Tropical Storm Iselle

Post Tropical Storm Iselle Track 1500 Hours August 30 2020
Post Tropical Storm Iselle Track 1500 Hours August 30 2020

Tropical Storm Iselle Wind Speed FieldTropical Storm IselleNWS National Hurricane Center Miami FL 300 PM MDT Sun Aug 30, 2020 ( see video below)

Iselle hasn’t produced organized deep convection for about the past day, so it no longer meets the definition of a tropical cyclone. Thus, it is now a post-tropical remnant low, and this is the last advisory.

The initial wind speed is set to 25 kt, which matches a recent ASCAT-B pass. Models are in good agreement on the low gradually turning from its current northward heading to north- northwest tomorrow and slowly weakening due to a poor environment. The low is expected to degenerate into a trough of low pressure on Tuesday, and no changes were made from the previous forecast.

FORECAST POSITIONS AND MAX WINDS

INIT  30/2100Z 23.7N 113.3W   25 KT  30 MPH - Post Tropical (WSW San Carlos, Mexico)
 12H  31/0600Z 24.7N 113.4W   20 KT  25 MPH - Low (WSW San Carlos, Mexico)
 24H  31/1800Z 25.8N 113.8W   20 KT  25 MPH - Low (WSW San Carlos, Mexico)
 36H  01/0600Z...DISSIPATED

NWS National Hurricane Center Miami FL 900 AM MDT Sun Aug 30, 2020

Iselle is skirting the line between tropical cyclone and remnant low, producing a small amount of deep convection during the past few hours. While this isn’t particularly organized thunderstorm activity, for now it is enough to keep advisories going another 6 hours. The initial intensity is set to 30 kt, near the latest TAFB T-number. Iselle is still expected to degenerate into a remnant low later today due to persistent easterly shear and a dry and stable environment. Model guidance all show the system becoming a trough by Tuesday, so the dissipation phase has been pushed ahead from the last forecast. The guidance also remains in good agreement on a northward track today, followed by a north-northwest turn overnight. The track is shifted a bit to the east because of a more eastward initial position, otherwise it is unchanged.

ORECAST POSITIONS AND MAX WINDS

INIT  30/1500Z 23.0N 113.3W   30 KT  35 MPH - Depression (WSW Todos Santos, Mexico)
 12H  31/0000Z 24.0N 113.4W   25 KT  30 MPH - Low (WSW San Carlos, Mexico)
 24H  31/1200Z 25.0N 113.7W   20 KT  25 MPH - Low (WSW San Carlos, Mexico)
 36H  01/0000Z...DISSIPATED

NWS National Hurricane Center Miami FL 900 PM MDT Sat Aug 29, 2020

Most of the deep convection associated with Iselle has been displaced to the west of the center due to strong easterly shear caused by a large upper-level anticyclone to the northeast. Based on the decreased organization, it is assumed that the cyclone has been gradually spinning down this evening, and the current intensity estimate is set at 35 kt. Although the shear may relax a bit in a day or so, Iselle will be moving over increasingly cooler waters during that time. Therefore, continued weakening is anticipated, and the system will likely degenerate into a remnant low in 12 to 24 hours. The official intensity forecast is at the high end of the model guidance.

The cyclone has been moving north-northeastward, at around 020/8 kt, on the west side of a mid-level high pressure area. The weakening and increasingly shallow system should gradually turn to the left under the influence of the lower-level flow. The official track forecast is about in the middle of the guidance envelope.

FORECAST POSITIONS AND MAX WINDS

INIT  30/0300Z 21.8N 114.2W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 12H  30/1200Z 22.9N 114.3W   30 KT  35 MPH - Depression  (WSW Todos Santos, Mexico)
 24H  31/0000Z 23.9N 114.6W   30 KT  35 MPH - Low (WSW Todos Santos, Mexico)
 36H  31/1200Z 24.5N 114.9W   25 KT  30 MPH - Low (WSW San Juanico, Mexico)
 48H  01/0000Z 25.0N 115.3W   20 KT  25 MPH - Low (WSW San Juanico, Mexico)
 60H  01/1200Z 25.5N 115.7W   20 KT  25 MPH - Low (WSW San Juanico, Mexico)
 72H  02/0000Z...DISSIPATED

NWS National Hurricane Center Miami FL 900 AM MDT Sat Aug 29, 2020 

Iselle has changed little in organization since the last advisory, as the storm has a classic shear pattern in satellite imagery with the low-level center near the northeastern edge of the convective mass. Satellite intensity estimates have changed little in the last 6 h, so the initial intensity remains 40 kt.

The cyclone has tracked a little to the right with the initial motion now north-northeastward or 025/6 kt. This should be short lived, and Iselle is expected to resume a northward motion later today or tonight. The northward motion should continue for another day or so, followed by a turn toward the northwest as Iselle becomes a weak and shallow system. The new forecast track is tweaked a little from the previous forecast based on the initial position and motion.

Iselle is moving over decreasing sea surface temperatures, and it continues to experience easterly vertical shear. This combination should cause steady weakening, and the new intensity forecast follows the previous forecast in calling for the system to weaken to a depression by 24 h, to degenerate to a remnant low by 36 h, and to weaken to a trough by 72 h. The new intensity forecast lies at the upper edge of the intensity guidance.

