MY 2023 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #81

*******Note that forecast 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 SEPTEMBER 20 2023 6:25 PM EDT...

See Hurricane Nigel section below for an update on the latest hurricane in the open central Atlantic. See area of interest #36 and #38 sections below for a pair of tropical waves of low pressure being monitored for signs of development as they both move toward the central tropical Atlantic in the days ahead. See area of interest #37 section below for information on a subtropical disturbance with development potential expected to develop near the southeastern United States coast by late this week and into the weekend... impacts from this system could also spread as far north as Delaware.

HURRICANE NIGEL... Nigel is undergoing its long-anticipated northeastward acceleration in the deep-layer flow ahead of the current frontal low moving across Atlantic Canada and its supporting upper trough currently over eastern North America. The previous forecast track performed well and so my updated one below is essentially unchanged. Regarding intensity... Nigel is close to my previous forecast while having weakened recently from its earlier peak of 100 mph maximum sustained winds. I initially anticipated the weakening would be from the beginnings of southwesterly shear to be imparted by the approaching upper trough. However on colorized infrared satellite imagery the strongest thunderstorms are weighted to the west side of the eye instead of toward the east or north as traditionally seen with a southweserly shear vector... therefore the recent weakening is more likely due to a bout of dry air ingestion. Looking forward... southwesterly shear notably increases by 24 hours as the approaching upper trough gets closer... and my current intensity forecast point for this timeframe is the same as previous as I do not expect Nigel to go below category 1 hurricane status for two reasons... (1) the rapid forecast northeast motion of Nigel is parallel to and hence mitigates the effect of the southwesterly shear vector... (2) the upper divergence zone on the east side of the trough will be aiding Nigel. By 48 hours... transition into a remnant frontal cyclone supported by the upper trough's divergence zone should be completed with Nigel over cooler waters.

Nigel and the intensifying frontal low that will soon move into the north Atlantic from Atlantic Canada will be moving northeastward in tandem and generating a large area of surf. Expect surf to reach the shores of Newfoundland by tonight and tomorrow... with surf reaching the shores of southeast Greenland... Iceland... the Azores... and the British Isles by this weekend. Gusty winds will be likely across the British Isles and Iceland during the weekend and into Monday as well.

******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********

0 Hr Position (1200Z Sep 20)... 90 mph maximum sustained wind hurricane centered in the central Atlantic at 36.1N-54.4W

IOH 24 Hr Forecast (1200Z Sep 21)... 75 mph maximum sustained wind hurricane centered in the north-central Atlantic at 41.5N-44W

IOH 48 Hr Forecast (1200Z Sep 22)... Frontal cyclone centered in the northeastern Atlantic at 47.5N-34W

******National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT***************************

Loss of Tropical Cyclone Status (1200Z Sep 22)... 65 mph maximum sustained wind frontal cyclone centered in the northeast Atlantic at 47.6N-31N

5-Day Position (1200Z Sep 25)... 45 mph maximum sustained wind frontal cyclone centered south-southwest of Iceland at 58N-23W

AREA OF INTEREST #36... The tropical wave of low pressure that was departing the west coast of Africa last evening has become a little better organized with the cloud pattern suggesting a center of rotation near 11.5N-19W as of 1200Z earlier this morning. Despite this... I cap my peak development odds by day 5 at the 50% mark as various global model runs either do not develop this wave... or develop it slowly in the 5 day window. The models may be picking up on the fact that during the 5-day forecast period that vorticity from the western of the two current eastern Altantic upper troughs will be pushed southward toward this tropical wave by the current central Atlantic upper ridge cell... with this upper vorticity perhaps suppressing the outflow of this wave as it nears. In addition the tropical wave appears to be ingesting a swath of dry Saharan air to the northwest and may lose thunderstorms and oragnization within the next 24 hours. By days 4 and 5 the tropical wave enters the central tropical Atlantic away from the potentially suppressing upper vorticity and where dry Saharan air concentrations tend to be lower... and this is when I reserve the peak 50% odds of development in the 5-day forecast window. Note that although tropical cyclone formation is unlikely by the time the wave passes the Republic of Cabo Verde Islands in the next 24 hours... its north side may produce periods of heavy rainfall and gusty winds for the islands.

