*******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.**********
...WEDNESDAY OCTOBER 27 2021 12:47 PM EDT...
See area of interest #1 section below for more details on the surface cyclone located just offshore of the United States northeast coast being monitored for acquisition of tropical characteristics. See area of interest #2 section below more information on the tail end of a frontal system currently over northeastern Mexico... which is expected to arrive in the Caribbean Sea in a few days and potentially develop into a tropical disturbance.
Elsewhere... satellite imagery suggests a tropical wave of low pressure has recently exited the west coast of Africa and entered the far eastern tropical Atlantic. In the short-term...conditions for this wave will be hostile for development due to westerly shear induced by the string of upper vorticity currently spanning the central and eastern tropical Atlantic. Longer range model runs have trended toward a more amplified upper ridge in the northwest Atlantic...which could cause the upper vortex associated with area of interest #1 to be more amplified while the area of interest moves into the open central Atlantic. The stronger eastern divergence zone of the more amplified upper vortex may cause the surface low pressure circulation of area of interest #1 to be stronger...resulting in more northward warm air transport in the eastern Atlantic on the east side of area of interest #1 which would result in amplfication of warm core upper ridging over this tropical wave. This upper ridging could reduce the shear over the tropical wave and allow it to develop in the eastern tropical Atlantic as today's 0600Z GFS model run hinted at. In lieu of the latest long range model solutions...will monitor this situation and declare another area of interest if needed.
AREA OF INTEREST #1...The surface frontal cyclone that has been parked offshore of the United States east coast is undergoing a cyclonic loop toward its supporting upper trough axis at a location offshore of Cape Cod Massachusetts... a typical event for a mid-latitude cyclone reaching its mature phase. The upper trough is amplifying further into a cold core upper vortex due to the cold air pulled southward by the frontal cyclone's west side and also due to adjacent amplification of upper ridging over eastern North America in the warm southerly flow ahead of the current central US frontal low. So far thunderstorm activity has reduced near the surface cyclone's center as the cyclone has decoupled from the eastern divergence zone of the upper trough while undergoing its typical whirl toward the upper trough axis. The surface cyclone ventured north of 40N latitude for a time overnight where water temperatures are in the mid-teens of deg C...which are too cold for thunderstorm generation even with the aid of the cold temperatures of the upper trough. Once the upper trough amplifies into a upper vortex and shifts east in the next 24 hours...the surface cyclone will likewise shift east. The surface cyclone has returned to warmer waters south of 40N latitude in its cyclonic loop...and as it shifts east will be near the north wall of warm Gulf stream waters in 24 to 36 hours. Combined with the cold temperatures of the upper vortex... there is still a window for the surface cyclone to develop instability and a core of thunderstorms needed for tropical characteristics. However I have lowered short-term subtropical cyclone development odds to 50% to acknowledge the decrease in thunderstorms that has taken place in the last 24 hours.
After 48 hours... the current north Atlantic upper trough will be in the northeast Atlantic and amplifying due to the amplified eastern North America upper ridge to shift east into the northwest Atlantic. The amplifying northeast Atlantic upper trough is likely to grab and stretch the overhead upper vortex of the surface cyclone at 72 hours... with the upper vortex and surface cyclone diving southeast into the open central Atlantic while pivoting around the southwest side of the upper trough. The stretched vortex may produce a belt of shearing winds over the surface cyclone. In addition... the upper vortex is likely to warm/weaken while remaining cut-off from high latitude cold air courtesy of the aforementioned warm core upper ridge to enter the northwest Atlantic. The potential for shear and loss of instability as the upper vortex warms suggest less favorable conditions for subtropical development... and I lower development odds to 10% by 96 hours accordingly. It should be noted that in the longer range... models have trended with a more amplified northwestern Atlantic upper ridge which results in the upper vortex separating from the northeastern Atlantic upper trough and potentially becoming re-amplified enough to reduce shear. In addition by 120 hours the southeast track of the surface cyclone takes it toward the warm 26 deg C sea surface temperature isotherm which would be enough to counteract the stabilization caused by the warming upper vortex. As a result I trim subtropical development odds back up to 20% for the 120-hour forecast point.
