands, Puerto Rico, and the Dominican Republic.
  • 2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands, Puerto Rico, and portions of the Dominican Republic beginning Wednesday and spreading westward through Thursday. Tropical Storm Warnings are in effect for these areas. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system.
  • 3. The details of the long-range track and intensity forecasts are more uncertain than usual since the system does not have a well-defined center and it could move over portions of the Greater Antilles later this week. However, this system could bring some rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida by the end of the week. Interests there should monitor its progress and updates to the forecast over the next few days.

NWS National Hurricane Center Miami FL 1100 AM AST Tue Jul 28, 2020

Shower and thunderstorm activity has increased overnight and this morning in association with a broad area of low pressure located several hundred miles east of the Lesser Antilles. Recent visible satellite imagery and ASCAT data show that the system’s circulation is quite elongated and lacks a well-defined center. Observations from NOAA buoy 41040 and ASCAT suggest that the system is already producing winds of 30-35 kt, and the systems’s initial intensity has been set to 35 kt. Dry air located just to the north of the system has been hindering development over the past couple of days, but environmental conditions are expected to be more conducive for development over the next couple of days. Therefore, the system is likely to become a tropical storm before it reaches the Leeward Islands and advisories are being initiated in order to issue Tropical Storm Warnings for a portion of the Leeward Islands, the U.S. and British Virgin Islands, and Puerto Rico. A U.S. Air Force Reserve reconnaissance aircraft is scheduled to investigate the system early this afternoon, and should provide additional information on the intensity and structure of the low pressure area.

It cannot be stressed enough that since the system is still in the formative stage, greater than average uncertainty exists regarding both the short-term and longer-term track and intensity forecasts. A subtropical ridge that extends westward from the central Atlantic is expected to be the dominant steering mechanism over the next several days, and the flow around this ridge should steer the low-pressure area generally west-northwestward. However, the details in the track forecast could change depending on exactly where within elongated circulation the center forms.

Regardless of the exact track, the system is expected to bring locally heavy rainfall to much of the Lesser Antilles, and tropical-storm-force winds to portions of the Leeward Islands, the Virgin Islands, and Puerto Rico within the next 24-48 hours. After that time, a general west- northwestward heading should continue but as mentioned before, uncertainty exists as to how close the system tracks to Hispaniola, Cuba, the Bahamas, and Florida. The NHC track forecast is in best agreement with the HFIP corrected consensus model. It should be noted that a stronger cyclone is likely to favor a more northern track, while a weaker system is likely to remain more equatorward. Users should remember that the long-term average NHC track forecast errors at days 4 and 5 are 140 and 175 n mi, respectively.

Given the current structure of the system, only gradual strengthening is predicted during the next day or two, however the system is expected to become a tropical storm when it is near the Leeward Islands on Wednesday. After 48 hours, possible land interaction with the Greater Antilles, and increasing south to southwesterly shear from an upper-level trough could temper further strengthening. The global models generally weaken the system due to these negative factors and the NHC forecast calls for little change in strength at the longer range. Interests in Hispaniola, the Bahamas, Cuba, and Florida should continue to monitor forecasts as changes to both track and intensity are likely.

Key Messages:

  • 1. Potential Tropical Cyclone Nine will produce heavy rains and potentially life-threatening flash flooding and mudslides across the northern Leeward Islands, the Virgin Islands and Puerto Rico.
  • 2. Tropical storm conditions are likely across portions of the Leeward Islands, the Virgin Islands, and Puerto Rico Wednesday through Thursday, and Tropical Storm Warnings are in effect. Do not focus on the details of the track forecast, as rainfall and wind hazards will extend far from the center of the system.
  • 3. The details of the long-range track and intensity forecasts are more uncertain than usual since the system does not have a well-defined center and could move over portions of the Greater Antilles later this week. However, this system could bring some rainfall and wind impacts to portions of Hispaniola, Cuba, the Bahamas, and Florida by the end of the week. Interests there should monitor its progress and updates to the forecast over the next few days.

NWS National Hurricane Center Miami FL 800 PM EDT Mon Jul 27, 2020 

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

  • 1. Shower activity associated with a low-pressure area located about 850 miles east of the Windward Islands has again become a little better organized, however the low’s circulation remains broad without a well-defined center of circulation. Environmental conditions are expected to become somewhat more conducive for development, and a tropical depression or tropical storm is likely to form during the next couple of days while the system moves west-northwestward at 15 to 20 mph and approaches the Leeward Islands. Regardless of development, locally heavy rains and gusty winds are likely across portions of the Leeward Islands on Wednesday and will spread westward to the Virgin Islands and Puerto Rico Wednesday night and Thursday. Interests on these islands should continue to monitor the progress of this system, and tropical storm watches or warnings could be required for portions of the area on Tuesday. * Formation chance through 48 hours…high…70 percent. * Formation chance through 5 days…high…80 percent.

NWS National Hurricane Center Miami FL 800 AM EDT Mon Jul 27, 2020

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

The Weather Prediction Center has issued the last advisory on the remnants of Hanna, located inland over northern Mexico.