******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (1200Z Sep 21)... 5% chance of tropical cyclone formation (south of the Republic of Cabo Verde Islands near 12.2N-24W)

IOH 48 Hr Outlook (1200Z Sep 22)... 15% chance of tropical cyclone formation (eastern tropical Atlantic near 12.8N-29W)

IOH 72 Hr Outlook (1200Z Sep 23)... 35% chance of tropical cyclone formation (eastern tropical Atlantic near 13.2N-34W)

IOH 96 Hr Outlook (1200Z Sep 24)... 50% chance of tropical cyclone formation (eastern tropical Atlantic near 13.8N-39W)

IOH 120 Hr Outlook (1200Z Sep 25)... 50% chance of tropical cyclone formation (central tropical Atlantic near 14.2N-44W)

******National Hurricane Center (hurricanes.gov) official outlook as of 2 PM EDT***************************

Formation chance through 48 hours... 10%

Formation chance through 7 days... 70%

AREA OF INTEREST #37... The southern part of the current eastern North America upper trough remains pinned in place in the vicinity of the Florida peninsula while trapped between the current central Atlantic and Mexico upper ridge cells. In addition some upper vorticity has recently rolled into the south-central US from the southwestern US while skirting across the north side of the Mexico upper ridge cell. This upper vorticity is getting ready to take a southeast dive into the stuck upper trough over the Florida peninsula over the next 48 hours... resulting in either a more amplified upper trough or vortex over the Florida peninsula. The increased amplification of the upper trough... or formation of a vortex... will result in increased upper air divergence and low shear over the warm waters offshore of the southeastern US coast and to the north of the northwestern Bahamas such that a subtropical surface low pressure disturbance with development potential is expected to form over the next 48 hours.

By 48 hours... the northern half of the current eastern North America upper trough will have shifted into the northwestern Atlantic... with the western convergence zone of this upper trough creating a strong surface ridge over the northeastern US. Therefore the subtropical disturbance is expected to swing north into the Carolinas in the flow between the east side of the Florida upper trough/vortex and southwest side of the northeastern US surface ridge. My forecast track in the updated outlook below is shifted east toward eastern North Carolina as the models have trended with a more east position for the materializing upper trough/vortex. By 72 hours... as is typically seen by a surface system supported by an upper trough/vortex... the parent upper vortex and surface low cyclonically whirl toward each other and become vertically stacked... and a northward track from eastern North Carolina to the northeastern US coastal region is anticipated while the vertically stacked system becomes captured by an amplified upper trough to move across Canada (this upper trough will materialize from upper vorticity currently just east of Alaska and over northwest Canada). There is some uncertainty in the decay rate of this system after 72 hours as the parent upper vortex merges with the passing upper trough... for example if the surface low pressure center is initially aligned with the supportive eastern divergence zone of the resulting merged upper trough it will take longer to weaken but will eventually do so as it whirls toward the axis of the merged upper trough where upper divergence is less. However if the surface low pressure center becomes initially aligned with the axis of the upper trough... it will decay faster. A slower decay rate will mean gusty winds and coastal surf will reach further up the northeastern US coastline. Either way... this system will lose tropical characteristics after 72 hours while transitioning into a non-tropical frontal low associated with the merged upper trough. The transition to non-tropical will also be aided as this system moves away from the heat source of warm Gulf stream waters and into cooler waters and/or the northeastern United States landmass.

Regarding odds of subtropical cyclone formation... two scenarios with this subtropical disturbance are possible... (1) the supporting upper trough/vortex has a consolidated upper divergence zone to allow for a consolidated surface center and subtropical cyclone status... (2) the Florida upper trough/vortex has a more elongated upper divergence zone that produces an elongated surface subtropical low or trough. With global models still trending toward the first scenario... I keep my peak odds of subtropical cyclone formation just above the 50% mark. Interests along the United States east coast from northeastern Florida to Delaware should be aware of this disturbance for the follwoing impacts to begin later this week and through the weekend as follows:

(1) A strong surface pressure gradient is expected to setup between the north side of the subtropical disturbance and south side of the northeastern US surface ridge... resulting in winds blowing toward shore and hence an increase in strong coastal surf for northeastern Florida and Georgia.

