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BIRDSEYE VIEW POSTS

Since 2012 on the now retired Weather Underground blogs, I have been posting annotated "birdseye view" charts of the Atlantic basin, with a detailed explanation and forecasting that references the chart. From there you may know me as "NCHurricane2009." While I now do these "birdseye view" posts here, I will continue to do comments at Yale Climate Connections via Disqus where the former Weather Underground community has moved to. Feel free to reply to me there, at my Disqus feed at this link, or via e-mail at IOHurricanes@outlook.com 

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MY 2023 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #79

Updated: Sep 19, 2023

*******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.**********


...MONDAY SEPTEMBER 18 2023 5:05 PM EDT...

See Hurricane Nigel section below for an update on the latest hurricane in the open central Atlantic. Elsewhere... see area of interest #36 section below for an update on the tropical wave over western Africa which has potential to develop later this week when it moves offshore into the eastern tropical Atlantic. See area of interest #37 section below for an update on a subtropical disturbance with development potential expected to develop near the southeastern United States coast by late this week and into the weekend.


HURRICANE NIGEL... Nigel in the open central Atlantic has strengthened into the lastest and fifth hurricane of the 2023 Atlantic season. My forecast track is once again adjusted north... and also east... of my previous as Nigel is northeast of the prior track. Through 48 hours there will be a northward component to the westward track of Nigel as follows:

(1) Even though the western part of the Atlantic surface ridge has recovered as ex-Lee and ex-Margot are lifting northeastward and away... it has not recovered much as the next upper trough in the mid-latitude westerlies... currently inbound from eastern North America... has developed a sprawling surface frontal low pressure near the mid-Atlantic United States coast that will continue northeastward into Atlantic Canada. This frontal low will keep the western part of the Atlantic surface ridge weak such that Nigel will continue northwestward instead of westward through 48 hours.

(2) After 48 hours... Nigel is expected to recurve and accelerate northeastward due to the deep-layer southwesterly flow ahead of the frontal low that will be moving across Atlantic Canada and aforementioned next upper trough. My updated forecast track below for this timeframe is generally south and west of the recent 1200Z GFS model run positions as I expect there will be enough western Atalntic surface ridging to keep Nigel a little more to the south and west of the GFS solution during much of the next 48 hours.


Regarding intensity... Nigel strengthened into a hurricane earlier than I anticipated... perhaps as the further northeast position of the hurricane kept it further away from the current wave of upper vorticity to the west (consisting of an upper vortex trying to link with the current northwest Atlantic upper trough) and its shearing upper winds. However the core of the hurricane does not have a well-defined eye and therefore Nigel may struggle to strengthen rapidly in the low shear environment over the next 24 hours. Therefore I agree with the NHC on showing a minimal category 3 hurricane by 24 hours instead of anything stronger. Weakening is shown after 24 hours as the hurricane begins to become sheared by the recurving upper trough... with the shear progressively worsening with time as the trough moves closer to Nigel. However the weakening rate should be gradual as Nigel also becomes supported by the broad divergence zone of the trough... and by 96 hours as Nigel reaches cooler waters transition into a strong frontal cyclone supported by the upper trough should be completed.


The updated forecast track will still allow for surf generated by Nigel to reach the shores of Bermuda by tomorrow and Wednesday.

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

0 Hr Position (1200Z Sep 18)... 80 mph maximum sustained wind hurricane centered in the central Atlantic at 27.2N-51.4W

IOH 24 Hr Forecast (1200Z Sep 19)... 115 mph maximum sustained wind major hurricane centered in the central Atlantic at 30N-55W

IOH 48 Hr Forecast (1200Z Sep 20)... 100 mph maximum sustained wind hurricane centered in the central Atlantic at 35N-56.5W

IOH 72 Hr Forecast (1200Z Sep 21)... 75 mph maximum sustained wind hurricane centered in the north-central Atlantic at 41N-47W

IOH 96 Hr Forecast (1200Z Sep 22)... Frontal cyclone centered in the northeastern Atlantic at 49N-27.5W

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

Peak Strength (1200Z Sep 19)... 115 mph maximum sustained wind major hurricane centered in the central Atlnatic at 30N-53.9N

5-Day Position (1200Z Sep 23)... 60 mph maximum sustained wind frontal cyclone centered in the northeastern Atlantic at 53.5N-20W


AREA OF INTEREST #36... The tropical wave of low pressure over interior western Africa through 1200Z today displayed itself as a cloud swirl with scattered thunderstorms near 10.5N-7.5W. This wave continues to be close to another wave to the west which has recently moved offshore from the west coast of Africa. Although the spin of this wave has not moved north yet from 10N latitude... my updated forecast track in the outlook below has an increase in the northward angle of the westward track for the short-term (next 48 hours) while assuming that this wave's spin will undergo a fujiwhara-type interaction with the adjacent wave's spin offshore of Africa before absorbing the adjacent spin. Currently upper winds in the eastern tropical Atlantic are not conducive for development due to suppressing upper vorticity at the southern base of the current eastern Atlantic upper trough. However in the days ahead the north Atlantic upper ridge cell will push the suppressing upper vorticity westward and away... allowing the current western Africa upper ridge cell with favorable low shear and outflow to expand into the eastern tropical Atlantic. Despite the favorable upper wind outlook... I continue to only gradually ramp up odds of tropical cyclone formation to what is now 50% by day 5 as the models now agree on this system being initially broad for rapid development... probably as this wave will absorb the adjacent wave to the west as noted above (often broad systems lack a well-defined center with focused thunderstorms needed for more rapid development). Note the Republic of Cabo Verde Islands may see heavy rainfall and gusty winds from this system by Thursday.

