*******Note that forecast and outlooks in this post are NOT the official forecast from the National Hurricane Center (NHC). They are my own detailed views on the Atlantic tropics based on current observations and latest computer model runs. As such do not make decisions based on my posts...consult news media...watches and warnings from your local weather office...and any evacuation orders issued by local governments to make the most informed and best decisions. Visit the NHC website hurricanes.gov (hurricanes dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.*********
…UPDATE… WEDNESDAY SEPTEMBER 25 2024 11:20 PM EDT…
A bout of rapid tropical cyclone formation has occurred in the open North Atlantic… the NHC has already upgraded new area of interest #33 to Tropical Storm Isaac with 50 mph maximum sustained winds… and the NHC forecast calls for Isaac to reach a peak of 70 mph maximum sustained winds while passing north of the Azores in the days ahead. I will have a forecast for Isaac in my next update.
...WEDNESDAY SEPTEMBER 25 2024 11:15 PM EDT...
The Atlantic tropics are rather active this evening with Hurricane Helene threatening large swaths of Florida and the southeastern United States… and the following areas of interest:
(1) See area of interest #32 section below for the eastern Atlantic tropical wave with a high chance of tropical cyclone formation
(2) See area of interest #33 section below for the north Atlantic frontal low which also has a high chance of tropical cyclone formation
(3) See area of interest #34 for a tropical wave of low pressure which could develop in a few days while approaching Central America and the western Caribbean
(4) A tropical wave of low pressure with a distinct thunderstorm field is now departing the west coast of Africa… however not currently adding it as an area of interest as models tend to agree that area of interest #32 will tend to dominate the surface inflow and upper outflow of the region.
HURRICANE HELENE... The following is my forecast for Hurricane Helene completed around 1200Z earlier today... this forecast was released in special update #88C (https://www.infohurricanes.com/post/my-2024-atlantic-hurricane-season-birdseye-view-post-88c-special-update). Since then Helene has only slightly strengthened further to 85 mph maximum sustained winds while turning north-northeast across the southeast Gulf of Mexico… therefore making no changes to my track and intensity projection from 1200Z.
As documented in special update #88B (https://www.infohurricanes.com/post/my-2024-atlantic-hurricane-season-birdseye-view-post-88b-special-update)... tropical storm Helene's track took a west hook toward the northeastern Yucatan peninsula coast while its center was likely cyclonically swinging around the remainder southern portion of the broad tropical low pressure field it formed within. During this time the adjacent Yucatan peninsula upper vorticity shifted west and away toward the eastern Bay of Campeche while likely shoved by the previous southeastern US upper ridge wave... which helped relax the shear over Helene such that it developed a more symmetric core of thunderstorms covering its center that allowed it to strengthen a little quicker than my previous intensity forecast showed. The latent heat release of the strong thunderstorm core may have also helped in shoving the adjacent cool-core upper vorticity into the eastern Bay of Campeche in recent hours. From 8 AM to 11 AM EDT the track has returned to the previously expected north turn which has just kept the center from making landfall over the northeastern Yucatan peninsula... and during this time Helene strengthened further to a category 1 hurricane with 80 mph maximum sustained winds. Recently (around 2 PM EDT) the thunderstorm core has relaxed in intensity on colorized infrared satellite pictures... perhaps while the strengthening circulation inhaled some of the dry sinking air associated with the western convergence zone of the adjacent eastern Bay of Campeche upper vorticity. Likewise the NHC update at 2 PM EDT shows that Helene has not strengthened between 11 AM EDT and 2 PM EDT.
