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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 #77

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


...UPDATE...FRIDAY SEPTEMBER 15 2023 12:45 PM EDT...

The following are recent events in the Atlantic tropics:

(1) The central tropical Atlantic low pressure area... tagged as area of interest #34 in this update... was upgraded by the National Hurricane Center to Tropical Depression Fifteen as of 11 AM EDT. Fifteen now joins Lee and Margot as the third active tropical cyclone currently in the Atlantic basin. The 11 AM EDT NHC advisory on Fifteen also calls for a 5-day forecast track toward the waters southeast of Bermuda... but with a slightly stronger intensity forecast than what I showed in the 10:45 AM full update below.

(2) Lee is now centered northwest of Bermuda while continuing to accelerate northward away from Bermuda and toward the eastern part of the northeastern United States and Atlantic Canada... and has weakened to 80 mph maximum sustained winds as of the 11 AM EDT NHC advisory. However Lee remains a formidable cyclone due to its large size. The strongest wind measured on Bermuda from Lee so far was sustained at 43 mph... gusting to 62 mph... as of 20:55 local time Thursday. Gusts to tropical storm force (40+ mph) are still occuring but will reduce with time as Lee continues to pull away from Bermuda (source... http://www.weather.bm/observations.asp).

(3) As of the 11 AM EDT NHC advisory... Margot located in the eastern Atlantic has weakened to 65 mph maximum sustained winds.


...FRIDAY SEPTEMBER 15 2023 10:45 AM EDT...

See Hurricane Lee and Tropical Storm Margot sections below for the two currently active tropical cyclones in the Atlantic basin. The largest concern in the Atlantic basin for land areas is Hurricane Lee which will strike the eastern part of the northeastern United States and Atlantic Canada this weekend as a large and vigorous cyclone with significant impact. See Lee section below for more information on expected impacts to land areas.


In addition... a broad tropical low pressure in the central tropical Atlantic is being monitored for signs of tropical cyclone formation... see area of interest #34 section below for more information.


Elsewhere... satellite imagery shows a pair of well-defined tropical waves of low pressure to the east of Area of Interest #34 as follows:

(1) A large/broad tropical wave of low pressure has spent the last couple of days emerging from western Africa and passing south of the Republic of Cabo Verde Islands. Suppressing upper vorticity from the current northwest Africa and northeast Atlantic upper troughs has sagged southward toward this tropical wave thanks to increasing north Atlantic upper ridging bolstered by the outflows of tropical cyclones Lee and Margot. As the north Atlantic upper ridging continues east in the mid-latitude westerlies... the suppressing upper vorticity will be pushed westward by this upper ridging and in tandem with this tropical wave... and the global models for the most part agree on not developing this wave. Therefore this tropical wave is not considered an area of interest at this time.

(2) An additional tropical wave of low pressure is currently moving into western Africa from central Africa with some signs of organization... and computer models agree on developing this wave once it emerges into the eastern tropical Atlantic in about five days. See area of interest #36 section below for more information.


HURRICANE LEE... Lee... now centered west-northwest of Bermuda... has spent the last 36 hours weakening to a category 1 hurricane due to increased shear imparted by the currently approaching amplified eastern North America upper trough which has dug into Lee. The hurricane has also grown in size due to the large area of divergence on the east side of the upper trough. The hurricane is accelerating northward near 67.5W longitude in deep-layer southerly flow ahead of the upper trough and the surface front driven by the trough. Noting the models have shifted east while now showing the landfall of Lee's center either over southwestern Nova Scotia or New Brunswick instead of the Maine/New Brunswick border. However I am keeping my forecast track line similar to the previous... which assumes a landfall near the Maine/New Brunswick border... as the incoming upper trough did not shove Lee east to 66.5W longitude through 0600Z today as I previously expeceted. Also the west side of the hurricane is now pulling down cold air associated with the amplified upper trough... which will create a cold core upper vortex just west of Lee whose northeast side will try to bend the track slightly west in the short-term. Note the post-landfall east lean in track as Lee and the upper vortex become stacked... because the upper vortex and Lee get pushed in unison by the next North America amplified upper trough that approaches in the mid-latitude westerlies.