The surface cyclone has produced notable impacts to land including coastal sea swells along the mid-Atlantic US...northeast US...and Atlantic Canada coasts...as well as gusty winds across the mid-Atlantic and northeastern US coasts. Wind damage and power outages have been reported in eastern Massachusetts...Connecticut...and Rhode Island. Winds affecting the northeastern US and mid-Atlantic US coasts will subside over the next 24 hours as the surface cyclone shifts eastward away from land. Coastal sea swells will take a couple of days to subside. The following are National Weather Service station observations of wind (mph) generated by the surface cyclone over the last several hours:
**Bar Harbor ME...sustained 24...gust 33 (now)
**Portland ME...sustained 18...gust 30 (now)
**Portland ME...sustained 24...gust 38 (5:51 AM EDT)
**Portsmouth NH...sustained 15...gust 28 (now)
**Portsmouth NH...sustained 25...gust 40 (2:56 AM EDT)
**Boston MA...sustained 28...gust 36 (now)
**Boston MA...sustained 43...gust 56 (5:54 AM EDT)
**Barnstable MA (Cape Cod Region)...sustained 33...gust 61 (5:56 AM EDT...weather station not currently reporting)
**Newport RI...sustained 33...gust 52 (5:53 AM EDT...weather station not currently reporting)
**New London CT...sustained 28...gust 54 (7:56 AM EDT)
**New London CT...sustained 25...gust 39 (now)
**Bridgeport CT...sustained 25...gust 37 (now)
**Montauk NY (East End of Long Island)...sustained 21...gust 43 (now)
**New York NY...sustained 10...gust 23 (now)
**Atlantic City NJ...sustained 15...gust 28 (now)
**Ocean City MD...sustained 12...gust 25 (now)
**Virginia Beach VA...sustained 21...gust 29 (now)
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Oct 28)... 50% chance of subtropical cyclone formation (northwest Atlantic near 40N-62.5W)
IOH 48 Hr Outlook (1200Z Oct 29)... 50% chance of subtropical cyclone formation (north Atlantic near 40N-56W)
IOH 72 Hr Outlook (1200Z Oct 30)... 30% chance of subtropical cyclone formation (north Atlantic near 39.5N-47.5W)
IOH 96 Hr Outlook (1200Z Oct 31)... 10% chance of subtropical cyclone formation (central Atlantic near 34.5N-44W)
IOH 120 Hr Outlook (1200Z Nov 1)... 20% chance of subtropical cyclone formation (central Atlantic near 31N-42.5W)
AREA OF INTEREST #2...The current highly amplified cold core upper trough moving across western North America is expect to later cross eastern North America and drive a surface cold front as far south as the Caribbean Sea in a few days. The tail end of this front may evolve into a tropical disturbance supported by the outflow and low shear of the ongoing upper ridge in the Caribbean. Although computer model support showing the formation of a well-defined Caribbean disturbance has reduced... the forecast arrival of the front while upper winds remain favorable in the Caribbean has not changed...therefore I have declared a new area of interest for tropical development for this situation. However I assign low 10% odds of tropical cyclone development by day 5 given the latest model trends. Anything that does develop will likely drift west toward Nicaragua or Honduras during that timeframe as a steering surface ridge builds over North America in the wake of the amplified upper trough.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Oct 28)...0% chance of tropical cyclone formation (central Gulf of Mexico near 25N-90W)
IOH 48 Hr Outlook (1200Z Oct 29)...0% chance of tropical cyclone formation (northwestern Caribbean Sea near 19N-85W)
IOH 72 Hr Outlook (1200Z Oct 30)...0% chance of tropical cyclone formation (central Caribbean Sea near 15N-80W)
IOH 96 Hr Outlook (1200Z Oct 31)...5% chance of tropical cyclone formation (central Caribbean Sea near 15N-80W)
IOH 120 Hr Outlook (1200Z Nov 1)...10% chance of tropical cyclone formation (offshore of the Nicaragua/Honduras border near 15N-81W)
...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... frontal cyclone makes a cyclonic loop just offshore of the northeastern United States over the next 12 hours... while beginning to accelerate the cyclone develops a circular and possibly tropical core from 24 to 42 hours... accelerates southeast into the open central Atlantic and reaches 30.5N-44W at 120 hours while potentially gaining tropical characteristics for a second time
**For area of interest #2...no development shown
0000Z ECMWF Model Run...
**For area of interest #1... accelerates east and then southeast into the open central Atlantic while potentially acquiring tropical characteristics...reaches 33N-45W by 120 hours.
**For area of interest #2...tail end of cold front evolves into a broad southern Caribbean tropical low pressure near 12N-79W at 120 hours.
0600Z GFS Model Run...
**For area of interest #1... accelerates east and develops a circular and potentially tropical core near 40.2N-61W at 30 hours... dives southeast as a non-tropical and more elongated low pressure and reaches 32.2.N-41W at 120 hours.
**For area of interest #2...no development shown
**Tropical wave which has recently entered the eastern tropical Atlantic from western Africa develops into a compact tropical low near 10N-23W at 102 hours...drifts north and becomes a compact tropical cyclone near 12.5N-24W at 120 hours.
0600Z NAVGEM Model Run...
**For area of interest #1...accelerates east-southeast into the open central Atlantic while potentially acquiring tropical characteristics...reaches 36N-45W by 120 hours.
**For area of interest #2...Tail end of cold front evolves into a southern Caribbean tropical low pressure near 12.5N-80W at 126 hours.
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