The track forecast still calls for a north-northeast acceleration across the eastern Gulf of Mexico and into the southeastern United States from northern Florida as the current eastern North America upper trough and eastern Bay of Campeche upper vorticity merge into a lengthy upper trough that steers Helene. The track forecast is nudged westward to account for Helene's prior westward swing toward the Yucatan peninsula noted in the prior paragraph and now calls for the center to make landfall over the far eastern Florida panhandle instead of the northwestern part of the Florida peninsula. After landfall... the south part of the steering upper trough is forecast to become a cut-off upper vortex to the east of an amplified upper ridge... with Helene arcing north then eventually north-northwest around the east side of the upper vortex... a path that takes the inland remnant system across the eastern US. During this time Helene transitions into a remnant frontal cyclone supported by the eastern divergence zone of the steering upper vortex... and likewise pulls cold air associated with the upper vortex toward itself. Because the intensity forecast has trended stronger as noted below... Helene will be able to pull the cold core air associated with the upper vortex closer to the warm air it was previously under... resulting in a stronger steering upper jet in the air mass contrast zone. This is why the center of ex-Helene is now forecast to reach the vicinity of the southwestern North Carolina border sooner than I previously showed. Regarding intensity... I have increased the forecast and now show a top-end category 2 for landfall based on Helene strengthening a little faster than the prior forecast... however I still trend below the latest NHC intensity forecast as of this writing (which calls for a category 3 peak) due to the recent pause in strengthening noted in the prior paragraph. There are two scenarios possible regarding intensity up until landfall... either the southwesterly wind shear associated with the steering upper trough keeps Helene on the weaker side... or Helene re-develops another strong thunderstorm core whose latent heat release can develop a local warm core upper anticyclone that deflects the shear and allows the southwesterly upper flow to enhance the northern outflow of the anticyclone which could result in a surprise amount of intensification. I see my current intensity forecast as the middle ground between the two possibilities. Note that Helene is a larger sized storm as it was born within a broad low pressure field... and will continue to remain large due to the size of the eastern divergence zone of the steering upper trough/vortex. The large wind field will drive a life-threatening widespread coastal storm surge toward the landfall region regardless of the exact category number assigned to Helene's peak intensity. Final note regarding intensity... the post-landfall decay rate of Helene could be slower than normal due to the aforementioned field of upper divergence.
Regarding impact to land areas:
(1) For the northeastern Yucatan peninsula... interests should remain sheltered in place until the gusty winds and coastal surf relax as Helene pulls away later today. Heavy rainfall with flash flooding remains a concern for today.
(2) For the Cayman Islands… periods heavy rainfall will continue from the southeastern outer bands of Helene over the next several hours.
(3) For western Cuba… heavy rainfall... gusty tropical storm force winds... and coastal surf are upon the area. Weather conditions will improve gradually over the next 24 hours once Helene accelerates north-northeast and away
(4) For the Florida Keys... expect periods of heavy rainfall from Helene's outer eastern rain bands and coastal surf over the next 36 hours. The west shift in the forecast track has reduced the potential for gusty tropical storm force winds here... however tropical storm force gusts cannot be ruled out for Key West.
(5) The southwest Florida peninsula coast will be subject to coastal surf by tomorrow. For the remainder of the Florida peninsula west coast... Florida panhandle... and Alabama coast... coastal surf arrives by late tomorrow and early Friday… becoming life-threatening for the eastern Florida panhandle and northwestern Florida peninsula no later than early Friday (listen to the advice of local government officials and news media... particularly evacuation orders). Damaging hurricane to tropical storm force winds are expected to spread onshore across the northern half of Florida peninsula... eastern half of the Florida panhandle... and southern Georgia no later than early Friday… with the most severe wind toward the eastern Florida panhandle coast and/or far northwestern Florida peninsula coast. Heavy rainfall with flooding potential is expected to spread across central and northern Florida… then across much of the southeast US Friday through Saturday. Due to the slowed inland decay rate mentioned at the end of the prior paragraph… gusty winds are also possible across much of the southeast US Friday through Saturday. The inland southeastern US region that maybe subject to gusty winds (with scattered damage) and heavy rainfall potential includes Georgia... Alabama... Mississippi... eastern Arkansas... southeastern Missouri... Tennessee... Kentucky... southern Illinois... southern Indiana... southern Ohio... the western Virginias (including most of West Virginia)... and the western Carolinas.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Sep 25)... 80 mph maximum sustained wind hurricane centered just offshore of the northeastern Yucatan peninsula at 21.6N-86.3W