FORECAST POSITIONS AND MAX WINDS

INIT  29/1500Z 20.3N 114.7W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 12H  30/0000Z 21.3N 114.6W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 24H  30/1200Z 22.8N 114.7W   30 KT  35 MPH - Depression (WSW Todos Santos, Mexico)
 36H  31/0000Z 23.9N 115.1W   25 KT  30 MPH - Low (WSW Todos Santos, Mexico)
 48H  31/1200Z 24.7N 115.5W   20 KT  25 MPH - Low (WSW San Juanico, Mexico)
 60H  01/0000Z 25.4N 116.0W   20 KT  25 MPH - Low (WSW San Juanico, Mexico)
 72H  01/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 800 PM PDT Fri Aug 28, 2020

Persistent bursts of deep convection continue to develop just to the west of Iselle’s center, which occasionally becomes hidden beneath the cirrus outflow (as it is now). Dvorak final-T numbers from TAFB and SAB are 3.0/45 kt based on a shear pattern, and along with what was shown by an afternoon ASCAT pass, Iselle’s intensity remains 45 kt. Strong shear from the east-northeast is forecast to continue for the next 24 hours or so. After that time, the shear should relax, but then the thermodynamic environment becomes more limiting with Iselle moving over sub-26C waters and into a much more stable environment. All told, Iselle is expected to steadily weaken during the next several days, and it is likely to degenerate into a remnant low on Sunday. The NHC forecast is very similar to the previous prediction and is close to HCCA and the IVCN intensity consensus aid. The remnant low is expected to dissipate in about 4 days.

Iselle is moving slowly north-northeastward (015/5 kt) on the western periphery of a low- to mid-level area of high pressure centered over Mexico. This high, along with a trough extending off the coast of California, should cause Iselle to gradually turn toward the north and then north-northwest with some increase in forward speed during the next couple of days. There is a typical amount of spread among the track models, and they generally agree on this northward motion with a slight bend to the left through the forecast period. The regional hurricane models appear almost in a cluster all to themselves on the eastern side of the guidance envelope, and the NHC forecast therefore closely follows the TVDG model consensus aid, which gives double weight to the global models.

ORECAST POSITIONS AND MAX WINDS

INIT  29/0300Z 19.3N 115.2W   45 KT  50 MPH - Tropical Storms (WSW Isla Clarion, Mexico)
 12H  29/1200Z 20.2N 115.0W   40 KT  45 MPH - Tropical Storms (WSW Puerto Vallarta, Mexico)
 24H  30/0000Z 21.7N 115.0W   35 KT  40 MPH - Tropical Storms (WSW Cabo San Lucas, Mexico)
 36H  30/1200Z 23.1N 115.4W   30 KT  35 MPH - Depression (WSW Todos Santos, Mexico)
 48H  31/0000Z 24.2N 115.9W   25 KT  30 MPH - Low (WSW San Carlos, Mexico)
 60H  31/1200Z 24.9N 116.4W   20 KT  25 MPH - Low (WSW San Carlos, Mexico)
 72H  01/0000Z 25.6N 116.8W   20 KT  25 MPH - Low (WSW San Carlos, Mexico)
 96H  02/0000Z...DISSIPATED

NWS National Hurricane Center Miami FL 200 PM PDT Fri Aug 28, 2020 

First light visible imagery shows that Iselle’s center of circulation has once again reappeared to the northeast of the deep convective mass. Last night’s briefly diminished shear interlude probably resulted in Iselle’s peak intensity. For this advisory, the initial intensity is lowered to 45 kt which is based on a blend of the Dvorak intensity estimates from TAFB and SAB, a METOP-B ASCAT overpass containing 40-45 kt winds and a SATCON analysis of 42 kt.

The northeasterly shear is forecast to persist through the 24 hour period and then decrease and veer from the southeast. At that time, however, Iselle will be traversing sub 25C sea surface temperatures and will be moving into a more dry and stable surrounding environment. As a result, Iselle should weaken to a tropical depression Saturday night and degenerate to a remnant low on Sunday.

The initial motion is estimated to be north-northeastward, or, 020/6 kt. A northward direction should commence Saturday morning, then a turn north-northwestward is forecast Sunday morning. As Iselle continues to weaken and degenerates to a remnant low, a turn toward the northwest, well offshore of the southern coast of the Baja California peninsula, is expected to occur Sunday late night. The NHC forecast is an update of the previous advisory and is close to the GFEX (FV3/ECMWF mean) and the NOAA HCCA consensus model.