(2) A similar setup for strong coastal surf is expected for the coastal Carolinas. With the latest forecast track shift... the threat of gusty winds with damage potential has decreased for eastern and central South Carolina but remains for coastal and inland parts of northeastern South Carolina and the eastern half of North Carolina.

(3) As discussed above... the decay rate of this system after 72 hours is uncertain. A slower decay rate will mean gusty winds with some damage potential along with coastal surf for southeastern Virginia... eastern Maryland... and Delaware.

(4) These impacts are expected regardless of whether or not the subtropical disturbance has a consolidated enough center to be classified as a subtropical cyclone.

******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (1200Z Sep 21)... 0% chance of subtropical cyclone formation (offshore of northeastern Florida near 30N-79.5W)

IOH 48 Hr Outlook (1200Z Sep 22)... 50% chance of subtropical cyclone formation (offshore of the southeastern United States near 31N-77.5W

IOH 72 Hr Outlook (1200Z Sep 23)... 60% chance of subtropical cyclone formation (just east of Cape Fear North Carolina near 34N-77.5W)

IOH 96 Hr Outlook (1200Z Sep 24)... 0% chance of subtropical cyclone formation (New Jersey near 40N-74.5W)

******National Hurricane Center (hurricanes.gov) official outlook as of 2 PM EDT***************************

Formation chance through 48 hours... 10%

Formation chance through 7 days... 40%

AREA OF INTEREST #38... Over the last several hours... poleward (northward) outflow in the vicnity of 10N-33W was enhanced by the current central Atlantic upper vorticity that is retrograding westward. Based on the appearance of organized curved thunderstorm bands in the region... this appears to have led to the formation of a tropical low pressure rotation. In addition a tropical wave to the west of Area of Interest (AOI) #36 has moved westward from 25W longitude and has merged with this new disturbance... and this system is being analyzed as a tropical wave of low pressure in the NHC TAFB surface analysis charts. Because the upper vorticity is expected to retrograde northwestward and out of the way while revolving around the southwest side of the current central Atlantic upper ridge cell... the vorticity is not expected to shear this disturbance and hence I assess it has development potential. Therefore in this update cycle I am initiating this new disturbance as the thirty-eighth Atlantic tropical area of interest tracked on this site this year. For track... I slightly increase the north angle of the westward track through 72 hours due to the surface ridge weakness of the subtropical disturbance to be located near the southeast United States coast... tagged as area of interest (AOI) #37 in this update. Once the subtropical disturbance lifts northward... the Atlantic surface ridge recovers and the forecast track is bent more westward and less northward by 96 and 120 hours. Regarding odds of tropical cyclone formation... I have already set them as high as 30% by day 5 despite global models essentially not developing this system due to the organization of this system's curved thunderstorm bands.

The global models may not be developing this system while suggesting an interaction between AOI #36 and this system... and two closely spaced tropical systems tend to fight each other while competing for surface inflow and upper outflow... or alternatively merge into a broad system that takes time to consolidate and develop. For instance the 0000Z CMC model run attempts to develop this system in the short-term... then shuts down development as it merges with AOI #36 and becomes too broad. The 0600Z GFS model run has AOI #36 accelerate westward while pulled in by this disturbance... and then takes a while to develop AOI #36 after it becomes broadened after absorbing this disturbance. For now I assume AOI #36 and this system will be seperate entities... however noting the forecast track below is of low confidence as any interaction between the two systems will result in changes in the forecast track.

I recommend that interests in the Lesser Antilles should be aware of this disturbance as it could be making an approach just after day 5 while conditions remain favorable for its development through that time.