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

IOH 24 Hr Outlook (1200Z Sep 19)... 0% chance of tropical cyclone formation (western Africa near 11.5N-12.5W)

IOH 48 Hr Outlook (1200Z Sep 20)... 0% chance of tropical cyclone formation (just offshore of the west coast of Africa near 12.5N-17.5W)

IOH 72 Hr Outlook (1200Z Sep 21)... 10% chance of tropical cyclone formation (south-southeast of the Republic of Cabo Verde Islands near 13N-22.5W)

IOH 96 Hr Outlook (1200Z Sep 22)... 30% chance of tropical cyclone formation (eastern tropical Atlantic near 13.5N-27.5W)

IOH 120 Hr Outlook (1200Z Sep 23)... 50% chance of tropical cyclone formation (eastern tropical Atlantic near 14N-32.5W)

******National Hurricane Center (hurricanes.gov) official outlook as of 2 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... while the warm southerly flow ahead of the current broad frontal low currently over the US east coast promotes warm core upper ridging over the western Atlantic. The southern part of the current eastern North America upper trough is expected to become trapped between these upper ridges and in the vicinity of the Florida peninusla. Models also show a portion of the upper vorticity currently offshore of California swinging eastward around the north side of the Mexico upper ridge... followed by a southeastward dive into the stuck upper trough over the Florida peninsula by days 4 and 5... 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 4 to 5 days. With models remaining in good agreement on a subtropical disturbance materializing... we now have our thirty-seventh tropical Atlantic area of interest tracked on this site this year.


By the 4 to 5 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 any subtropical disturbance that develops is likely 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. Global models are split on subtropical cyclone formation... with about half of them showing the Florida upper trough/vortex having a consolidated upper divergence zone to allow for a consolidated surface center and subtropical cyclone status... and the other half of the models showing a more elongated upper divergence zone that produces an elongated surface subtropical low or trough. With half of the models onboard and the other half not... I support 50% odds of subtropical cyclone formation by day 5. Interests across coastal northeast Florida... coastal Georgia... and coastal and inland area of the Carolinas should now 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 (1200Z Sep 19)... 0% chance of subtropical cyclone formation (offshore of northeastern Florida near 29N-80W)

IOH 48 Hr Outlook (1200Z Sep 20)... 0% chance of subtropical cyclone formation (offshore of northeastern Florida near 29N-80W)

IOH 72 Hr Outlook (1200Z Sep 21)... 0% chance of subtropical cyclone formation (offshore of northeastern Florida near 29N-80W)

IOH 96 Hr Outlook (1200Z Sep 22)... 30% chance of subtropical cyclone formation (offshore of the Florida/Georgia border near 30.5N-79.5W)

IOH 120 Hr Outlook (1200Z Sep 23)... 50% chance of subtropical cyclone formation (coastal South Carolina near 32.5N-79.5W)

******National Hurricane Center (hurricanes.gov) official outlook as of 2 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 18) CMC Model Run...

**For Hurricane Nigel... recurves northward and then northeastward and reaches the northeast Atlantic near 50.5N-26W at 120 hours as a remnant frontal cyclone.

**For Area of Interest #36... emerges from the west coast of Africa at 42 hours and organizes into a tropical low southeast of the Republic of Cabo Verde Islands by 60 hours... tropical cyclone formation suggested near 15N-28W at 90 hours... reaches 18N-37W at 126 hours as a tropical storm.

**For Area of Interest #37... surface low forms north of the northwestern Bahamas near 29.8N-77W at 102 hours... rapid subtropical cyclone formation suggested near 31N-78W at 120 hours.


0000Z (Sep 18) ECMWF Model Run...

**For Hurricane Nigel... recurves northward and then northeastward and reaches the northeast Atlantic near 49N-25W at 120 hours as a remnant frontal cyclone.

**For Area of Interest #36... emerges from the west coast of Africa at 54 hours and organizes into a broad tropical low just southeast of the Republic of Cabo Verde Islands by 72 hours... tropical cyclone formation suggested near 16.5N-34W at 120 hours

**For Area of Interest #37... broad subtropical surface low forms offshore of northeastern Florida and near 29.5N-79.5W by 120 hours.


1200Z (Sep 18)... GFS Model Run...

**For Hurricane Nigel... recurves northward and then northeastward and reaches the northeast Atlantic near 53.8N-19W at 120 hours as a remnant frontal cyclone.

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

**For Area of Interest #37... broad subtropical surface low forms offshore of the southeastern United States at 102 hours near 29.5N-77W... the strengthening subtropical surface low develops a consolidated center and hence becomes a subtropical cyclone at 126 hours while making landfall on the southeastern North Carolina coast.


0600Z (Sep 18) NAVGEM Model Run...

**For Hurricane Nigel... recurves northward and then northeastward and reaches the northeast Atlantic near 51N-20.5W at 120 hours as a remnant frontal cyclone

**For Area of Interest #36... emerges from the west coast of Africa at 54 hours and organizes into a broad tropical low whose center passes just south of the Republic of Cabo Verde Islands at 90 hours... strengthens into a broad tropical cyclone near 14.8N-30W at 120 hours.

**For Area of Interest #37... no development shown

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