FORECAST POSITIONS AND MAX WINDS

INIT  28/2100Z 18.8N 115.2W   45 KT  50 MPH - Tropical Storm (WSW Isla Clarion, Mexico)
 12H  29/0600Z 19.6N 114.9W   40 KT  45 MPH - Tropical Storm (WSW Puerto Vallarta, Mexico)
 24H  29/1800Z 21.0N 114.7W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 36H  30/0600Z 22.6N 115.0W   30 KT  35 MPH - Depression (WSW San Carlos, Mexico)
 48H  30/1800Z 23.7N 115.7W   25 KT  30 MPH - Low (WSW San Carlos, Mexico)
 60H  31/0600Z 24.4N 116.3W   25 KT  30 MPH - Low (WSW San Carlos, Mexico)
 72H  31/1800Z 25.1N 117.0W   20 KT  25 MPH - Low (WSW San Carlos, Mexico)
 96H  01/1800Z...DISSIPATED

NWS National Hurricane Center Miami FL 800 AM PDT Fri Aug 28, 2020 

GOES-16 shortwave infrared imagery and a recent AMSR2 microwave overpass indicate that deep convection has been developing near the surface center during the past several hours. In fact, the microwave image showed a small, compact inner core defined by a partially closed eye-like feature. My initial thoughts were that this cloud feature is in the mid-portion of the atmosphere, but the lower 37 GHZ frequency confirmed very little vertical tilt. Subjective and objective satellite intensity estimates are based on the shear scene-type which would yield a slightly lower intensity estimation. Consequently, the initial intensity is held at 50 kt, but it could certainly be a little stronger based on the aforementioned polar low-orbiter pass.

The FV3/GFS and ECMWF SHIPS statistical-dynamical intensity models show 20 to 30 kt of northeasterly shear persisting through the next few days, however, the UW-CIMSS shear analysis reveals less than 20 kt. For now, based on what the large-scale models and the SHIPS models agree on, gradual weakening should begin by early Saturday morning and continue through Monday morning as the cyclone traverses decreasing oceanic temperatures and moves into a more thermodynamically stable surrounding environment. The NHC intensity is an update of the previous advisory, and calls for Iselle to weaken to a tropical depression on Sunday and degenerate into a remnant low on Sunday evening.

Based on the 0920 UTC AMSR2 pass, the initial position was adjusted to the northwest of the previous position and the forward motion is estimated to be northeastward, or 035/4 kt. A northward direction should commence by early Saturday morning, then a turn north-northwestward to northwestward is forecast during the 48-60 hr period. As Iselle continues to weaken and become a more shallower system, a turn toward the west-northwest, well offshore of the southern coast of the Baja California peninsula, is forecast to occur Monday morning. The NHC forecast is nudged to the left of the previous track forecast due the adjusted position, and is based on the HCCA and TVCE multi-model consensus aids.

FORECAST POSITIONS AND MAX WINDS

INIT  28/1500Z 18.3N 115.3W   50 KT  60 MPH - Tropical Storm (WSW Isla Clarion, Mexico)
 12H  29/0000Z 19.2N 114.8W   50 KT  60 MPH - Tropical Storm (WSW Puerto Vallarta, Mexico)
 24H  29/1200Z 20.6N 114.6W   45 KT  50 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 36H  30/0000Z 22.2N 114.9W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 48H  30/1200Z 23.5N 115.7W   30 KT  35 MPH - Depression (WSW San Carlos, Mexico)
 60H  31/0000Z 24.1N 116.6W   25 KT  30 MPH - Low (WSW San Carlos, Mexico)
 72H  31/1200Z 24.4N 117.3W   20 KT  25 MPH - Low (WSW San Carlos, Mexico)
 96H  01/1200Z...DISSIPATED

NWS National Hurricane Center Miami FL 800 PM PDT Thu Aug 27, 2020

Iselle continues to produce bursts of deep convection in an environment of moderate easterly shear, with the latest round of convection completely covering the low-level center. Since satellite classifications haven’t changed much since the previous advisory, the initial intensity is left at 45 kt based on the earlier ASCAT data. There is some uncertainty in exactly how strong the shear currently is, with SHIPS diagnostics showing nearly 30 kt while UW-CIMSS is analyzing a little under 20 kt.

A recent SSMIS pass showed some defined curved banding, so I’m inclined to think the shear is closer to the lower estimate. With that said, the SHIPS diagnostics do show the shear relaxing just a bit during the next 12-24 hours, which could allow Iselle to strengthen a little, and this scenario is supported by the latest HWRF, HMON, COAMPS-TC, HCCA, and GFS model solutions. After that time, the shear is expected to strengthen again, and then Iselle will be moving over cooler waters. Therefore, the new NHC intensity forecast is very close to the previous one from 24 hours and beyond. Iselle is likely to lose all of its convection and become a remnant low in about 3 days and then open up into a trough by day 5.

Iselle is embedded in an elongated mid-level trough which extends northeastward to Tropical Storm Hernan near the coast of Mexico, and its initial motion is north-northeastward, or 030/3 kt. The interaction between the two tropical storms is expected to cause Iselle to continue moving north-northeastward or northward during the first 2 days. Once Hernan dissipates and Iselle weakens, a low-level ridge over northern Mexico should cause Iselle to turn toward the northwest and west on days 3 and 4. The track models have shifted a bit to the northeast after 48 hours, leaving the previous forecast near the left side of the guidance envelope, so the new NHC forecast was adjusted closer to the multi-model consensus aids during that period.

