*******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 12 2021 1:42 PM EDT...
See area of interest #1 section below for an update on the potential for subtropical development to the south of the Azores in the days ahead. It is quiet elsewhere in the Atlantic tropics.
AREA OF INTEREST #1...The current upper trough spanning the northwestern and central Atlantic is continuing to amplify in response to a developing and amplifying warm deep-layer ridge to the west (this deep-layer ridge is supported by the warm sector of the current central US frontal cyclone). The amount of upper divergence beneath the east side of the amplifying upper trough has increased...which has allowed for a surface frontal low pressure in the north Atlantic to explosively intensify into a cyclone. This north Atlantic cyclone has driven a surface cold front that is making its way into the Azores as of this writing. The upper trough is expected to continue to amplify...with the developing deep-layered ridge to the west expected to cut-off the south part of the upper trough into a cold core upper vortex over or near the Azores at 48+ hours. The eastern divergence zone of the upper vortex is then expected to generate a surface low pressure along the current cold front running through the Azores.
In the 48 to 72 hour window... the surface frontal low and upper vortex may become a more tropical feature of interest as they shift south away from the Azores and toward milder water temps in the mid-20s of deg C while pushed by the deep-layered ridge. The upper vortex is expected to be sufficiently cold (200 mb heights below 1200 dekameters) for instability at these water temps...which could result in thunderstorm activity and potential acquisition of tropical characteristics. After 72 hours...the deep-layered ridge is forecast to have its upper layer to the northwest... and surface layer (supported by the eastern convergence zone of the upper layer) passing to the north. This will result in the surface layer pushing the surface low on a more westward angle in track while the upper layer keeps the upper vortex more toward the east... and the surface low is expected to migrate beneath the west side of the upper vortex as a result. Conditions beneath the west side of the upper vortex will tend to be hostile for subtropical development with northerly wind shear and upper convergence. By 120 hours the current upper trough just offshore of western Canada will have made its way to the north Atlantic and generate its own suface frontal cyclone...with this cyclone creating a weakness in the surface layer of the ridge and slowing down the southwest track of the surface low. However this slow down will be too late..with the surface low remaining beneath the west side of the upper vortex. As a result I trim down subtropical development odds to 0% by 120 hours from prolonged exposure to upper convergence on the west side of the upper vortex which will have weakened the surface low by that timeframe...and all models are in agreement with this idea. However I have raised peak subtropical development odds to 40% for the 72 hour timeframe as models have maintained forecasting a sufficiently cold upper vortex being stacked over the surface frontal low...which is a classic recipe for possible subtropical development. The 40% odds are on the medium-side...a compromise between the favorable thermodynamic profile caused by the upper vortex and the short window of time this feature will have for subtropical development given that it will migrate beneath the hostile west side of the upper vortex soon after 72 hours.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Nov 13)... 0% chance of subtropical cyclone formation (over the Azores near 38.5N-30W)
IOH 48 Hr Outlook (1200Z Nov 14)... 0% chance of subtropical cyclone formation (over the eastern Azores near 38N-26W)
IOH 72 Hr Outlook (1200Z Nov 15)... 40% chance of subtropical cyclone formation (eastern Atlantic near 33N-30W)
IOH 96 Hr Outlook (1200Z Nov 16)... 20% chance of subtropical cyclone formation (eastern Atlantic near 30N-35W)
IOH 120 Hr Outlook (1200Z Nov 17)... 0% chance of subtropical cyclone formation (eastern Atlantic near 27.5N-37.5W)
...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... south part of current northwest Atlantic upper trough becomes cut-off by deep-layered ridge to the west as a vortex... surface frontal low forms with support from the divergence zone of the vortex at a location just southeast of the Azores at 54 hours...frontal low shifts southwest and becomes a subtropical storm near 30.5N-31.5W at 90 hours...continues southwest and weakens to a remnant low near 26.5N-34W at 120 hours
0000Z ECMWF Model Run...
**For Area of Interest #1... south part of current northwest Atlantic upper trough becomes cut-off by deep-layered ridge to the west as a vortex... surface frontal low forms with support from the divergence zone of the vortex at a location over the eastern Azores at 48 hours...frontal low shifts southwest and becomes a subtropical storm near 32N-33W at 96 hours...continues southwest and weakens to a remnant trough near 26N-34.5W at 120 hours
1200Z GFS Model Run...
**For Area of Interest #1... south part of current northwest Atlantic upper trough becomes cut-off by deep-layered ridge to the west as a vortex... surface frontal low forms with support from the divergence zone of the vortex at a location just southeast of the Azores at 42 hours... frontal low shifts southwest and potentially acquires tropical characteristics near 32.5N-33W at 81 hours...continues southwest and weakens to a remnant low near 25N-35W at 120 hours.
0600Z NAVGEM Model Run...
**For Area of Interest #1...south part of current northwest Atlantic upper trough becomes cut-off by deep-layered ridge to the west as a vortex... surface frontal low forms with support from the divergence zone of the vortex at a location just east of the Azores at 54 hours...frontal low shifts south-southwest and reaches peak strength near 31.5N-29W at 90 hours....continues south-southwest while weakening and reaches 26N-31W at 120 hours.