*******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.**********
...TUESDAY SEPTEMBER 19 2023 11:55 PM EDT...
See Hurricane Nigel section below for an update on the latest hurricane in the open central Atlantic. See area of interest #36 section below for an update on the tropical wave of low pressure now entering the eastern tropical Atlantic from western Africa... and 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.
Elswhere... the remnant low pressure of Margot drifting eastward toward the Azores has seen an increase in thunderstorms boosted by the eastern divergence zone of the current upper trough sliding eastward across the northeast Atlantic. However the thunderstorms have recently weakened and become seperated southeastward from ex-Margot's center as the upper trough is beginning to dive southeastward and away while revolving around the current central Atlantic upper ridge cell... this will leave ex-Margot to weaken under the western convergence zone of the departing upper trough. Therefore ex-Margot is not expected to redevelop while moving toward the Azores... and in fact ex-Margot has already weakened enough that the potential for short-term Azores coastal surf from this system has ended. The next round of coastal surf for the Azores is expected late this week from what will be the remnant frontal cyclone of Nigel... see Nigel section below for more info.
In addition... the current upper vorticity retrograding westward across the eastern tropical Atlantic is enhancing the poleward (northward) outflow of thunderstorms in the vicinity of 10N-35W... to the west of Area of Interest #36 and the tropical wave near 25W longtidue... and to the east of another tropical wave near 45W longitude. So far the CIMSS 850 mb vorticity product (https://tropic.ssec.wisc.edu/real-time/windmain.php?basin=atlantic&sat=wg8&prod=vor&zoom=&time=) does not show much low-level rotation near 10N-35W... however the enhanced poleward outflow regime may result in the formation of a rotating tropical low pressure disturbance in this region that then moves westward toward the Lesser Antilles over the next five days. 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 would not unfavorably shear such a disturbance and it would bear watching. Therefore will add an area of interest for the central tropical Atlantic in future updates should a disturbance materialize in the region.
HURRICANE NIGEL... As of 5 PM EDT today Nigel has made its turn to the north in the deep-layer flow ahead of the current frontal low heading toward Atlantic Canada and its supporting upper trough currently over eastern North America... and for the remainder of its life cycle Nigel will be accelerating northeastward in the flow ahead of these features. My updated forecast track below is adjusted eastward in the short-term as Nigel was slightly east of the previous forecast as of 1200Z earlier today... with the eastward error of margin in the previous forecast growing now that the north turn has already occurred. My longer-term forecast track is shifted westward as the latest model data shows the upper trough moving slower to the east. Regarding intensity... over the last 30 hours the north side of the hurricane's core ingested dry air which caused the earlier developing small eye to collapse... and since then a new large eye has taken shape and solified. This dry air disruption caused a delay in Nigel's intensification and put the hurricane behind my and the NHC official intensity forecasts that were issued yesterday. As of 5 PM EDT aircraft recon found Nigel had reached 100 mph maximum sustained winds. By 24 hours I forecast Nigel to be slightly weaker than this intensity as southwesterly shear imparted by the approaching upper trough begins to increase... however it would not surprise me if Nigel acheived a peak a little stronger than 100 mph max sustained winds between now and 24 hours and before the shear develops. Even though the shear increases by 48 hours as the upper trough gets closer... I have not lowered the 48-hour intensity point relative to the prior forecast as the shear may not be enough to weaken Nigel below a category 1 hurricane by then 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 72 hours... transition into a remnant frontal cyclone supported by the upper trough's divergence zone should be completed with Nigel over cooler waters.