  • 1. Shower activity associated with a broad area of low pressure located a little more than 1000 miles east of the Windward Islands has become a little less organized since last night. However, environmental conditions are still expected to become more favorable for development in a day or two and a tropical depression or tropical storm will likely form within the next couple of days. The system is forecast to move westward to west-northwestward at 15 to 20 mph and could begin to affect portions of the Lesser Antilles on Wednesday or Wednesday night. Interests on those islands should continue to monitor the progress of this system. * Formation chance through 48 hours…high…80 percent. * Formation chance through 5 days…high…90 percent.

NWS National Hurricane Center Miami FL 800 PM EDT Sun Jul 26, 2020

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

The National Hurricane Center has issued its final advisory on recently downgraded Tropical Depression Hanna, located inland over northeastern Mexico. Future advisories on Hanna will be issued by the NOAA Weather Prediction Center in College Park, Maryland.

  • 1. Shower activity is becoming a little better organized in association with a broad area of low pressure located over the central tropical Atlantic about midway between the coast of Africa and the Lesser Antilles. Environmental conditions are expected to become increasingly conducive for development of this system, and a tropical depression or tropical storm is likely to form within the next day or two while moving westward to west-northwestward at 15 to 20 mph. This system is expected to begin affecting portions of the Lesser Antilles on Wednesday or Wednesday night, and interests on those islands should continue to monitor its progress. * Formation chance through 48 hours…high…80 percent. * Formation chance through 5 days…high…90 percent.

NWS National Hurricane Center Miami FL 800 AM EDT Sun Jul 26, 2020

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

The National Hurricane Center is issuing advisories on Tropical Storm Hanna, located inland near the border of Texas and Mexico.

  • 1. A broad area of low pressure associated with a tropical wave is located about 1000 miles west-southwest of the Cabo Verde Islands. This system is producing a wide area of disorganized showers and thunderstorms as it moves westward around 20 mph across the tropical Atlantic. Environmental conditions are forecast to be conducive for development and a tropical depressions is likely to form within the next few days as the system nears the Lesser Antilles. Interests in the Lesser Antilles should monitor the progress of this system. * Formation chance through 48 hours…medium…60 percent. * Formation chance through 5 days…high…90 percent.

NWS National Hurricane Center Miami FL 800 PM EDT Sat Jul 25, 2020 

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

The National Hurricane Center is issuing advisories on Hurricane Hanna, located near the coast of south Texas. The National Hurricane Center has issued the last advisory on the remnants of Gonzalo, located over the southeastern Caribbean Sea.

  • 1. A broad area of low pressure associated with a tropical wave located several hundred miles west-southwest of the Cabo Verde Islands is producing a large area of cloudiness and disorganized showers and thunderstorms. Environmental conditions appear conducive for development, and a tropical depression is likely to form early next week while the wave moves westward at around 15 mph across the tropical Atlantic. * Formation chance through 48 hours…medium…50 percent. * Formation chance through 5 days…high…80 percent.

NWS National Hurricane Center Miami FL 200 PM EDT Sat Jul 25, 2020

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

The National Hurricane Center is issuing advisories on Tropical Depression Gonzalo, located just of northwest of Trinidad. The National Hurricane Center is also issuing advisories on Hurricane Hanna, located over the western Gulf of Mexico just offshore the southern coast of Texas.

  • 1. A tropical wave is producing a broad area of cloudiness and disorganized showers and thunderstorms several hundred miles southwest of Cabo Verde Islands. The wave is expected to move westward at about 15 mph during the next several days, and a tropical depression is likely form early next week when the system reaches the central tropical Atlantic. * Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…high…70 percent.

NWS National Hurricane Center Miami FL 800 AM EDT Sat Jul 25, 2020

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

The National Hurricane Center is issuing advisories on Tropical Storm Gonzalo, located about 100 miles east of Trinidad. The National Hurricane Center is also issuing advisories on recently upgraded Hurricane Hanna, located over the western Gulf of Mexico near the southeast coast of Texas.

  • 1. A tropical wave is producing an area of disorganized showers and thunderstorms a few hundred miles southwest of Cabo Verde Islands. The wave is expected to move westward at about 15 mph during the next several days, and a tropical depression could form by early next week when the system reaches the western tropical Atlantic. * Formation chance through 48 hours…low…10 percent. * Formation chance through 5 days…medium...60 percent.

NWS National Hurricane Center Miami FL 200 AM EDT Sat Jul 25, 2020

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

The National Hurricane Center is issuing advisories on Tropical Storm Gonzalo, located a couple of hundred miles east of the southern Windward Islands. The National Hurricane Center is also issuing advisories on Tropical Storm Hanna, located over the western Gulf of Mexico.

  • 1. A tropical wave is producing an area of disorganized showers and thunderstorms a few hundred miles south of Cabo Verde Islands. This wave is expected to move westward during the next several days, and it could become a tropical depression early next week when it reaches the western tropical Atlantic. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…medium…50 percent.

