*******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.**********
...TUESDAY DECEMBER 1 2020 10:45 PM EDT...
See area of interest sections below for areas of disturbed weather currently being monitored for tropical development in the Atlantic basin. Elsewhere...a subtropical surface low pressure over the central Atlantic previously induced by the divergence zone of a cut-off upper vortex in the region lacks thunderstorms due to the stability provided by water temps below 26 deg C and relatively warm upper vortex. In addition the surface low has been pushed westward by the surface layer of the deep-layer ridge currently in place across the Atlantic and into the suppressing western convergence zone of the upper vortex. Subtropical cyclone formation of this surface low is not expected.
AREA OF INTEREST #1...Continuing to monitor a northeastern Atlantic deep-layered low pressure system...consisting of an upper vortex and surface cyclone...for acquisition of tropical characteristics. Even though water temps in the region are currently running at 20 deg C...the upper vortex (measuring at 1175 dekameters in height at 200 mb) has been plenty cold enough to raise instability at these water temps...and as a result bands of showers and thunderstorms continue to persist around the surface cyclone center. However the National Hurricane Center has deemed this activity not concentrated enough to declare a subtropical cyclone. I have lowered odds of subtropical cyclone formation to 30% as the surface cyclone will continue to weaken directly below the center of the upper vortex where there is a lack of divergence as we typically see with post-mature mid-latitude storms. I have also nudged the track forecast track points south and west due to the current position of the surface cyclone.
A deep-layer ridge to the west...bolstered in the warm sector of the strong frontal cyclone over the eastern US...has begun to push this system to the southwest. Odds of development in the outlook below are continuously tapered downward as the surface cyclone will continue to weaken beneath the center of the upper vortex as stated in the previous paragraph. In addition while remaining cut-off from high latitude cold air...the cold core upper vortex will increasingly weaken and warm to stabilizing temps (measure towards 1200+ dekameters in height at 200 mb) by 48+ hours...which would put an end to any tropical development especially as the southwest track will not go south enough to the needed warm 26 deg C waters to otherwise sustain tropical development. Thus the odds of tropical development are dropped to 0% by 72 hours.
*******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Dec 2)...30% chance of subtropical cyclone formation (eastern Atlantic near 32N-22.5W)
IOH 48 Hr Outlook (1800Z Dec 3)...10% chance of subtropical cyclone formation (eastern Atlantic near 29N-24W)
IOH 72 Hr Outlook (1800Z Dec 4)...0% chance of subtropical cyclone formation (eastern Atlantic near 25N-25W)
AREA OF INTEREST #2...The upper-level anticyclonic center of the warm western Atlantic upper ridge...supported by the warm sector of the strong eastern US frontal cyclone...continues producing showers and thunderstorms in the southern Caribbean Sea with an area of weak cyclonic turning near 11N-80W. Despite a lack of computer model support...I am maintaining the southern Caribbean as an area of interest for possible tropical development due to the current observation of cyclonic turning and favorable upper wind outlook...which in addition to the upper-level anticyclonic outflow includes possible poleward outflow enhancement induced by the upper southwesterly flow ahead of the eastern US frontal cyclone’s upper trough. The track of this disturbance has been stationary in weak steering currently in place to the south of the eastern US frontal cyclone...but the forecast track below shows a westward turn into Costa Rica and southern Nicaragua as the surface ridge building behind the frontal cyclone will soon take over the steering. I have lowered odds of development to 10% due to the current lack of computer model support and lack of development thus far with less time over water remaining on the current forecast track. Regardless of tropical cyclone formation or not...heavy rainfall with flash flooding potential is possible across southern Nicaragua...Panama...and Costa Rica over the next couple of days..but thankfully this is south of the devastation zone produced by Hurricanes Eta and Iota earlier this month.
*******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Dec 2)...10% chance of tropical cyclone formation (southern Caribbean near 11N-82.5W)
IOH 48 Hr Outlook (1800Z Dec 3)...0% chance of tropical cyclone formation (inland over southeastern Nicaragua just north of the Costa Rica border near 11N-84.5W)
...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...center of surface cyclone drifts southwest while weakening and dissipates near 26N-24W by 60 hours. For area of interest #2...no tropical cyclone formation shown. Elsewhere...current central Atlantic subtropical low dissipates near 30.5N-59.5W at 30 hours.
1200Z ECMWF Model Run...For area of interest #1...center of surface cyclone drifts southwest while weakening and dissipates near 29.5N-24W just after 48 hours. For area of interest #2...no tropical cyclone formation shown. Elsewhere...current central Atlantic subtropical low dissipates near 30N-59W just after 24 hours.
1200Z GFS Model Run...For area of interest #1...center of surface cyclone drifts southwest while weakening and dissipates near 28N-24W by 54 hours. For area of interest #2...no tropical cyclone formation shown. Elsewhere...current central Atlantic subtropical low dissipates near 30N-59W at 48 hours.
1200Z NAVGEM Model Run...For area of interest #1...center of surface cyclone drifts southwest while weakening and dissipates near 28.5N-21.5W by 54 hours. For area of interest #2...no tropical cyclone formation shown.. Elsewhere...current central Atlantic subtropical low dissipates near 28N-59W just after 54 hours.
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