*******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 DECEMBER 18 2024 1:53 PM EDT...
Continuing daily posts on the Atlantic tropics due to potential subtropical to tropical cyclone formation in the open central Atlantic ocean over the next couple of days. See area of interest #51 section below for details.
AREA OF INTEREST #51... As typically seen with out-of-season Atlantic tropical development... monitoring the possibility that a forecast mid-latitude deep-layer cyclone gains tropical characteristics... initiating an area of interest for this situation as the major global models have come into remarkable agreement on this situation. The incipient cold core upper trough is now diving southeast into the open central Atlantic under the force of warm core upper ridging developing to the north and west... and is expected to itself amplify into a cut-off upper vortex as the surrounding upper ridging continues to amplify. The upper ridge to the north is supported by warm surface southerly flow ahead of the current eastern Canada frontal low. The upper ridge to the west is supported by warm surface southerly flow between the west side of the current northwest Atlantic surface ridge and new frontal low over the eastern US that soon will eject northeast into Atlantic Canada. The eastern divergence zone of the incipient upper trough has begun to produce a concentrated area of showers and thunderstorms in the vicinity of 32N-42W... and surface pressures will soon fall resulting in a surface low. As the incipient upper trough becomes a quasi-stationary cut-off upper vortex at that location in 24 hours... it will reel in the intensifying surface low... resulting in effectively a deep-layer cyclone. Even though sea-surface temps in the area are currently running at a mild 22 deg C... the upper vortex is forecast to be cold (200 mb heights below 1200 dekameters) resulting in a thermal profile conducive for instability... showers... and thunderstorms. Most of the major global models remain in agreement in at least transition into subtropical status... and perhaps fully tropical status should the thunderstorms release enough latent heat to produce a warm core outflow layer above the surface (but below the upper vortex) which would help with surface pressure falls further. Therefore I have raised peak odds of subtropical cyclone formation to 40% in this update... and will issue higher odds in future updates should future satellite observations warrant.
Beyond 48 hours... this system becomes increasingly entangled with a strong upper trough and surface frontal system that encroaches from the western Atlantic. The southerly flow ahead of the surface front should pull the surface low north into increasingly cooler waters less conducive for tropical development... and I lower tropical development odds from the peak in as soon as 72 hours (note the upper vortex over the surface low opens into a NW/SE tilting upper trough while attempting to merge with the encroaching western Atlantic upper trough... should the surface low line up with the western convergence zone of the titling upper trough that would aid in the surface low's weakening). By 96 hours the surface low... if it is still around during that time... should lose its identity to the low pressure field of the incoming western Atlantic surface front. The updated forecast track in the 72 to 96 hour window has a slower forward speed as the model consensus has shifted to a slightly more delayed arrival of the surface front.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
IOH 24 Hr Outlook (1800Z Dec 19)... 25% chance of subtropical cyclone formation (central Atlantic near 32N-42W)
IOH 48 Hr Outlook (1800Z Dec 20)... 40% chance of subtropical cyclone formation (central Atlantic near 32N-42W)
IOH 72 Hr Outlook (1800Z Dec 21)... 15% chance of subtropical cyclone formation (central Atlantic near 33.5N-42W)
IOH 96 Hr Outlook (1800Z Dec 22)... 0% chance of subtropical cyclone formation (central Atlantic near 38.5N-41.5W)
*****National Hurricane Center (hurricanes.gov) official outlook as of 1 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 (Dec 18) CMC Model Run...
**For area of interest #51... surface low forms near 30N-40W at 42 hours... moves west through 84 hours while not gaining much strength... dissipates near 31N-46W at 114 hours
0000Z (Dec 18) ECMWF Model Run...
**For area of interest #51... well-defined surface trough develops near 30.5N-39.5W at 42 hours... rapid subtropical to tropical cyclone formation suggested near 31.2N-44W at 54 hours... the quasi-stationary tropical cyclone then begins to gradually weaken through 90 hours... while turning north weakens to a remnant low near 33.5N-44W at 108 hours... remnant low loses identity to developing frontal low to the west by 126 hours
1200Z (Dec 18) GFS Model Run...
**For area of interest #51... surface low forms near 30.5N-39W at 21 hours... rapid subtropical to tropical cyclone formation suggested near 32N-41W at 30 hours... tropical cyclone moves west-southwest to 31.5N-45W through 54 hours... while turning north the tropical cyclone weakens to a remnant low near 32.5N-44W by 90 hours... remnant low loses identity ahead of incoming frontal system to the west shortly thereafter
0600Z (Dec 18) NAVGEM Model Run...
**For area of interest #51... surface low forms near 30.5N-40W at 36 hours... rapid subtropical cyclone formation suggested near 30.5N-45W at 48 hours... moves west-southwest then turns north where it weakens to a remnant low near 34N-46W at 114 hours... remnant low loses identity ahead of incoming frontal system to the west shortly thereafter
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