******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (1200Z Sep 21)... 5% chance of tropical cyclone formation (eastern tropical Atlantic near 10.5N-38W)

IOH 48 Hr Outlook (1200Z Sep 22)... 10% chance of tropical cyclone formation (central tropical Atlantic near 11.5N-42.5W)

IOH 72 Hr Outlook (1200Z Sep 23)... 20% chance of tropical cyclone formation (central tropical Atlantic near 13N-47.5W)

IOH 96 Hr Outlook (1200Z Sep 24)... 30% chance of tropical cyclone formation (central tropical Atlantic near 13.5N-52.5W)

IOH 120 Hr Outlook (1200Z Sep 25)... 30% chance of tropical cyclone formation (east of the Lesser Antilles near 14N-57.5W)

******National Hurricane Center (hurricanes.gov) official outlook as of 2 PM EDT***************************

Not in the official outlook

...COMPUTER MODEL SUMMARY...

Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields(http://moe.met.fsu.edu/tcgengifs/).

0000Z (Sep 20) CMC Model Run...

**For Hurricane Nigel... recurves northeastward in tandem with frontal cyclone ejecting from Atlantic Canada and reaches the northeast Atlantic near 50.5N-25W at 72 hours as a remnant frontal cyclone... the two cyclones orbit each other with ex-Nigel becoming the dominant and centered near 57.5N-20.5W at 120 hours.

**For Area of Interest #36... no development shown

**For Area of Interest #37... surface low forms offshore of northeastern Florida and near 29.5N-79.5W at 54 hours with subtropical cyclone formation suggested near 31N-79W at 72 hours... subtropical cylcone makes landfall just east of the North Carolina/South Carolina border just after 84 hours with the center of the weakening inland remnant low reaching central Pennsylvania by 120 hours.

**For Area of Interest #38... compact tropical depression forms near 9.8N-36W at 54 hours... the tropical depression weakens to a broad low pressure near 11N-37.5W by 78 hours while absorbing AOI #36 passing to the north... the broad low pressure weakens further to a tropical wave crossing 46.5W longitude by 120 hours

0000Z (Sep 20) ECMWF Model Run...

**For Hurricane Nigel... recurves northeastward in tandem with frontal cyclone ejecting from Atlantic Canada and reaches the northeast Atlantic near 50.5N-24W at 72 hours... the two cyclones orbit each other with ex-Nigel centered at 53N-21.5W at 120 hours

**For Area of Interest #36... tropical wave does not develop into a tropical cyclone thorugh 120 hours with the wave axis reaching 43W at the 120-hour mark.

**For Area of Interest #37... surface low forms offshore of South Carolina and near 31.5N-77W at 60 hours... surface low then makes a cyclonic loop and then moves north-northeast to Cape Hatteras North Carolina through 102 hours... surface low then transitions into a non-tropical frontal low that reaches 39.5N-69.5W at 120 hours.

**For Area of Interest #38... no development shown

0600Z (Sep 20) GFS Model Run...

**For Hurricane Nigel... recurves northeastward in tandem with frontal cyclone ejecting from Atlantic Canada and reaches the northeast Atlantic near 51.5N-24W at 69 hours... the two cyclones orbit each other with ex-Nigel becoming the dominant and centered near 55.5N-20.5W at 120 hours.

**For Area of Interest #36... compact tropical cyclone formation suggested near 14.5N-50.8W at 120 hours

**For Area of Interest #37... compact subtropical cyclone formation suggested near 31.2N-75.2W at 45 hours... the strengthening subtropical cyclone makes landfall near Cape Lookout North Carolina at 72 hours... transitions into a gradually weakening non-tropical frontal low that shifts north to the southeast coast of New Jersey through 120 hours.

**For Area of Interest #38... no development shown

0600Z (Sep 20) NAVGEM Model Run...

**For Hurricane Nigel... recurves northeastward in tandem with frontal cyclone ejecting from Atlantic Canada and reaches the northeast Atlantic near 51N-20W at 72 hours... the two cyclones orbit each other with ex-Nigel becoming the dominant and centered near 59.5N-18.5W at 120 hours.

**For Area of Interest #36... no development shown

**For Area of Interest #37... rapid subtropical cyclone formation shown offshore of southeastern North Carolina and near 33.8N-76W at 66 hours... transitions into a non-tropical frontal cyclone that moves northeast close to the northeastern United States coast and into Nova Scotia through 120 hours.

**For Area of Interest #38... no development shown