*******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.**********
...SUNDAY NOVEMBER 22 2020 10:03 PM EDT...
See area of interest section below for an update on the surface trough of low pressure located northeast of the Bahamas.
AREA OF INTEREST #1...A surface trough of low pressure persists northeast of the Bahamas under the supportive upper divergence on the east side of an amplified upper trough in the region. The comma shaped cloud mass associated with this disturbance had a hook signature near 27.5N-70W as of 1800Z...and this is probably the location of the upper divergence maximum and lowest surface pressure along the surface trough. Using this location as the initial point of this disturbance...the short-term track forecast in the outlook below is nudged northward as this current position is north of the previous outlook. In the next 24 hours...the supporting upper trough of this disturbance will de-amplify as it merges with fast-moving and incoming upper trough from central Canada...resulting in an increase in wind shear. Given the lack of thunderstorms within the cloud mass of this disturbance as seen in colorized infrared satellite imagery...I forecast a 0% chance of subtropical cyclone formation in the short term due to the expected increase in shear and as the disturbance loses tropical characteristics while merging with the cold front to be driven by the incoming large scale upper trough. The divergence zone of the incoming upper trough could strengthen this system into a defined non-tropical frontal low after it merges with the front by 48 hours...therefore even as this disturbance loses tropical characteristics heavy rains and gusty winds will be possible across Bermuda during that time.
The south end of the cold front containing the remnant frontal low of this disturbance will likely evolve into a subtropical disturbance and located in the central Atlantic by 72+ hours as the base of the front’s upper trough becomes cut-off into a vortex in response to amplifying upstream deep-layered ridge to the west. About half of the models forecast the remnant non-tropical low pressure of this disturbance to transition into a subtropical low pressure supported by the eastern divergence zone of the forecast upper vortex...and the other half forecast the remnant non-tropical low to lose its identity along the cold front with a separate subtropical surface low pressure forming. However with half of the models onboard with revitalizing this disturbance as a subtropical entity in the central Atlantic...I have joined the NHC outlook strategy in assuming that the possible central Atlantic subtropical disturbance in 72+ hours will be a continuity of this disturbance. Regardless of selecting a model run which dissipates this disturbance along the cold front in the short-term or not...there is agreement (with the exception of the NAVGEM) that the subtropical surface low will track southwest under the influence of the deep-layered ridge to the west at a location over or just north of warm 26 deg C waters. The 1200Z GFS also forecasts the upper vortex to be at 1205 dekameters in height at 200 mb at 96 hours...an indication of somewhat cold upper air temps that may help in adding instability needed for thunderstorms. Therefore I forecast 30% odds of subtropical cyclone formation by 96+ hours...slightly higher than the NHC tropical weather outlook. I have not gone with higher odds as the models show a weak subtropical surface low/trough in the long range instead of explicitly indicating subtropical cyclone formation.
At 120 hours I switch the designation in the outlook below from subtropical to tropical as the the upper vortex is forecast to weaken to a southwest-to-northeast upper trough axis with warm core anticyclonic outflow being a possibility north of the upper trough axis and over this disturbance. The southwest track is forecast to slow down due to a blocking surface ridge to the west to be supported by an approaching shortwave upper trough.
IOH 24 Hr Outlook (1800Z Nov 23)...0% chance of subtropical cyclone formation (western Atlantic near 30N-69W
IOH 48 Hr Outlook (1800Z Nov 24)...0% chance of subtropical cyclone formation (east of Bermuda near 32.5N-61W)
IOH 72 Hr Outlook (1800Z Nov 25)...0% chance of subtropical cyclone formation (central Atlantic near 30N-55W)
IOH 96 Hr Outlook (1800Z Nov 26)...30% chance of subtropical cyclone formation (central Atlantic near 28N-57W)
IOH 120 Hr Outlook (1800Z Nov 27)...30% chance of tropical cyclone formation (central Atlantic near 28N-59W)
...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...surface trough evolves into a surface low near 26.5N-71W at 24 hours...passes just southeast of Bermuda as a non-tropical frontal low by 48 hours...separates from front and reaches 28N-59W by 120 hours.
1200Z ECMWF Model Run... For area of interest #1...surface trough evolves into a surface low near 27.5N-70W at 24 hours...passes just east of Bermuda as a strengthening non-tropical frontal low by 48 hours...separates from front and reaches 28N-58W by 120 hours.
1200Z GFS Model Run...For area of interest #1...surface trough evolves into a surface low near 26N-70.5W at 24 hours...passes just southeast of Bermuda as a non-tropical frontal low by 42 hours...loses it’s identity along front by 66 hours. Elsewhere...south end of front that absorbed area of interest #1 evolves into a surface low near 29.5N-56W by 84 hours...surface low weakens to a surface trough by 102 hours near 27.5N-59W...trough reaches 26N-60.5W by 120 hours.
1200Z NAVGEM Model Run...For area of interest #1...surface trough absorbed by cold front by 30 hours. Elsewhere...south end of front that absorbed area of interest #1 evolves into a surface low near 32.5N-52.5W by 84 hours...reaches 31.5N-55W by 120 hours.