For intensity... my previous forecast has done well and the updated one remains the same. Intensity forecast philosophy is to only have Lee gradually weaken in the short-term due to the tremendous eastern divergence zone of the forming upper vortex to the west which will aid Lee. Transition into a non-tropical frontal cyclone is anticipated around 24 hours once Lee reaches the cool northwest Atlantic waters well below 26 deg C... with the weakening rate of the frontal cyclone increasing between 24 and 48 hours as ex-Lee and the upper vortex become vertically stacked as traditionally seen with a post-mature frontal cyclone... with a lack of divergence beneath the core of the upper vortex causing the increased weakening rate. From 48 to 72 hours models show ex-Lee maintaining strength while moving northeastward near southeastern Quebec... the northern Gulf of St. Lawrence... and into northwestern Newfoundland or southern Labrador as the overhead upper vortex opens into a trough due to the approach of the next amplified upper trough in the mid-latitudes... with ex-Lee aided by the eastern divergence zone of this next trough.


Regarding impact to land areas:

(1) Expect wide-reaching surf from this large-sized storm to continue reaching the shores of the mid-Atlantic and northeastern United States coast... Bermuda... and Atlantic Canada (Nova Scotia... New Brunswick... and Newfoundland) through this weekend.

(2) The strongest coastal surf and tropical storm force winds are or have already occurred for Bermuda... expect gradually improving weather with coastal surf lingering thorugh the weekend as noted above.

(3) Interests across Nova Scotia... New Brunswick... Prince Edward Island... Maine... New Hampshire... central and eastern Massachusetts... eastern Connectictut... Rhode Island... and eastern Long Island New York are expected to see more direct impacts such as substantial coastal surf and damaging winds this weekend... I recommend to finish preparing for Lee by tonight. Even though Lee has been slowly weakening during its approach... damaging winds and strong coastal surf are expected to affect a large area due to the recent growth in the size of the storm. Flash flooding heavy rainfall will be another hazard to contend with.

(4) Expect gusty winds with some damage potential... coastal surf... and heavy rainfall from what will be the remnant frontal cyclone of Lee for southeastern Quebec... Newfoundland... and Labrador by Monday.

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

0 Hr Position (0600Z Sep 15)... 85 mph maximum sustained wind hurricane centered west-northwest of Bermuda at 33.7N-67.6W

IOH 24 Hr Forecast (0600Z Sep 16)... 70 mph maximum sustained wind tropical storm transitioning into a frontal cyclone while centered midway between Cape Cod Massachusetts and the southwest coast of Nova Scotia at 41N-67.5W

IOH 48 Hr Forecast (0600Z Sep 17)... 50 mph maximum sustained wind frontal cyclone centered over south-central New Brunswick at 46N-66.5W

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

4-day Position (0000Z Sep 19)... 40 mph maximum sustained wind dissipating frontal low centered in the north-central Atlantic at 54.5N-42.5W


HURRICANE MARGOT (RECENTLY DOWNGRADED TO TROPICAL STORM MARGOT)... Margot is currently located in the open eastern Atlantic in between a re-building Atlantic surface ridge to the north and passing high-latitude upper trough to the northeast. The northwest side Margot has pulled down enough high-latitude cold air to make a local upper trough also to the northwest. The local upper trough to the west has sheltered Margot from hostile northwesterly shear that would have been imparted by the high-latitude upper trough to the northeast and thus the tropical cyclone has been slow to weaken from a hurricane to a tropical storm. The southwesterly flow ahead of the local upper trough has also won the steering battle against the re-building Atlantic surface ridge while dragging Margot northeast of my previous forecast. However the combo of the Atlantic surface ridge and a warm core upper ridge that builds to the northwest in the warm sector of what will be ex-Lee is expected to cause Margot to undergo a clockwise loop turn over the next few days. By days 3 and 4 the warm core upper ridge and Atlantic surface ridge will have continued eastward in the mid-latitude westerlies such that Margot will continue the clockwise loop around the west sides of these features... resulting in a northeastward recurvature of Margot itself into the mid-latitude westerlies and northeastern Atlantic. My updated intensity forecast remains similar to my previous as I still call for more short-term weakening as Margot becomes sandwiched between the passing warm core upper ridge to the north and low-latitude tropical upper ridging to the south... an environment where the outflow of both upper ridges converges over Margot and tries to weaken the tropical storm. By 72 hours what will be remaining of the current central Atlantic upper vortex will be flung northeastward toward Margot by the upper vortex to be over ex-Lee... resulting in increased upper divergence over Margot on the east side of the incoming upper vortiicty. This upper divergence pattern continues through 96 hours as what will be remaining of the upper vortex over ex-Lee then shifts east toward Margot in the mid-latitude westerlies. Therefore I call for Margot to re-strengthen as a tropical storm by 72 hours while still situated over marginal 26 deg C waters... followed by transition into a remnant non-tropical frontal cyclone supported by the incoming upper vorticity by 96 hours once the updated forecast track takes Margot over waters below 26 deg C.