FORECAST POSITIONS AND MAX WINDS

INIT  28/0300Z 17.6N 115.5W   45 KT  50 MPH - Tropical Storm (WSW Manzanillo, Mexico)
 12H  28/1200Z 18.2N 115.0W   50 KT  60 MPH - Tropical Storm (WSW Clarion Island, Mexico)
 24H  29/0000Z 19.1N 114.5W   45 KT  50 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 36H  29/1200Z 20.1N 114.2W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 48H  30/0000Z 21.7N 114.2W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 60H  30/1200Z 23.2N 114.7W   30 KT  35 MPH - Depression (WSW Cabo San Lucas, Mexico)
 72H  31/0000Z 23.9N 115.7W   25 KT  30 MPH - Low (WNW Todas Santos, Mexico)
 96H  01/0000Z 23.9N 117.0W   20 KT  25 MPH - Low (WNW Todas Santos, Mexico)
120H  02/0000Z...DISSIPATED

NWS National Hurricane Center Miami FL 200 PM PDT Thu Aug 27, 2020

Since the previous advisory, a new burst of deep convection developed near the center of Iselle the persisted for much of the morning. It was not until recently that the easterly shear has begun to push the convection to the west of the center. The latest Dvorak estimates from TAFB and SAB as well as data from a recent ASCAT overpass all agree that the initial intensity has increased to 45 kt. Moderate-to-strong easterly to northeasterly vertical wind shear should prevent Iselle from further strengthening over the next couple of days. After that time, the shear is forecast to relax somewhat. However, by that time the cyclone will then be moving over SSTs of only 26 C and into a more stable atmospheric environment.

This should cause the deep convection to gradually dissipate, and the cyclone is expected to weaken late this weekend into early next week before degenerating into a remnant low. The latest NHC intensity forecast is close to the LGEM guidance. Iselle continues moving northeastward at 4 kt, embedded in a large monsoon gyre. The steering flow is forecast to become more southeasterly this weekend inside the gyre, resulting in a gradual turn to the northwest. As the cyclone breaks north of the gyre and becomes shallow, a turn to the west is anticipated while the system becomes steered by a low- to mid- level ridge to its north. The latest NHC track forecast was nudged just a little to the right of the previous one due to a shift in the guidance, and lies in between the previous forecast and the various track consensus aids.

FORECAST POSITIONS AND MAX WINDS

INIT  27/2100Z 17.2N 115.8W   45 KT  50 MPH - Tropical Storm (WSW Manzanillo, Mexico)
 12H  28/0600Z 17.8N 115.3W   45 KT  50 MPH - Tropical Storm (WSW Clarion Island, Mexico)
 24H  28/1800Z 18.6N 114.7W   45 KT  50 MPH - Tropical Storm (ENE Clarion Island, Mexico)
 36H  29/0600Z 19.6N 114.4W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 48H  29/1800Z 20.8N 114.3W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 60H  30/0600Z 21.9N 114.6W   30 KT  35 MPH - Depression (WSW Cabo San Lucas, Mexico)
 72H  30/1800Z 22.8N 115.4W   25 KT  30 MPH - Depression (WSW Cabo San Lucas, Mexico)
 96H  31/1800Z 23.2N 116.7W   25 KT  30 MPH - Low (WNW Todas Santos, Mexico)
120H  01/1800Z 23.1N 117.8W   20 KT  25 MPH - Low (WSW Todas Santos, Mexico)

NWS National Hurricane Center Miami FL 800 AM PDT Thu Aug 27, 2020

Iselle consists of a rather ragged looking area of deep convection being sheared to the southwest of a partially exposed low-level center. A blend of the latest satellite intensity estimates from TAFB and UW-CIMSS support keeping the initial intensity at 40 kt. Moderate-to-strong easterly to northeasterly vertical wind shear will likely prevent Iselle from strengthening over the next couple of days. After that time, the shear is forecast to relax somewhat. However, the cyclone will then be moving over SSTs of only 26 C and into a more stable atmospheric environment. This should cause the deep convection to gradually dissipate, and the cyclone is expected to weaken this weekend before degenerating into a remnant low early next week. The latest NHC intensity forecast is a blend of the IVCN consensus and SHIPS guidance.

Iselle is moving northeastward at 4 kt, embedded in a large monsoon gyre. The steering flow is forecast to become more southerly this weekend inside the gyre, resulting in a more northward motion. As the cyclone breaks north of the gyre, a turn to the northwest then west is anticipated while the system becomes steered by a low- to mid- level ridge to its north. The latest NHC track forecast is little changed from the previous one, and lies in between the HFIP corrected consensus HCCA and the TVCX track consensus.

FORECAST POSITIONS AND MAX WINDS

INIT  27/1500Z 17.0N 115.8W   40 KT  45 MPH - Tropical Storm (WSW Puerto Vallarta, Mexico)
 12H  28/0000Z 17.5N 115.4W   40 KT  45 MPH - Tropical Storm (WSW Manzanillo, Mexico)
 24H  28/1200Z 18.2N 115.0W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 36H  29/0000Z 18.9N 114.6W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 48H  29/1200Z 19.9N 114.2W   40 KT  45 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 60H  30/0000Z 21.2N 114.4W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 72H  30/1200Z 22.2N 115.3W   30 KT  35 MPH - Depression (WSW Cabo San Lucas, Mexico)
 96H  31/1200Z 22.5N 116.6W   25 KT  30 MPH - Low (WSW Cabo San Lucas, Mexico)
120H  01/1200Z 22.3N 117.3W   20 KT  25 MPH - Low (WSW Cabo San Lucas, Mexico)

NWS National Hurricane Center Miami FL 800 PM PDT Wed Aug 26, 2020

Satellite imagery indicates that a convective burst has formed a little closer to the center of Iselle during the past few hours, although the circulation remains at least somewhat elongated. Various subjective and objective satellite intensity estimates are near 35 kt, so that remains the initial intensity. The imagery, as well as model analyses, show that Iselle continues to be undergoing strong easterly shear.