NWS National Hurricane Center Miami FL 200 AM EDT Sat Jul 25, 2020

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

The National Hurricane Center is issuing advisories on Tropical Storm Gonzalo, located a couple of hundred miles east of the southern Windward Islands. The National Hurricane Center is also issuing advisories on Tropical Storm Hanna, located over the western Gulf of Mexico.

A tropical wave is producing an area of disorganized showers and thunderstorms a few hundred miles south of Cabo Verde Islands. This wave is expected to move westward during the next several days, and it could become a tropical depression early next week when it reaches the western tropical Atlantic. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…medium…50 percent.

NWS National Hurricane Center Miami FL 800 PM EDT Fri Jul 24, 2020

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: The National Hurricane Center is issuing advisories on Tropical Storm Gonzalo, located a few hundred miles east of the southern Windward Islands. The National Hurricane Center is also issuing advisories on Tropical Storm Hanna, located over the western Gulf of Mexico less than 200 miles east of Corpus Christi, Texas.

  • 1. A tropical wave is producing an area of disorganized showers and thunderstorms a few hundred miles south of Cabo Verde Islands. This wave is expected to move westward during the next several days, and it could become a tropical depression early next week when it reaches the western tropical Atlantic. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…medium…50 percent.

NWS National Hurricane Center Miami FL 200 PM EDT Fri Jul 24, 2020

For the North Atlantic…Caribbean Sea and the Gulf of Mexico: The National Hurricane Center is issuing advisories on Tropical Storm Gonzalo, located less than 500 miles east of the southern Windward Islands. The National Hurricane Center is also issuing advisories on Tropical Storm Hanna, located over the western Gulf of Mexico a little more more than 200 miles east of Corpus Christi, Texas.

  • 1. A tropical wave located about 300 miles south-southeast of the Cabo Verde Islands continues to produced an area of cloudiness and disorganized showers and thunderstorms. The disturbance is expected move westward across the tropical Atlantic during the next several days, and some gradual development of this system is possible by early next week when it reaches the western tropical Atlantic. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…medium…40 percent.

NWS National Hurricane Center Miami FL 200 PM EDT Fri Jul 24, 2020

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

The National Hurricane Center is issuing advisories on Tropical Storm Gonzalo, located less than 500 miles east of the southern Windward Islands. The National Hurricane Center is also issuing advisories on Tropical Storm Hanna, located over the western Gulf of Mexico a little more more than 200 miles east of Corpus Christi, Texas.

  • A tropical wave located about 300 miles south-southeast of the Cabo Verde Islands continues to produced an area of cloudiness and disorganized showers and thunderstorms. The disturbance is expected move westward across the tropical Atlantic during the next several days, and some gradual development of this system is possible by early next week when it reaches the western tropical Atlantic.  1. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…medium…40 percent.

NWS National Hurricane Center Miami FL 800 AM EDT Fri Jul 24, 2020

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

The National Hurricane Center is issuing advisories on Tropical Storm Gonzalo, located less than 600 miles east of the southern Windward Islands. The National Hurricane Center is also issuing advisories on Tropical Storm Hanna, located over the west-central Gulf of Mexico less than 300 miles east of Corpus Christi, Texas.

  • 1. A tropical wave located about 300 miles southeast of the Cabo Verde Islands is producing an area of cloudiness and disorganized thunderstorms. The disturbance is expected move westward across the tropical Atlantic during the next several days, and some gradual development of this system is possible by early next week when it reaches the western tropical Atlantic. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…low…30 percent.

NWS National Hurricane Center Miami FL 200 AM EDT Fri Jul 24, 2020

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

The National Hurricane Center is issuing advisories on Tropical Storm Gonzalo, located several hundred miles east of the southern Windward Islands. The National Hurricane Center is also issuing advisories on recently upgraded Tropical Storm Hanna, located over the central Gulf of Mexico.

  • 1. A tropical wave located near the west coast of Africa is expected to move westward across the tropical Atlantic during the next several days. Some development of this system is possible early next week when it reaches the western tropical Atlantic. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…low…30 percent.

NWS National Hurricane Center Miami FL 800 PM EDT Thu Jul 23, 2020

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

The National Hurricane Center is issuing advisories on Tropical Storm Gonzalo, located several hundred miles east of the southern Windward Islands. The National Hurricane Center is also issuing advisories on Tropical Depression Eight, located over the central Gulf of Mexico.

  • 1. A tropical wave located near the west coast of Africa is expected to move westward across the tropical Atlantic during the next several days. Some development of this system is possible early next week when it reaches the western tropical Atlantic. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…low…30 percent.

NWS National Hurricane Center Miami FL 200 PM EDT Thu Jul 23, 2020

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

The National Hurricane Center is issuing advisories on Tropical Storm Gonzalo, located about 850 miles east of the southern Windward Islands. The National Hurricane Center is also issuing advisories on Tropical Depression Eight, located over the central Gulf of Mexico.

  • 1. A tropical wave near the coast of Africa is expected to move westward across the tropical Atlantic for the next several days. Some development of this system is possible when it reaches the western tropical Atlantic early next week. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…low…20 percent.

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