IOH 24 Hr Forecast (1200Z Sep 26)... 100 mph maximum sustained wind hurricane in the eastern Gulf of Mexico at 25.5N-85.5W
IOH 36 Hr Forecast (0000Z Sep 27)... 110 mph maximum sustained wind hurricane centered over the far eastern Florida panhandle coast at 29.8N-83.5W
IOH 48 Hr Forecast (1200Z Sep 27)... Remnant frontal cyclone centered over the NC/ SC/ GA border intersection at 35N-83W
*****National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT*************************
Landfall (0000Z Sep 27)… 125 mph maximum sustained wind major hurricane centered over the eastern Florida panhandle coast at 29.7N-84.6W
4-Day Position (1200Z Sep 29)… 15 mph maximum sustained wind dissipating remnant low centered over the western Kentucky/Tennesee border at 36.6N-87.9W
AREA OF INTEREST #32... The current eastern Atlantic tropical wave of low pressure has not become a tropical cyclone so far as it’s central area of rotating thunderstorms decreased in intensity from ingestion of dry Saharan air just to the northeast. However the wave remains well-organized… computer model support showing development is high… and a regional tropical upper ridge axis featuring low shear and upper outflow remains in place. Therefore I have begun a specific track and intensity forecast assuming that this system will eventually become a tropical cyclone. Track forecast is classic for an Atlantic tropical cyclone expected to stay over open water… with the system initially moving west while pushed by the surface layer of the eastern Atlantic mid-latitude deep layer ridge… follows by a north then northeast turn in the flow ahead of an upper trough. The upper trough will be a merger between the current eastern North America upper trough and northwest Atlantic upper vorticity… and as such will have a mid-ocean broad surface ridge weakness composed of area of interest #33 and current Great Lakes frontal low. The mid-ocean surface weakness will pass well north of this area of interest… however the upper flow of the upper trough should be enough to keep the north and northeast speed decent as this system is expected to be strong/tall enough in the latter part of the forecast period to be guided by upper winds. Through 72 hours I forecast gradual intensification into a minimal hurricane… the slow intensification rate being a reflection on the aforementioned dry Saharan air environment. I then cap the intensification at 96 hours and show weakening by 120 hours due to forecast wind shear imparted by the steering upper trough.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 25)... Tropical low centered in the eastern tropical Atlantic at 14N-32.5W
IOH 24 Hr Forecast (1800Z Sep 26)... 35 mph maximum sustained wind tropical depression centered in the eastern tropical Atlantic at 15N-37.5W
IOH 48 Hr Forecast (1800Z Sep 27)... 50 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 16N-41.2W
IOH 72 Hr Forecast (1800Z Sep 28)... 75 mph maximum sustained wind hurricane centered in the central tropical Atlantic at 18.5N-44W
IOH 96 Hr Forecast (1800Z Sep 29)... 75 mph maximum sustained wind hurricane centered in the central Atlantic at 22.5N-45W
IOH 120 Hr Forecast (1800Z Sep 30)... 65 mph maximum sustained wind tropical storm centered in the central Atlantic at 27N-43W
*****National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT*************************
Formation chance through 48 hours... 70%
Formation chance through 7 days... 80%
AREA OF INTEREST #33… Over the last couple of days the mid-latitude frontal low that has been in the western Atlantic has developed into a duplex of centers… and old southwestern center decaying underneath the lack of divergence beneath the overhead upper vorticity and a new northeastern center that became the dominant while spawned by the eastern divergence zone of the upper vorticity. During my recent Saturday to Monday vacation period (previous posts #88A and #88B) models were hinting at tropical development of the northeastern center… and by Tuesday evening this center was indeed developing increased thunderstorm activity and was upgraded to an area of interest by the NHC by early Wednesday… this marks the thirty-third tropical Atlantic area of interest tracked on this site this year (not tagging this as area of interest #31 as the low pressure center of interest is now absorbing what is left of area of interest #31).