There is some potential for surf generated by Nigel to reach the shores of Bermuda within the next 24 hours. After that Nigel and the intensifying frontal low that will move across Atlantic Canada and the north Atlantic will be moving northeastward in tandem and generating a large area of surf. Expect surf to reach the shores of Newfoundland by late tomorrow and Thursday.... with surf reaching the shores of southeast Greenland... Iceland... the Azores... and the British Isles by late this week and through the weekend. Gusty winds will be possible across the British Isles and Iceland during the weekend as well.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 19)... 100 mph maximum sustained wind hurricane centered in the central Atlantic at 31.8N-54.7W
IOH 24 Hr Forecast (1800Z Sep 20)... 95 mph maximum sustained wind hurricane centered in the central Atlantic at 36.5N-54.5W
IOH 48 Hr Forecast (1800Z Sep 21)... 75 mph maximum sustained wind hurricane centered in the north-central Atlantic at 42N-43W
IOH 72 Hr Forecast (1800Z Sep 22)... Frontal cyclone centered in the northeastern Atlantic at 48.5N-32.5W
******National Hurricane Center (hurricanes.gov) official forecast as of 5 PM EDT***************************
Peak Strength (0600Z Sep 20)... 110 mph maximum sustained wind hurricane centered in the central Atlnatic at 33.7N-54.9N
5-Day Position (1800Z Sep 24)... 45 mph maximum sustained wind frontal cyclone centered south-southwest of Iceland at 58N-21.5W
AREA OF INTEREST #36... The tropical wave of low pressure that has been tracked over western Africa over the last few days was near the west coast of Africa in the vicinity of 11.5N-15W as of 1800Z... and per the latest satellite imagery is already entering the eastern tropical Atlantic from the west coast of Africa. Suppressing upper vorticity in the eastern tropical Atlantic has been retrograding westward and away under the influence of the current central Atlantic upper ridge cell... allowing for the favorable western Africa tropical upper ridge cell with low shear and upper outflow to expand into the eastern tropical Atlantic. Despite this favorable upper wind evolution... my updated outlook below more slowly raises odds of tropical cyclone formation with time... thus still keeping odds of development by day 5 capped at 50%. This is because the wave appears less organized on satellite pictures compared to the prevoius update despite an increase in thunderstorm activity... and because models are more slowly developing the wave. The models may be picking up on the fact that during the 5-day forecast period that vorticity from the current northeast Atlantic upper trough is expected to 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. However by days 4 and 5 the tropical wave will be pulling westward and away from this upper vorticity... and this is when I steepen the rate at which I increase development odds in the updated outlook below. Note the Republic of Cabo Verde Islands may see heavy rainfall and gusty winds from this system by Thursday... even without tropical cyclone formation by then.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 20)... 0% chance of tropical cyclone formation (eastern tropical Atlantic near 12N-20W)
IOH 48 Hr Outlook (1800Z Sep 21)... 5% chance of tropical cyclone formation (south-southwest of the Republic of Cabo Verde Islands near 12.5N-25W)
IOH 72 Hr Outlook (1800Z Sep 22)... 15% chance of tropical cyclone formation (eastern tropical Atlantic near 13N-30W)
IOH 96 Hr Outlook (1800Z Sep 23)... 35% chance of tropical cyclone formation (eastern tropical Atlantic near 13.5N-35W)
IOH 120 Hr Outlook (1800Z Sep 24)... 50% chance of tropical cyclone formation (central tropical Atlantic near 14N-40W)
******National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT***************************
Formation chance through 48 hours... 0%
Formation chance through 7 days... 70%
AREA OF INTEREST #37... Over the next few days a warm core upper ridge over Mexico will be bolstered by the warm southerly flow ahead of a broad frontal low pressure area that develops over the western United States. The southern part of the current eastern North America upper trough is expected to be pinned in place by this upper ridge and in the vicinity of the Florida peninusla. In addition... some of the upper vorticity rolling into the southwestern US from California is racing eastward across the north side of the Mexico upper ridge... with this vorticity expected to take a southeastward dive into the stuck upper trough over the Florida peninsula in about 3 days... 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 waters offshore of the southeastern US coast and to the north of the northwestern Bahamas such that a subtropical disturbance with development potential is likely to form in about 3 to 4 days.
By the 3 to 4 day window... 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. Two scenarios with this subtropical disturbance are possible... (1) the Florida 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 more models trending toward the first scenario... I have increased my peak odds of subtropical cyclone formation to 60% in this update. Interests across coastal northeast Florida... coastal Georgia... and coastal and inland area of the Carolinas should be aware of this area of interest for the following impacts expected by late this week and into 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. In addition gusty winds with damage potential may spread northward across both coastal and inland areas of the Carolinas.
