MY 2020 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #188
*******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.**********
...MONDAY NOVEMBER 23 2020 10:20 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 upper trough in the region. A closed surface low has formed along the trough near 27.5N-69W which is confirmed by ASCAT-B and ASCAT-C descending passes of surface winds with a maximum wind strength around minimal tropical depression force. My forecast track in the updated outlook below is adjusted south of the previous outlook due to the current position of the surface low. The supporting upper trough has recently de-amplified and merged with a larger upper trough exiting North America...which has increased the wind shear over the new surface low. Thunderstorms have recently intensified with the surface low...but are not organizing into bands around the surface low and are biased to the east of the surface low due to the increasing shear. This...combined with the fact that the surface low will soon lose its tropical status while merging with the cold front to the northwest tied to the incoming upper trough...means that the surface low will likely not be upgraded to a tropical depression in the short term.
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 48+ hours as the base of the front’s upper trough becomes cut-off into a vortex in response to an amplifying upstream deep-layered ridge to the west. Models (with the exception of the CMC) now agree that the remnant non-tropical low pressure of this disturbance will transition into a subtropical low pressure supported by the eastern divergence zone of the forecast upper vortex. Models have also increased the intensity of the forecast subtropical low pressure. However I have not yet raised odds of subtropical cyclone formation above 30% as the CMC...ECMWF...and NAVGEM do not agree with the more south forecast track shown in the outlook below...thus there is no guarantee at this time the subtropical low will track over or near the 26 deg C warm water near 27.5N latitude. The subtropical low is forecast to reverse to a southwest track under the influence of the deep-layered ridge to the west. By 96+ hours...the upper vortex is still forecast to weaken to a southwest-to-northeast upper trough axis...but the axis is now forecast to be over instead of south of the subtropical low such that the potential for full tropical characteristics in the long range has reduced. I have also nudged odds of development downward beginning at 120 hours as the subtropical low has potential to weaken under the lack of divergence directly beneath the forecast upper trough axis. Also during this time...the southwest track is forecast to slow down to a stall due to a blocking surface ridge to the west to be supported by an approaching shortwave upper trough crossing the northwest Atlantic.
IOH 24 Hr Outlook (0000Z Nov 25)...0% chance of subtropical cyclone formation (western Atlantic near 30.5N-60W)
IOH 48 Hr Outlook (0000Z Nov 26)...0% chance of subtropical cyclone formation (central Atlantic near 29.5N-54.5W)
IOH 72 Hr Outlook (0000Z Nov 27)...30% chance of subtropical cyclone formation (central Atlantic near 27.5N-54.5W)
IOH 96 Hr Outlook (0000Z Nov 28)...30% chance of subtropical cyclone formation (central Atlantic near 27.5N-56W)
IOH 120 Hr Outlook (0000Z Nov 29)...20% chance of subtropical cyclone formation (central Atlantic near 27.5N-56W)
...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 low passes southeast of Bermuda as a non-tropical frontal low by 24 hours...loses it’s identity along front by 54 hours. Elsewhere...south end of front that absorbed area of interest #1 evolves into a surface low near 31N-52.5W by 72 hours...reaches 29.8N-54W by 120 hours.
1200Z ECMWF Model Run...For area of interest #1...passes southeast of Bermuda as a strengthening non-tropical frontal low by 48 hours...separates from front by 72 hours and reaches 29N-56W by 120 hours.
1800Z GFS Model Run...For area of interest #1...passes southeast of Bermuda as a non-tropical frontal low by 18 hours...separates from front by 66 hours and reaches 26.5N-56.5W by 120 hours.
1200Z NAVGEM Model Run...For area of interest #1...passes southeast of Bermuda as a non-tropical frontal low by 24 hours...separates from front by 60 hours...weakens to a surface trough near 33N-52.5W by 84 hours.