As of 1800Z the low pressure center was near 37N-56.5W. This system and overhead upper vorticity are forecast to shift east through 72 hours. During and after that time the upper vorticity merges with the current eastern North America upper trough… with the merged cold core upper trough axis being west of this area of interest as the upper trough fragment from North America is colder than the overhead upper vorticity. This system’s track therefore bends more north after 72 hours in the southwesterly flow ahead of the merged upper trough axis. There is some discrepancy in the models for the 5+ day track forecast… with the CMC and ECMWF favoring this system continuing east-northeast toward Europe while caught by the southwest side of a high-latitude upper trough that dives in from northern Canada and Greenland… and the GFS favoring the merged upper trough axis to the west amplifying into an upper vortex that pulls this system on a westward retrogression toward Canada. For now my day 5 position splits the difference between these two extremes. For odds of tropical cyclone formation… despite a pause in the thunderstorm activity by Wednesday early afternoon the odds of tropical development have rapidly escalated upward as this system has rapidly developed organized thunderstorms near its center (the NHC has rapidly increased odds of tropical cyclone formation to 80%). I have set my peak odds at 50% in case the thunderstorms dwindle again in the overnight… but I also wouldn’t be surprised if the NHC upgrades this system to a tropical cyclone in the overnight should the thunderstorms persist. The current forecast track has this system over 26+ deg C for 72 hours… and places this system under boosting upper divergence on the east side of the upper trough by 96 hours which may help to counteract the movement into waters below 26 deg C. By 120 hours my forecast position places this system over even cooler waters around 20 deg C and under suppressive convergence on the southwest side of the high-latitude upper trough… and I drop my odds of tropical development to 0% by then.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
Formation chance through 4 days... 50% (well west-northwest of the Azores near 42N-36.5W)
Formation chance days 4 to 5… 0% (northwest of the Azores near 44N-31.5W)
*****National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT*************************
Formation chance through 48 hours... 40%
Formation chance through 7 days... 40%
Updated to 70% odds for 48 hours and 80% odds for 7 days in a special tropical weather outlook at 10:45 PM EDT
AREA OF INTEREST #34… A tropical wave of low pressure recently crossed 56W longitude as of 1800Z earlier this evening on its way toward the Lesser Antilles and eastern Caribbean. Models have increasingly suggested the wave may evolve into a broad tropical low over Central America and western Caribbean in about 5 to 7 days… and I have added the wave as an area of interest for tropical development.
Over the last couple of days the wave saw some increase in thunderstorms while taking advantage of outflow beneath an expanding local upper ridge as noted in the upper air panel of the above birdseye view chart. However through day 4… as the surface wave crosses the eastern and central Caribbean… it is likely to be suppressed as (1) the warm core upper ridge over Helene eventually matures into a west Atlantic deep-layer ridge… (2) the deep-layer ridge spreads the upper vorticity currently to the west of the wave across the southern half of the Caribbean… (3) the deep-layer ridge pushes some of the upper vorticity over area of interest #33 southwest into the southeast Caribbean by late in the 5 day forecast period… (4) the tropical wave becomes suppressed by a lack of divergence beneath the upper vorticity. By day 5… the upper air pattern over the wave begins to improve as the upper vorticity in step (2) above fades and allows the southwest side of the deep-layer ridge to potentially increase the outflow over the wave… however I assign a low 10% chance of development at day 5 as the models prefer to show development after that time. Noting that long-term development after day 5 may be briefly interrupted by the upper vorticity in step (3) above sliding west over the surface wave. Noting the upper vorticity may split into two upper waves as it does so… for instance this past 1200Z GFS model run already has the upper vorticity split into one upper wave over the southern Lesser Antilles and another upper wave over the south-central Caribbean by day 5. When the gap between the two decaying upper waves… featuring potential upper anticyclonic divergent flow… aligns with the surface wave the surface wave may try to develop in the 7+ day window.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
Formation chance through 5 days... 10% (Caribbean Sea just east of the Nicaragua/Honduras border near 15N-82W)
*****National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT*************************
Not in the official outlook
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/).
0000Z (Sep 25) CMC Model Run...
**For Tropical Storm Helene... while strengthening into a large and potentially intense hurricane accelerates north-northeast across the eastern Gulf of Mexico with the center making landfall over the eastern Florida panhandle just after 48 hours... while transitioning into a large and vigorous inland remnant frontal cyclone the center whirls north-northwest into western Kentucky at 72 hours while reaching the core of the regional overhead upper vorticity... while weakening into a large remnant frontal low due to a lack of divergence beneath the overhead upper vorticity both ex-Helene and the upper vorticity proceed to shift east-southeast onto the coast of North Carolina through 120 hours then into the waters just northwest of Bermuda through 168 hours
**For area of interest #32... tropical cyclone formation suggested at 17N-44W at 66 hours... while strengthening into a hurricane turns north and reaches 27.5N-45.5W by 168 hours
**For area of interest #33... frontal low cyclonically loops southeast through 18 hours while possibly acquiring tropical status... frontal low then proceeds to move east across the mid-latitudes of the central Atlantic while becoming less tropical and more elongated through 72 hours and eventually loses identity to another elongated frontal low developing to the northeast after that time
**Tropical wave currently near west coast of Africa proceeds to move west-northwest across the eastern tropical Atlantic and becomes absorbed by northeast side of area of interest #32 by 102 hours
**Additional tropical wave emerges from west coast of Africa at 78 hours and proceeds to gradually evolve into a tropical low that reaches the central tropical Atlantic near 15N-43W by 168 hours
**Tail end of surface cold front driven by ex-Helene reaches waters just northwest of the Yucatan peninsula through 96 hours while simultaneously the tropical wave currently east of the Lesser Antilles proceeds to move into and across the Caribbean... tail end of front and tropical wave then proceed to merge into a broad tropical low over Central America and adjacent Caribbean waters through 168 hours
0000Z (Sep 25) ECMWF Model Run...
