MY 2020 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #186
*******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.**********
...SATURDAY NOVEMBER 21 2020 9:12 PM EDT...
See area of interest section below for an update on the surface trough of low pressure left behind by a cold front at a location northeast of the Bahamas.
AREA OF INTEREST #1...The tail end of the cold front that has recently moved into the eastern Atlantic from the central Atlantic has left behind a surface trough of low pressure located just northeast of the eastern Bahamas. Showers and thunderstorms have increased in association with the surface trough in an environment of reducing shear and increasing upper divergence induced by the east side of cold core upper vorticity left behind by the aforementioned front’s upper trough...which has recently become an amplified upper trough. The formation of the amplified cold core upper trough is in response to adjacent warm upper ridging to the north and west associated with warm air ahead of another cold front pivoting southeast across central Canada. The forecast positions in the updated outlook below are shifted west as the location of the upper divergence maximum in the region is further west of previous forecasts. After 24 hours...the amplified upper trough in the region will de-amplify as it merges with incoming upper trough tied to the front from central Canada...resulting in an increase in wind shear. Thus I drop odds of development to 0% by 48 hours. I forecast a very low 5% chance of tropical cyclone formation by 24 hours as there is only a narrow window of low wind shear and the surface trough is currently not well organized for tropical development. Although models have converged on the surface trough evolving into a surface low by 48 hours...this will likely be too late for tropical development as wind shear will be increasing...and not long after the surface low will be merging with the cold front arriving from central Canada once the front moves into the Atlantic which would transition this system to non-tropical. The divergence zone of the front’s upper trough will likely strengthen the surface low after it merges with the front by 72 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 could evolve into another subtropical disturbance and located in the central Atlantic by 120 hours as the base of the front’s upper trough becomes cut-off into a vortex near 25N-52.5W in response to amplifying upstream upper ridging to the west. I have not extended the current outlook below to 120 hours as it is difficult to know if the remnant frontal low of this disturbance will become entangled with the forecast central Atlantic upper vortex or if the remnant frontal low will zip northward along the front and a new separate disturbance will form with the support of the upper vortex. Meanwhile the current NHC outlook is currently assuming that the possible central Atlantic subtropical disturbance by 120 hours (day 5) will be a continuity of this disturbance.
IOH 24 Hr Outlook (1800Z Nov 22)...5% chance of tropical cyclone formation (western Atlantic near 26N-70W)
IOH 48 Hr Outlook (1800Z Nov 23)...0% chance of tropical cyclone formation (western Atlantic near 29N-70W)
...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 an elongated closed circulation near 26N-72W at 48 hours... quickly transitions to a strengthening non-tropical low along an incoming front to the northwest by 66 hours while passing over Bermuda.
1200Z ECMWF Model Run...For area of interest #1...surface trough evolves into a surface low near 28N-69W at 48 hours...quickly transitions to a strengthening non-tropical low along an incoming front to the northwest by 72 hours while passing just southeast of Bermuda.
1800Z GFS Model Run...For area of interest #1...surface trough evolves into a surface low near 28N-68.5W at 48 hours...quickly transitions to a non-tropical low along an incoming front to the northwest by 60 hours while passing over Bermuda.
1200Z NAVGEM Model Run...For area of interest #1...surface trough evolves into a surface low near 28N-69W at 54 hours...quickly transitions to a strengthening non-tropical low along an incoming front to the northwest by 66 hours while passing over Bermuda.