Iselle is embedded in a large monsoon gyre, and the southwesterly flow on the south side of the gyre should be the main steering influence for the next 60 h or so. This should result in the cyclone continuing its current slow motion toward the northeast. After that time, the cyclone should separate from the gyre, allowing a mid-level ridge to the north and northeast of the system to become the dominant steering mechanism. This should cause Iselle to turn to the north, then northwest, then eventually to the west. The track guidance is in good agreement with the general scenario, although there is some spread on when and where there turns will occur. The new forecast track is close to the HCCA corrected consensus mode and has only minor adjustments from the previous forecast.

Present indications are that the current strong shear will persist for several days, and after 72 h the forecast track takes the cyclone over cooler sea surface temperatures. Based on this, significant strengthening appears unlikely, although short-lived spin ups due to convective bursts are possible. The new intensity forecast is unchanged from the previous forecast in showing little change of strength for 72 h, followed by weakening to a depression and a remnant low. An alternative scenario is that the shear causes Iselle to weaken to a trough at any time during the next 120 h.

FORECAST POSITIONS AND MAX WINDS

INIT  27/0300Z 16.2N 116.2W   35 KT  40 MPH - Tropical Storm (WSW Zihuatanejo, Mexico)
 12H  27/1200Z 16.7N 115.8W   35 KT  40 MPH - Tropical Storm (WNW Acapulco, Mexico)
 24H  28/0000Z 17.3N 115.3W   35 KT  40 MPH - Tropical Storm (WNW Acapulco, Mexico)
 36H  28/1200Z 17.9N 114.7W   35 KT  40 MPH - Tropical Storm (WSW Clarion Island, Mexico)
 48H  29/0000Z 18.6N 114.1W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 60H  29/1200Z 19.4N 113.6W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 72H  30/0000Z 20.7N 113.7W   30 KT  35 MPH - Depression (WSW Cabo San Lucas, Mexico)
 96H  31/0000Z 22.5N 115.5W   30 KT  35 MPH - Low (WSW Cabo San Lucas, Mexico)
120H  01/0000Z 22.5N 116.5W   25 KT  30 MPH - Low (WSW Cabo San Lucas, Mexico)

NWS National Hurricane Center Miami FL 200 PM PDT Wed Aug 26, 2020

A recent ASCAT overpass revealed that the cyclone barely has a closed center, and is embedded in a northeast-southwest oriented monsoon gyre feature. However, this same overpass sampled a decent sized area of 33-34 kt winds over the southeastern quadrant, and therefore the system has been upgraded to 35 kt Tropical Storm Iselle.

Assuming that the storm does not get absorbed into the monsoon gyre and open back into a trough, it should maintain its intensity over warm waters, while battling 20-30 kt of easterly to northeasterly shear over the next few days. After that time, the shear persists but Iselle should begin to move over waters with a lower oceanic heat content. This should cause the cyclone to weaken and eventually degenerate into a remnant low late in the forecast period. The latest NHC intensity forecast is near the various intensity consensus guidance.

The depression is moving northeast at 4 kt. This motion is expected to continue for the next day or so as it moves around the cyclonic gyre stretching southwestward from Hurricane Laura over central Mexico and into the eastern Pacific. Afterward, a gradual turn toward the north-northwest and west-northwest is expected as high pressure currently over the southwestern U.S. builds westward over the Baja California peninsula and adjacent eastern Pacific. The NHC forecast track is little changed from the previous one, and lies near the HFIP corrected consensus, or HCCA.

FORECAST POSITIONS AND MAX WINDS

INIT  26/2100Z 15.5N 116.8W   35 KT  40 MPH - Tropical Storm (WSW Acapulco, Mexico)
 12H  27/0600Z 16.0N 116.3W   35 KT  40 MPH - Tropical Storm (WSW Zihuatanejo, Mexico)
 24H  27/1800Z 16.8N 115.7W   35 KT  40 MPH - Tropical Storm (WNW Acapulco, Mexico)
 36H  28/0600Z 17.3N 115.1W   35 KT  40 MPH - Tropical Storm (WNW Acapulco, Mexico)
 48H  28/1800Z 17.9N 114.4W   35 KT  40 MPH - Tropical Storm (WSW Clarion Island, Mexico)
 60H  29/0600Z 18.7N 113.7W   35 KT  40 MPH - Tropical Storm (WSW Cabo San Lucas, Mexico)
 72H  29/1800Z 19.9N 113.7W   30 KT  35 MPH - Depression (WSW Cabo San Lucas, Mexico)
 96H  30/1800Z 21.4N 114.9W   30 KT  35 MPH - Low (WSW Cabo San Lucas, Mexico)
120H  31/1800Z 22.1N 116.1W   25 KT  30 MPH - Low (WSW Cabo San Lucas, Mexico)

NWS National Hurricane Center Miami FL 800 AM PDT Wed Aug 26, 2020

Earlier scatterometer data and GOES-16 Proxy-VIS satellite imagery indicate that the area of low pressure located several hundred miles southwest of the southern tip of the Baja California peninsula has acquired sufficient convective organization and a well-defined surface circulation, and is now classified as a tropical cyclone. Dvorak satellite intensity estimates and the aforementioned scatterometer data support an initial wind speed of 30 kt for this advisory.