(3) These impacts are likely 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 (1800Z Sep 20)... 0% chance of subtropical cyclone formation (offshore of northeastern Florida near 29N-80W)
IOH 48 Hr Outlook (1800Z Sep 21)... 10% chance of subtropical cyclone formation (offshore of northeastern Florida near 30N-79.5W)
IOH 72 Hr Outlook (1800Z Sep 22)... 60% chance of subtropical cyclone formation (offshore of the Georgia/South Carolina coast near 31.5N-79.5W
IOH 96 Hr Outlook (1800Z Sep 23)... 0% chance of subtropical cyclone formation (northeastern corner of South Carolina near 34N-79.5W)
******National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT***************************
Formation chance through 48 hours... 0%
Formation chance through 7 days... 30%
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields(http://moe.met.fsu.edu/tcgengifs/).
0000Z (Sep 19) CMC Model Run...
**For Hurricane Nigel... recurves northward and then northeastward in tandem with frontal cyclone ejecting from Atlantic Canada and reaches the northeast Atlantic near 51.2N-24W at 96 hours as a remnant frontal cyclone... through 120 hours the adjacent frontal cyclone swings the remannt cyclone of Nigel northward to the waters southwest of Iceland near 58.5N-22.5W through 120 hours.
**For Area of Interest #36... passes just south of the Republic of Cabo Verde Islands as a tropical low through 54 hours and becomes a tropical cyclone near 16N-32.5W at 84 hours... reaches 17.5N-38.5W at 120 hours as a tropical storm
**For Area of Interest #37... surface low forms north of the northwestern Bahamas near 29N-78.5W at 78 hours... rapid subtropical cyclone formation suggested near 30N-78.5W at 90 hours... subtropical cyclone makes landfall just east of the North Carolina/South Carolina border just after 108 hours and the center reaches the eastern North Carolina/Virginia border by 120 hours.
0000Z (Sep 19) ECMWF Model Run...
**For Hurricane Nigel... recurves northward and then northeastward in tandem with frontal cyclone ejecting from Atlantic Canada and reaches the northeast Atlantic near 50.5N-25W at 96 hours as a remnant frontal cyclone... through 120 hours the adjacent frontal cyclone swings the remnant cyclone of Nigel northward to the waters southwest of Iceland near 58.5N-23W through 120 hours.
**For Area of Interest #36... passes just south of the Republic of Cabo Verde Islands as a tropical low through 54 hours and becomes a tropical depression near 17N-37.5W at 120 hours
**For Area of Interest #37... broad subtropical surface low forms offshore of northeastern Florida and near 29.5N-78W by 84 hours... subtropical cyclone formation suggested offshore of Georgia and near 31.2N-78.8W at 108 hours with subtropical cyclone making landfall near the North Carolina/South Carolina border at 126 hours
1200Z (Sep 19) GFS Model Run...
**For Hurricane Nigel... recurves northward and then northeastward in tandem with frontal cyclone ejecting from Atlantic Canada and reaches the northeast Atlantic near 50N-25W at 84 hours as a remnant frontal cyclone... through 120 hours the adjacent frontal cyclone swings the remnant cyclone of Nigel northward and then westward to the waters west-southwest of Iceland near 60N-29W through 120 hours.
**For Area of Interest #36... compact tropical cyclone formation suggested to the east of the Lesser Antilles and near 14N-56W at 153 hours.
**For Area of Interest #37... surface low forms offshore of the southeastern United States near 30.5N-77W at 60 hours... surface low has an initial north-south elongated structure that finally consolidates into a subtropical cyclone just offshore of southeastern North Carolina at 93 hours... subtropical cyclone makes landfall near Cape Lookout North Carolina at 96 hours... center of circulation continues north-northeast near the United States east coast and reaches the Maryland/Delaware border at 120 hours.
0600Z (Sep 19) NAVGEM Model Run...
**For Hurricane Nigel... recurves northward and then northeastward and reaches the waters offshore of the British Isles near 53.8N-15W at 120 hours
**For Area of Interest #36... passes just south of the Republic of Cabo Verde Islands as a broad tropical low through 60 hours... center of tropical low reaches 15.5N-35.5W at 120 hours
**For Area of Interest #37... no development shown