*******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.**********
...FRIDAY NOVEMBER 27 2020 7:29 PM EDT...
See area of interest sections below for areas currently being monitored for tropical development in the Atlantic basin. Elsewhere...the current upper trough over eastern Canada will enter the northwest Atlantic in a couple of days and re-enforce a northwest Atlantic frontal low such that the frontal low aborts area of interest #1. After that time...a warm deep-layered ridge to form in the warm sector of a forecast strong eastern US frontal cyclone will cut-off the south part of this upper trough into a central Atlantic upper vortex...which could produce yet another subtropical disturbance if the upper vortex gets pushed far south enough by the deep-layered ridge and into warm 26 deg C water in the long range.
And finally the upper-level anticyclonic center of the deep-layered ridge to form in the warm sector of the forecast eastern US frontal cyclone is expected to settle over the southern Caribbean Sea where it could enhance the upper outflow and thunderstorms in the region. The southwesterly flow ahead of the frontal cyclone’s upper trough may also enhance the poleward upper outflow in the region as well. The 1200Z GFS model run from today suggests possible tropical cyclone formation in the southern Caribbean in about 5 days. However the GFS has a recent history of incorrectly forecasting tropical development in this region nearly two weeks ago...therefore not adding a new area of interest in the southern Caribbean at the present time.
AREA OF INTEREST #1...The meandering western to central Atlantic surface low pressure that we have been monitoring for several days for tropical development has detached from the tail end of the current central Atlantic cold front while transitioning into a subtropical low pressure supported by an upper vortex in the region. Overnight the surface low has moved more southwestward than forecast under the combined influences of the deep-layer ridge now in the north-central Atlantic and another surface ridge to the west over Florida and the southeast US supported by the western convergence zone of the current northwest Atlantic shortwave upper trough. Meanwhile the supporting upper vortex of the subtropical surface low has been stationary while weakening to a northwest-to-southeast tilted upper trough...keeping the upper divergence and thunderstorm bands at a location further north such that the bands of activity are now separated from the southwest-drifting cloud swirl surface low which has reached 25N-59.5W...and subtropical cyclone formation has not occurred as a result. Surface pressure have and will continue to fall further north where the bands of showers and thunderstorms are present...especially as the upper divergence zone of the northwest Atlantic shortwave upper trough overspreads that area in the next 24 hours. This will result in a northern lobe of low surface pressure to compete with the southern center currently at 25N-59.5W...therefore the chances of a consolidated circulation acquiring tropical characteristics has reduced and I have dropped odds of subtropical cyclone formation to a very low 10%. I have not yet dropped odds to 0% in the event the southern center reforms to the north closer to the thunderstorms and region of upper divergence while remaining consolidated. Forecast track in the outlook below assumes the southern center turns north-northeastward toward the developing northern lobe of low surface pressure and also toward a frontal low to be supported by the northwest Atlantic shortwave upper trough. By 48 hours on this track...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...and this is when I drop odds of development to 0%. Although the eastern divergence zone of a second shortwave upper trough to enter the northwest Atlantic will also be present during this time...the divergence zone of this second upper trough to be located well to the north will favor the frontal low over this system.
IOH 24 Hr Outlook (1800Z Nov 28)...10% chance of subtropical cyclone formation (central Atlantic near 27.5N-57.5W)
IOH 48 Hr Outlook (1800Z Nov 29)...0% chance of subtropical cyclone formation (central Atlantic near 30N-55W)
AREA OF INTEREST #2...The current north-central Atlantic upper trough as of 1200Z has produced a frontal low pressure near 50N-20W with its eastern divergence zone. This trough is expected to become a cut-off upper vortex in the northeast Atlantic in the vicinity of Madeira Island over the next few 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 1160 dekameters in height at 200 mb)...the upper vortex will trigger a strong surface cyclone in the region. Some models suggest the frontal low currently at 50N-20W will intensify into the surface cyclone...while other models show the surface cyclone forming further south over the next 24 hours and absorbing the frontal low. Either way the end result will be the same...with a surface cyclone to whirl southward beneath the forecast upper vortex near Madeira Island. Even though water temps in the region 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 by 48+ hours are based on the position of the upper vortex in the 1200Z GFS model run. I begin to taper the odds of development down by 96+ 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 nearly stall over or nearby by 72 to 120 hours...which could result in a very long period of gusty potentially damaging winds starting late tomorrow. Albeit by 120 hours the deep-layer ridge which will be responsible for keeping this system cut-off from the mid-latitude westerlies will have moved into Europe by then...with the current upper trough over eastern Canada making an approach and potentially dragging this system east by 120+ hours...perhaps offering some hope to Madeira Island by then.
IOH 24 Hr Outlook (1800Z Nov 28)...0% chance of subtropical cyclone formation (northeast Atlantic near 44N-18W)
IOH 48 Hr Outlook (1800Z Nov 29)...40% chance of subtropical cyclone formation (north-northwest of Madeira Island near 35.5N-17.5W)
IOH 72 Hr Outlook (1800Z Nov 30)...40% chance of subtropical cyclone formation (just northwest of Madeira Island near 34.5N-17.5W)
IOH 96 Hr Outlook (1800Z Dec 1)...30% chance of subtropical cyclone formation (just west of Madeira Island near 33N-17.5W)
IOH 120 Hr Outlook (1800Z Dec 2)...10% chance of subtropical cyclone formation (Madeira Island near 33N-16.5W)
...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 30 hours. For area of interest #2...surface cyclone forms near 45N-14.5W at 36 hours...center of cyclone passes over Madeira Island by 78 hours...whirls to a location north of Madeira Island by 108 hours...located northwest of Madeira Island at 120 hours.
0000Z ECMWF Model Run....For area of interest #1...degenerates to elongated circulation along 59W longitude at 24 hours. For area of interest #2...surface cyclone forms near 40.5N-20W at 48 hours...center of cyclone stalls just northwest of Madeira Island between 72 and 96 hours...drifts west to 35N-20W by 120 hours.
1200Z GFS Model Run...For area of interest #1...degenerates into a surface trough near 29N-56W by 30 hours. For area of interest #2...surface cyclone forms near 46N-17.5W at 18 hours...center of cyclone stalls over Madeira Island from 60 to 120 hours. Elsewhere...tropical low forms in the southern Caribbean Sea near 11N-78W at 102 hours...compact tropical cyclone formation suggested just after 120 hours.
0600Z NAVGEM Model Run...For area of interest #1...reaches 28N-57W at 30 hours while weakening to a surface trough. For area of interest #2...surface cyclone forms near 43.5N-15.5W at 24 hours...center of cyclone passes over Madeira Island by 78 hours and remains nearly stationary through 120 hours.