Moderate to strong east-northeasterly shear, the same inhibiting upper wind pattern that is affecting depression Thirteen-E, is forecast to hamper significant development through the entire forecast period. Therefore, only modest strengthening in the next 24 hours is expected, which agrees with the statistical- dynamical guidance and the large-scale models.

The depression is moving rather slowly toward the northeast, or 040/2 kt. The low to mid-level steering synoptic pattern consists of a cyclonic gyre stretching southwestward from Hurricane Laura over central Mexico and into the eastern Pacific. This feature should cause the depression to continue in a northeastward general direction at a relatively slow pace through Friday. Afterward, a gradual turn toward the north-northwest and west-northwest is expected as high pressure currently over the southwestern U.S. builds westward over the Baja California peninsula and adjacent eastern Pacific. The NHC official forecast is a reflection of a blend of the HCCA and TVCE consensus aids.

FORECAST POSITIONS AND MAX WINDS

INIT  26/1500Z 15.4N 117.2W   30 KT  35 MPH - Depression (WSW Acapulco, Mexico)
 12H  27/0000Z 15.8N 116.9W   30 KT  35 MPH - Depression (WSW Acapulco, Mexico)
 24H  27/1200Z 16.4N 116.4W   35 KT  40 MPH - Tropical Storm (WSW Zihuatanejo, Mexico)
 36H  28/0000Z 17.0N 115.7W   35 KT  40 MPH - Tropical Storm (WNW Acapulco, Mexico)
 48H  28/1200Z 17.8N 114.9W   35 KT  40 MPH - Tropical Storm (WSW Clarion Island, Mexico)
 60H  29/0000Z 18.6N 114.1W   30 KT  35 MPH - Depression (ESE Clarion Island, Mexico)
 72H  29/1200Z 19.6N 113.5W   30 KT  35 MPH - Depression (WSW Cabo San Lucas, Mexico)
 96H  30/1200Z 21.4N 114.3W   30 KT  35 MPH - Low (WSW Cabo San Lucas, Mexico)
120H  31/1200Z 22.4N 116.1W   25 KT  30 MPH - Low (ESE Cabo San Lucas, Mexico)

NWS National Hurricane Center Miami FL 500 PM PDT Tue Aug 25, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

  • 1. A low-pressure system located a few hundred miles south-southeast of the southern tip of the Baja California peninsula continues to show signs of organization. This system will likely become a tropical depression or tropical storm during the next day or two while it moves slowly east-northeastward or northeastward, near or just west of the southwestern coast of Mexico. For more information on this system, see High Seas forecasts issued by the National Weather Service. * Formation chance through 48 hours…high…90 percent. * Formation chance through 5 days…high…90 percent.
  • 2. An elongated area of low pressure near the coast of south-central Mexico continues to produce disorganized showers and thunderstorms. Development of this system has become unlikely due to unfavorable upper-level winds and close proximity to land. Regardless of development, areas of heavy rainfall and flash flooding are likely in southeastern Mexico and northern Central America during the next several days. * Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…low...20 percent.
  • 3. Another low-pressure system is located several hundred miles southwest of the southern tip of the Baja California peninsula. Although this system is currently producing limited shower and thunderstorm activity, its circulation has become better defined during the past several hours. Some additional development of the disturbance is expected and it will likely become a tropical depression during the next few days while it moves slowly east-northeastward to northeastward. * Formation chance through 48 hours…medium…60 percent. * Formation chance through 5 days…high…70 percent.

NWS National Hurricane Center Miami FL 1100 AM PDT Tue Aug 25, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

  • 1. A low-pressure system is located a few hundred miles south-southeast of the southern tip of the Baja California peninsula. Showers and thunderstorms have become better organized during the past several hours and the surface circulation has become a little better defined. Although upper-level winds are marginally conducive for further development, a tropical depression is likely to form during the next day or two while the system moves slowly east-northeastward to northeastward toward the southwestern coast of Mexico. * Formation chance through 48 hours…high…80 percent. * Formation chance through 5 days…high…90 percent.
  • 2. Satellite derived wind data indicate that the small low pressure system that was located just off of the coast of south-central Mexico near Puerto Angel has opened up into a trough of low pressure. Consequently, development of this system appears to be less likely. Regardless of development, areas of heavy rainfall and flash flooding are likely in southeastern Mexico and northern Central America during the next several days. * Formation chance through 48 hours…medium…40 percent. * Formation chance through 5 days…medium...40 percent.
  • 3. Another low-pressure system is located several hundred miles southwest of the southern tip of the Baja California peninsula. Gradual development of this system is possible during the next couple of days, and a tropical depression could form while it moves slowly east-northeastward. * Formation chance through 48 hours…medium…40 percent. * Formation chance through 5 days…medium…60 percent.

