*******Note that forecasts 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.**********
...WEDNESDAY SEPTEMBER 30 2020 5:35 PM EDT...
See area of interest #1 section below for the tail end of a cold front currenlty heading toward the western Caribbean Sea from the Gulf of Mexico being monitored for possible tropical cyclone formation over the next few days.
Long range computer model runs showing a tropical wave of low pressure emerging from western Africa and developing in the eastern tropical Atlantic underneath a fvorable upper ridge to persist over the region by next week have recently ceased...and currently there are no tropical waves over the eastern Atlantic or western Africa showing organized thunderstorm activity.
The tropical wave of low pressure currently in the open Atlantic near 53W longitude and approaching the Lesser Antilles may amplify (intensify) in a regime of split flow upper divergence upon entering the central Caribbean Sea in a few days. This upper air pattern will occur as the current north Atlantic upper vorticity dives southward toward the eastern Caribbean Sea due to the expansion of the northwestern Atlantic upper ridge to be induced by the warm sector of the current frontal system over the eastern US...with the split upper level flow occuring between the southwest side of the upper trough and southeast side of the Caribbean upper ridge to build over area of interest #1. The split flow upper divergence could result in the tropical wave intensifying into a possible tropical cyclone to the east of area of interest #1 by next week. Or alternatively if area of interest #1 does not consolidate and develop early...the strengthening tropical wave could merge with area of interest #1...resulting in a large and broad tropical disturbance in the western Caribbean Sea. Currently the latest model runs lean toward the latter idea...therefore this tropical wave has been incorporated into the area of interest #1 section at the present time.
AREA OF INTEREST #1...High-amplified upper trough energy over central North America has driven a surface cold front into the United States east coast and southeastern Gulf of Mexico...and the tail end of this cold front is expected to reach the western Caribbean Sea over the next 24 hours. Meanwhile over the next five days...upper-level winds over region will become more favorable for tropical development as the central North America upper trough regime relaxes...allowing the narrow strip of warm upper ridging ahead of the front and currently over western Cuba (featuring low shear and upper outflow) to expand in the trough regime's wake...therefore watching to see if the tail end of the front evolves into a tropical disturbance in the western Caribbean Sea. Given that the thunderstorm activity at the tail end of the front has decreased compared to yesterday...this is decreasing the chances that a tropical cyclone will form along the decaying front in the short term...and increasing the chances that the front will merge with incoming features to the east and become a much larger and broader system that will take time to consolidate into any kind of tropical cyclone. The first feature that will merge with the tail end of the front is the tropical wave of low pressure currently crossing the central Caribbean Sea...which has recently seen a large increase in thunderstorms and also over a large area due to upper divergence driven by upper vorticity that is retorgrading westward around the warm western Atlantic upper ridge. The second feature that may or may not merge with our developing disturbance is the tropical wave currently at 53W longitude heading toward the Lesser Antilles...which may later on intensify and develop thunderstorms when it reaches the central and western Caribbean due to split flow upper divergence to setup between a piece of upper vorticity to drop southward from the north Atlantic (thanks to the western Atalntic upper ridge) and the southeast side of the western Caribbean upper ridge to expand over our disturbance. My updated forecast track in the outlook below is adjusted southeastward toward the two tropical waves which may merge with the tail end of the front over the next five days....and also has a notably slower westward track around the central United States surface ridge as this system is more likely to become a large/broad disturbance that takes time to consolidate...and it will need to do so in order to be able to be moved around the surface ridge. My peak odds of development for the next five days have also been slightly lowered to 50% as it will take more time for a large/broad disturbance to consolidate into any kind of tropical cyclone...with odds trimmed down from the 50% by days 4 and 5 as the latest forecast track increasing land interaction with the Yucatan peninsula and southeastern Mexico by that timeframe. Although my forecast positions by that time are also over land...a large sprawling system may still be able to become a weak tropical cyclone centered over land due to thunderstorms bands reaching offshore...and so my odds of development are higher than 0% for days 4 and 5. Also the exact center of circulation is hard to predict for a large broad system that could undergo mutliple center reformations...and this is further complicated by the fact that the tropical waves incoming from the east may cause the center of this disturbance to relocate further east..so it is also possible the center is still over water (either in the western Caribbean or Bay of Campeche) by days 4 and 5.
Due to the increasing size potential of this emerging disturbance...I have expanded home page bulletins on this site to now include the Cayman Islands and inland areas across the entire Yucatan peninsula...Guatemala...and southeastern Mexico (this is in addition to western Cuba...Belize...and the east Yucatan peninsula coast of Mexico which have been mentioned on the home page bulletins on this site over the last couple of days). I am also mentioning the possiblity of flash flooding as this emerging disturbance is likely to be slow moving over many days...producing long periods of heavy rain.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 1)...0% chance of tropical cyclone formation (western Caribbean Sea near 17.5N-84W)
IOH 48 Hr Outlook (1800Z Oct 2)...10% chance of tropical cyclone formation (western Caribbean Sea near 17.5N-84W)
IOH 72 Hr Outlook (1800Z Oct 3)...50% chance of tropical cyclone formation (east of the Belize/Mexico border near 17.5N-86W)
IOH 96 Hr Outlook (1800Z Oct 4)...30% chance of tropical cyclone formation (Belize/Guatemala/Mexico border near 17.5N-89W)
IOH 120 Hr Outlook (1800Z Oct 5)...30% chance of tropical cyclone formation (just inland of Mexico's Bay of Campeche coast near 18N-91W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z CMC Model Run...For area of interest #1...large broad surface low develops in western Caribbean Sea by 150 hours.
1200Z ECMWF Model Run....For area of interest #1...no tropical cyclone formation shown.
1200Z GFS Model Run...For area of interest #1...surface low consolidates offshore of Belize and the eastern Yucatan peninsula coast by 48 hours...surface low moves northwest and tropical cyclone formation suggested on north coast of the Yucatan peninsula by 84 hours...possible tropical cyclone drifts southwestward to the west coast of the peninsula by 126 hours due to another broad low pressure in the western Caribbean Sea seeded by the tropical wave currently at 53W arriving into the region.
1200Z NAVGEM Model Run...For area of interest #1...surface low consolidates offshore of the east Yucatan peninsula coast at 60 hours...continues northwestward and reaches the north coast of the peninsula at 90 hours...becomes stationary with a slow southwest drift by 120 hours due to another broad low pressure area in the vicinity of Cuba seeded by the tropical wave currently at 53W arriving into the region.