*******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.**********
...MONDAY NOVEMBER 30 2020 5:10 PM EDT...
See area of interest sections below for areas of disturbed weather currently being monitored for tropical development in the Atlantic basin. Elsewhere...the south end of the current upper trough over the northwest Atlantic is becoming cut-off into a central Atlantic vortex by a warm upper ridge rapidly forming to the west with the support of the currently strong eastern US frontal cyclone. The divergence zone of this developing upper vortex has triggered a surface frontal low pressure...which as of this evening has a band of showers and thunderstorms just south of its center perhaps enhanced by the instability provided by the cold temperatures of the upper vortex (currently the upper vortex measures at 1197 dekameters in height at 200 mb). However I have not added this disturbance as another area of interest for subtropical development as the upper vortex is forecast to warm to above 1200 dekameters in height in the next 24 hours while remaining cut-off from high latitude cold air...and sea surface temperatures in the region are currently below 26 deg C...which will make thermodynamic conditions needed for thunderstorm activity less favorable.
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 1165 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 persistent 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 50% as the surface cyclone will begin 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 short-term track forecast track points northward due to the current position of the surface cyclone. Even though the current position is further north from Madeira Island compared to previous forecasts...gusty winds are likely occurring over the island due to the size of the surface cyclone's circulation.
After the deep-layer ridge currently to the north moves into Europe...another deep-layered ridge developing to the west in the warm sector of the strong frontal cyclone over the eastern US will push this system to the southwest. This would mean an end to gusty winds on Madeira Island by tomorrow. When the southwest drift of this system occurs...I taper the odds of development downward as the surface cyclone (if it fails to develop a tropical thunderstorm core with mid-level warm core upper outflow) will begin to weaken beneath the center of the upper vortex where there is a lack of divergence 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 72+ 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 96 hours.
*******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Dec 1)...50% chance of subtropical cyclone formation (eastern Atlantic near 35.5N-20W)
IOH 48 Hr Outlook (1800Z Dec 2)...30% chance of subtropical cyclone formation (eastern Atlantic near 33N-21.5W)
IOH 72 Hr Outlook (1800Z Dec 3)...10% chance of subtropical cyclone formation (eastern Atlantic near 30N-22.5W)
IOH 96 Hr Outlook (1800Z Dec 4)...0% chance of subtropical cyclone formation (eastern Atlantic near 25N-22.5W)
AREA OF INTEREST #2...The upper-level anticyclonic center of a rapidly developing warm western Atlantic upper ridge...supported by the warm sector of the strong eastern US frontal cyclone...is producing a concentrated area of thunderstorms in the southern Caribbean Sea as of today. Although the GFS no longer shows this area of disturbed weather developing and none of the other models show development at this location...I have added the southern Caribbean as another area of interest for possible tropical development in the coming days due to the current concentration of thunderstorm activity 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. Forecast track in the outlook below is stationary in the next 24 hours in weak steering currently in place to the south of the eastern US frontal cyclone...followed by a westward turn into Costa Rica and southern Nicaragua as a surface ridge to build behind the frontal cyclone takes over the steering. I have kept odds of development at a low 20% peak due to the current lack of computer model support. Regardless of tropical cyclone formation or not...heavy rainfall with flash flooding potential is possible across southern Nicaragua and Costa Rica starting tomorrow...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 1)...10% chance of tropical cyclone formation (southern Caribbean Sea near 11N-80.5W)
IOH 48 Hr Outlook (1800Z Dec 2)...20% chance of tropical cyclone formation (offshore of southern Nicaragua just northeast of the Nicaragua/Costa Rica border near 11N-83W)
IOH 72 Hr Outlook (1800Z Dec 3)...0% chance of tropical cyclone formation (inland over the Nicaragua/Costa Rica border near 11N-85W)
...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 25N-26W by 84 hours. For area of interest #2...no tropical cyclone formation shown. Elsewhere...current central Atlantic frontal low becomes possible subtropical cyclone near 32.5N-51.5W by 12 hours...dissipates near 30.5N-59.5W at 72 hours.
1200Z ECMWF Model Run...For area of interest #1...center of surface cyclone drifts southwest while weakening and dissipates near 29.8N-24W just after 72 hours. For area of interest #2...no tropical cyclone formation shown. Elsewhere...current central Atlantic frontal low dissipates near 27.5N-61.5W just after 48 hours without subtropical cyclone formation.
1200Z GFS Model Run...For area of interest #1...center of surface cyclone drifts southwest while weakening and dissipates near 26N-22W by 84 hours. For area of interest #2...no tropical cyclone formation shown. Elsewhere...current central Atlantic frontal low dissipates near 29.5N-56W at 26 hours without subtropical cyclone formation.
0600Z NAVGEM Model Run...For area of interest #1...center of surface cyclone drifts southwest while weakening and dissipates near 27.5N-21W by 90 hours. For area of interest #2...no tropical cyclone formation shown.. Elsewhere...current central Atlantic frontal low dissipates near 28N-59W just after 72 hours without subtropical cyclone formation.
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