NWS National Hurricane Center Miami FL 500 AM PDT Tue Aug 25, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

  • 1. A broad low-pressure system is located a few hundred miles south-southeast of the southern tip of the Baja California peninsula. Showers and thunderstorms are beginning to show some signs of organization, and environmental conditions appear conducive for a tropical depression to form during the next couple of days while the system moves slowly east-northeastward to northeastward toward the southwestern coast of Mexico. * Formation chance through 48 hours…high…80 percent. * Formation chance through 5 days…high…90 percent.
  • 2. A small low pressure system is located just offshore of the south-central Mexico coast near Puerto Angel. The low is forecast to meander or move slowly westward to west-northwestward over or near the southern coast of Mexico, and a tropical depression could still form if the system remains offshore over the next day or two. Regardless of development, areas of heavy rainfall and flash flooding are likely in southeastern Mexico and northern Central America during the next several days. * Formation chance through 48 hours…high…70 percent. * Formation chance through 5 days…high…70 percent.
  • 3. Another small low-pressure system has developed several hundred miles southwest of the southern tip of the Baja California peninsula. Some gradual development is possible during the next couple of days while the system moves slowly east-northeastward. * Formation chance through 48 hours…low…20 percent. * Formation chance through 5 days…low…30 percent.

NWS National Hurricane Center Miami FL 500 PM PDT Mon Aug 24, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

  • 1. An elongated zone of disturbed weather is located several hundred miles south-southwest of the southern tip of the Baja California peninsula. Environmental conditions appear conducive for a tropical depression to form later this week while the system meanders or drifts northward a few hundred miles offshore of the coast of southwestern Mexico. * Formation chance through 48 hours…high…70 percent. * Formation chance through 5 days…high…90 percent.
  • 2. A small low-pressure system located near the Gulf of Tehuantepec continues to show signs of organization. This low is forecast to meander or move slowly west-northwestward near the southern coast of Mexico, and a tropical depression is likely to form over the next day or two, assuming the center of circulation remains offshore. Regardless of development, areas of heavy rainfall and flash flooding are likely in southeastern Mexico and northern Central America during the next several days. * Formation chance through 48 hours…high…80 percent. * Formation chance through 5 days…high…80 percent.

NWS National Hurricane Center Miami FL 500 AM PDT Mon Aug 24, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

  • 1. An elongated zone of disturbed weather is located several hundred miles south-southwest of the southern tip of the Baja California peninsula. This weather is expected to consolidate in a couple of days, and environmental conditions appear conducive for a tropical depression to form later this week while the system meanders or drifts northward a few hundred miles offshore of the coast of southwestern Mexico. * Formation chance through 48 hours…high…70 percent. * Formation chance through 5 days…high…90 percent.
  • 2. A small low-pressure area located a couple of hundred miles southwest of the Gulf of Tehuantepec continues to produce disorganized shower and thunderstorm activity. Conditions appear conducive for additional development of this system if it does not move inland over Mexico, and a tropical depression is likely to form within the next couple of days while it moves slowly northwestward, near or parallel to the southwestern coast of Mexico. Regardless of development, areas of heavy rainfall and flash flooding are likely in southeastern Mexico and northern Central America during the next several days. * Formation chance through 48 hours…high…70 percent. * Formation chance through 5 days…high…80 percent.

NWS National Hurricane Center Miami FL 1100 AM PDT Sun Aug 23, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

  • 1. An elongated area of low pressure located several hundred miles south of the southern tip of the Baja California peninsula continues to produce a large area of disorganized cloudiness and thunderstorms. Conditions appear favorable for development, and a tropical depression is expected to form during the next couple of days while the system meanders or drifts northward a few hundred miles offshore the coast of southwestern Mexico. * Formation chance through 48 hours…high…70 percent. * Formation chance through 5 days…high…90 percent.
  • 2. Satellite images indicate that the circulation of a low-pressure area which formed earlier this morning a couple of hundred miles south-southwest of the Gulf of Tehuantepec is becoming better defined. Conditions appear conducive for additional development of this system, and a tropical depression is likely to form by the middle of the week while it moves slowly northwestward, near or parallel to the southwestern coast of Mexico. Regardless of development, areas of heavy rainfall and flash flooding are likely in southeastern Mexico and northern Central America during the next several days. * Formation chance through 48 hours…medium…60 percent. * Formation chance through 5 days…high…70 percent.

NWS National Hurricane Center Miami FL 500 AM PDT Sun Aug 23, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

  • 1. An elongated area of low pressure located several hundred miles south-southwest of the southern tip of the Baja California peninsula continues to produce a large area of disorganized cloudiness and thunderstorms. Conditions appear favorable for development, and a tropical depression is likely to form by the middle of this week while the system meanders or drifts northward a few hundred miles offshore the coast of southwestern Mexico. * Formation chance through 48 hours…medium…60 percent. * Formation chance through 5 days…high…80 percent.
  • 2. An area of low pressure appears to be forming a couple of hundred miles south-southwest of the Gulf of Tehuantepec. Conditions appear conducive for additional development of this system, and a tropical depression is likely to form by the middle of the week while it moves slowly northwestward, near or parallel to the southwestern coast of Mexico. Regardless of development, areas of heavy rainfall and flash flooding are likely in southeastern Mexico and northern Central America during the next several days. * Formation chance through 48 hours…medium…40 percent. * Formation chance through 5 days…high…70 percent.

