*******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.**********
...THURSDAY NOVEMBER 26 2020 8:54 PM EDT...
Although today is the Thanksgiving holiday for many...the Atlantic tropics refuse to go on vacation. See area of interest sections below for areas currently being monitored for tropical development in the Atlantic basin.
AREA OF INTEREST #1...The meandering western to central Atlantic surface low pressure that we have been monitoring for several days for tropical development remains attached to the tail end of a cold front tied to a large central Atlantic upper trough. The base of the upper trough has become cut-off into a vortex separated from the rest of the trough by a deep-layered ridge passing to the north...and the surface low pressure of interest is transitioning into a better organized subtropical low pressure supported by the eastern divergence zone of the upper vortex. The surface low pressure is currently located at 27.5N-57.5W with bands of showers and thunderstorms becoming better organized around the center...and recently pockets of activity are developing over the center. Based on this and today’s model runs which have trended a bit stronger with the surface low’s peak strength...I have raised peak odds of subtropical cyclone formation to 50%. In the next 24 hours...a west drift under the influence of the passing deep-layered ridge to the north is expected...with the track stalling from 24 to 48 hours due to a blocking surface ridge to the west to be supported by the western convergence zone of the shortwave upper trough currently approaching from the northeastern US. The 0600Z GFS suggests that during this time the supportive divergence zone of the shortwave upper trough may overspread the surface low...thus I keep the peak 50% odds of subtropical cyclone formation through 1800Z November 28. The frontal low pressure associated with this shortwave upper trough will likely drag the surface low northeastward by 72 hours...and this is when I lower odds of subtropical cyclone formation to 0% while the surface low will likely exposed to the back west side of the shortwave upper trough where northwesterly shear and upper convergence would be present.
*******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Nov 27)...50% chance of subtropical cyclone formation (central Atlantic near 27.5N-59W)
IOH 48 Hr Outlook (1800Z Nov 28)...50% chance of subtropical cyclone formation (central Atlantic near 27.5N-59W)
IOH 72 Hr Outlook (1800Z Nov 29)...0% chance of subtropical cyclone formation (central Atlantic near 30N-55W)
AREA OF INTEREST #2...Computer models remain in agreement that the current central Atlantic upper trough will become a cut-off upper vortex in the northeast Atlantic in the vicinity of Madeira Island in 2 to 5 days thanks to the strength of the deep-layered ridge currently west of the upper trough. Due to the tremendous amount of upper divergence to form on the east side of this strong and rather cold upper vortex (expected to measure 1155 to 1160 dekameters in height at 200 mb)...the upper vortex will trigger a strong surface cyclone in the region. Even though water temps are currently running at 18 to 20 deg C...the rather cold temps of the upper vortex may aid in the generation of thunderstorms with the surface cyclone such that acquisition of tropical characteristics will have to be watched for. Given the model consensus of a strong surface cyclone which may also help in the generation of thunderstorms with surface convergence...I retain elevated peak odds of subtropical cyclone formation of 40%. Forecast positions in the outlook below are based on the divergence maximum of the current central Atlantic upper trough in the 0600Z GFS model run which transitions into the forecast upper vortex just after 48 hours...followed by forecasting the surface cyclone to whirl beneath the upper vortex by 72+ hours (and the 72+ hour upper vortex positions from the 0600Z GFS are used in my forecast). I begin to taper the odds of development down by 120 hours as the surface cyclone could begin to weaken beneath the center of the upper vortex where there is a lack of divergence as we see with typical post-mature mid-latitude storms. See home page bulletins of this site for expected impacts to the Canary Islands and Portugal in the coming days. In particular...the biggest concern is for Madeira Island where the center of the surface cyclone and upper vortex is currently forecast to stall over or nearby by 96 to 120 hours...which could result in a very long period of gusty potentially damaging winds starting Saturday.
*******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Nov 27)...0% chance of subtropical cyclone formation (northeast Atlantic near 46N-20W)
IOH 48 Hr Outlook (1800Z Nov 28)...20% chance of subtropical cyclone formation (northeast Atlantic near 41.5N-17.5W)
IOH 72 Hr Outlook (1800Z Nov 29)...40% chance of subtropical cyclone formation (north-northwest of Madeira Island near 35.5N-17W)
IOH 96 Hr Outlook (1800Z Nov 30)...40% chance of subtropical cyclone formation (just northeast of Madeira Island near 34.5N-15W)
IOH 120 Hr Outlook (1800Z Dec 1)...30% chance of subtropical cyclone formation (just northeast of Madeira Island near 34.5N-15W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
0000Z CMC Model Run... For area of interest #1...degenerates to elongated circulation along 59W longitude at 48 hours. For area of interest #2...surface cyclone forms near 43N-16.5W at 48 hours...center of cyclone passes over Madeira Island by 108 hours...whirls to a location just north of Madeira Island by 120 hours.
0000Z ECMWF Model Run...For area of interest #1...possible subtropical cyclone suggested near 26.5N-59.2W at 24 hours...degenerates into a surface trough near 28N-58W by 72 hours. For area of interest #2...surface cyclone forms west of Portugal near 41.5N-14W at 72 hours...center of cyclone passes over Madeira Island by 96 hours...whirls to a location west of Portugal by 120 hours.
0600Z GFS Model Run...For area of interest #1...possible subtropical cyclone suggested near 27.5N-59.2W at 18 hours...degenerates into a surface trough near 28N-58W by 66 hours. For area of interest #2...surface cyclone forms near 43N-18W at 48 hours...center of cyclone passes just northeast of Madeira Island by 96 hours and reaches location midway between Madeira Island and Portugal by 120 hours.
1200Z NAVGEM Model Run...For area of interest #1...surface low reaches 26N-59W by 36 hours...reaches 30.5N-54W at 72 hours while weakening to a surface trough. For area of interest #2...surface cyclone forms west of Portugal near 42.5N-14W at 48 hours...center of cyclone passes over Madeira Island by 90 hours and whirls to a location north of Madeira Island by 120 hours.