**For Tropical Storm Helene... continues north-northeast across the eastern Gulf of Mexico while strengthening into a large and potentially intense hurricane with the center making landfall over the eastern Florida Panhandle just after 48 hours... center then whirls north-northwest then northwest into western Kentucky and Tennessee where core of regional upper vorticity is located through 78 hours while transitioning into a remnant frontal cyclone... due to lack of divergence beneath core of upper vorticity ex-Helene then weakens to a large frontal low centered over central Tennessee through 114 hours... ex-Helene then dissipates soon after while another frontal low materializing to the east becomes the dominant.
**For area of interest #32... tropical cyclone formation suggested near 17N-41.8W at 60 hours... while strengthening into a hurricane proceeds to turn north then northeast and reaches 33.5N-38.5W through 168 hours
**For area of interest #33... cyclonically loops southeast through 12 hours while potentially transitioning into a tropical cyclone... the possible tropical cyclone then continues east-northeast and passes just north of the Azores by 108 hours... subsequently loses tropical characteristics and continues east-northeast into the northwest coast of France by 150 hours as a remnant frontal low... remnant low becomes quasi-stationary over the northwest coast of France through 168 hours
**Tropical wave currently near west coast of Africa proceeds to move west-northwest across the eastern tropical Atlantic and becomes absorbed by northeast side of area of interest #32 by 102 hours
**Tail end of surface cold front driven by ex-Helene reaches waters just northwest of the Yucatan peninsula through 102 hours while simultaneously the tropical wave currently east of the Lesser Antilles proceeds to move into and across the Caribbean... tail end of front and tropical wave then proceed to merge into a broad tropical low over Central America... southeastern Mexico and adjacent eastern Bay of Campeche and Caribbean waters through 168 hours
1200Z (Sep 25) GFS Model Run...
**For Tropical Storm Helene... continues north-northeast across the eastern Gulf of Mexico while strengthening into a large and potentially intense hurricane with the center making landfall over the eastern Florida Panhandle just after 36 hours... the vigorous inland remnant circulation of Helene then swings north-northwest into eastern Kentucky by 51 hours while the divergence zone of a regional upper vorticity produces a western Kentucky/Tennessee border broad frontal low just to the west... ex-Helene then swings west across southern Indiana and Illinois through 66 hours while becoming absorbed by northwest quadrant of broad frontal low
**For area of interest #32... tropical cyclone formation suggested near 16N-38.8W at 27 hours... subsequently curves northwest... north... the northeast while strengthening into a hurricane and reaches waters just north of the Azores by 168 hours
**For area of interest #33... proceeds east-northeast as a possible strong tropical cyclone and reaches 43N-40W at 93 hours... the frontal low/upper trough currently over the Great Lakes then proceeds to become a deep-layer cyclone just west of this system which causes this system to cyclonically whirl northwest then west toward the waters east of Newfoundland through 168 hours while transitioning into a non-tropical remnant frontal cyclone supported by the divergence of the deep-layer cyclone.
**Tropical wave currently near west coast of Africa proceeds to move west-northwest across the eastern tropical Atlantic and becomes absorbed by east side of area of interest #32 by 99 hours
**Eastern US frontal low that absorbs ex-Helene proceeds to drive tail end of surface cold front into the waters northwest of the Yucatan peninsula through 102 hours while simultaneously the tropical wave currently east of the Lesser Antilles proceeds to move into and across the Caribbean... tail end of front and tropical wave then proceed to merge into a broad tropical low over Central America and adjacent Caribbean waters through 168 hours
1200Z (Sep 25) NAVGEM Model Run...
**Not available at above-mentioned source
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