NWS National Hurricane Center Miami FL 500 PM PDT Sat Aug 22, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

  • 1. An elongated area of low pressure located several hundred miles south-southwest of the southern tip of the Baja California peninsula continues to produce a large area of disorganized cloudiness and thunderstorms. Conditions appear favorable for gradual development, and a tropical depression is likely to form during the early- to-middle part of next week while the system drifts a few hundred miles offshore the coast of southwestern Mexico. * Formation chance through 48 hours…medium…50 percent. * Formation chance through 5 days…high…80 percent.
  • 2. An area of low pressure is likely to form south of the Gulf of Tehuantepec early next week. A tropical depression is likely to form by the middle of next week while the system moves slowly northwestward or northward, near or parallel to the southwestern coast of Mexico. Regardless of development, areas of heavy rainfall and flash flooding are likely in southeastern Mexico and northern Central America during the next several days. * Formation chance through 48 hours…low…10 percent. * Formation chance through 5 days…high…70 percent.

NWS National Hurricane Center Miami FL 1100 AM PDT Sat Aug 22, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

  • 1. An elongated area of low pressure located several hundred miles south-southwest of the southern tip of the Baja California peninsula continues to produce a large area of disorganized cloudiness and thunderstorms. This system is forecast to drift eastward for the next couple of days. Conditions appear favorable for gradual development, and a tropical depression is likely to form during the early to middle part of next week when the system is expected to move slowly off the coast of Mexico. * Formation chance through 48 hours…medium…40 percent. * Formation chance through 5 days…high...80 percent.
  • 2. Another area of low pressure is likely to form south of the Gulf of Tehuantepec early next week. A tropical depression could form by the middle of next week while the system moves slowly northwestward or northward, near or parallel to the southwestern coast of Mexico. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…medium…60 percent.

NWS National Hurricane Center Miami FL 500 AM PDT Sat Aug 22, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

  • 1. An elongated area of low pressure located several hundred miles south-southwest of the southern tip of the Baja California peninsula continues to produce a large area of disorganized cloudiness and thunderstorms. This system is forecast to move generally eastward for the next couple of days. Conditions appear favorable for gradual development and a tropical depression is likely to form during the early to middle part of next week when the system is expected to slow and meander off the coast of Mexico. * Formation chance through 48 hours…medium…40 percent. * Formation chance through 5 days…high…80 percent.
  • 2. Another area of low pressure is likely to form south of the Gulf of Tehuantepec early next week. A tropical depression could form by the middle of next week while the system moves slowly northwestward or northward, near or parallel to the southwestern coast of Mexico. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…medium…50 percent.

NWS National Hurricane Center Miami FL 500 PM PDT Fri Aug 21, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

The National Hurricane Center has issued its last advisory on Post-Tropical Cyclone Genevieve, located a couple hundred miles west of the central Baja California peninsula.

  • 1. An elongated area of low pressure located several hundred miles south-southwest of the southern tip of the Baja California peninsula continues to produce a large area of disorganized cloudiness and thunderstorms. This system is forecast to move generally eastward for the next couple of days. Conditions appear favorable for gradual development and a tropical depression is likely to form during the early to middle part of next week when the system is expected to slow down and meander off the coast of Mexico. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…high…70 percent.
  • 2. Another area of low pressure is likely to form south of the Gulf of Tehuantepec early next week. A tropical depression could form by the middle of next week while the system moves slowly northwestward or northward, near or parallel to the southwestern coast of Mexico. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…medium…40 percent.

NWS National Hurricane Center Miami FL 1100 AM PDT Fri Aug 21, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

The National Hurricane Center is issuing advisories on Tropical Storm Genevieve, located a couple of hundred miles west of Cabo San Lazaro, Mexico.

  • 1. An elongated area of low pressure located several hundred miles south-southwest of the southern tip of the Baja California peninsula is producing a large area of disorganized cloudiness and thunderstorms. Conditions appear favorable for some gradual development of this system, and a tropical depression is likely to form during the early to middle part of next week while it moves little. * Formation chance through 48 hours…low…30 percent. * Formation chance through 5 days…high...70 percent.
  • 2. Another area of low pressure is likely to form south of the Gulf of Tehuantepec early next week. Some slow development of this system is possible thereafter while it drifts northwestward or northward. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…low…30 percent.

NWS National Hurricane Center Miami FL 500 PM PDT Thu Aug 20, 2020

For the eastern North Pacific…east of 140 degrees west longitude:

The National Hurricane Center is issuing advisories on Tropical Storm Genevieve, located near the southwestern coast of the Baja California peninsula.

  • 1. An area of low pressure is expected to form this weekend several hundred miles southwest of the southwestern coast of Mexico. Conditions appear favorable for some slow development of this system, and a tropical depression could form by the middle of next week while it moves little. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…medium…50 percent.
  • 2. Another area of low pressure is likely to form south of the Gulf of Tehuantepec early next week. Some slow development of this system is possible thereafter while it drifts northward. * Formation chance through 48 hours…low…near 0 percent. * Formation chance through 5 days…low...20 percent.

If this system is named, it will become Tropical Storm Iselle.

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