Note because Margot moved northeastward more than prior forecasts... the Azores are already seeing or will likely see coastal surf from Margot over the next 24 hours until the storm begins its clockwise loop turn that takes it westward and away from the Azores. By day 4 the coastal surf is expected to return as Margot accelerates northeastward across the northeast Atlantic as a remnant frontal cyclone.

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

0 Hr Position (0600Z Sep 15)... 70 mph maximum sustained wind tropical storm centered in the eastern Atlantic at 36.7N-38.6W

IOH 24 Hr Forecast (0600Z Sep 16)... 50 mph maximum sustained wind tropical storm centered in the eastern Atlantic at 35N-38.5W

IOH 48 Hr Forecast (0600Z Sep 17)... 40 mph maximum sustained wind tropical storm centered in the central Atlantic at 35N-42W

IOH 72 Hr Forecast (0600Z Sep 18)... 50 mph maximum sustained wind tropical storm centered in the central Atlantic at 37.5N-43W

IOH 96 Hr Forecast (0600Z Sep 19)... Frontal cyclone centered in the northeastern Atlantic at 42N-39W

******National Hurricane Center (hurricanes.gov) official forecast as of 5 AM EDT***************************

5-day Position (0600Z Sep 20)... 45 mph maximum sustained wind frontal cyclone centered west-northwest of the Azores at 40N-34W


AREA OF INTEREST #34... The broad tropical low pressure now in the central tropical Atlantic has continued to see an increase in the organization and intensity of its thunderstorms... I am continuing a tropical cyclone formation forecast with a specific track and intensity projection as outlined below. The forecast track is adjsuted northward due to the current position of this system relative to the previous forecast... and it appears during the next five days there will be some northward component to the westward track of this system as follows:

(1) This system is already slightly north of the previous forecast while likely also dragged by the current central Atlantic upper vortex... and the latest model runs now indicate that although this upper vorticity is slated to be kicked northeastward and away by what will be the upper vortex over ex-Lee that some of this upper vorticity will remain behind and to the west of the system

(2) For the short-term the western part of the Atlantic surface ridge will remain weak due to the simulatenous presence of Lee and Margot. Although the surface ridge recovers in the middle of the 5-day forecast period as Lee and Margot lift northward and away the ridge does not recover enough to prevent a northward angle of the forecast track. This is because the next upper trough in the mid-latitude westerlies... currently over central North America... will drive a surface front into the western Atlantic.

(3) By day 5 the surface ridge weakness associated with the western Atlantic front will be narrow... however a sharper curve toward the north and toward this weakness is likely as the aforementioned next upper trough will also be nearing with a large area of steering upper southerly flow (this system should be strong/tall enough by day 5 to also be steered by upper winds).


Regarding intensity... upper winds through day 4 should be favorable for development as the current unfavorable central Atlantic upper vortex first pivots west and away from this system under the influence of a northwest Atlantic warm core upper ridge that builds in the warm sector of what will be ex-Lee... and then most of this upper vorticity pivots northeastward toward Margot and further away from this system while pulled around the upper vortex that will be on top of ex-Lee. Despite the 4-day favorable upper wind outlook... I only show gradual strengthening into a category 2 hurricane over the 4-day period as this system is currently broad in nature... and broad systems often lack a well-defined center of surface converence with focused thunderstorms needed for rapid development. Also broad systems tend to have a lax surface pressure gradient between the center and outside... requiring more of a central pressure drop to achieve an increase in wind speed. Also noting the forecast intensity over the next four days is lowered as this system has not quiet yet developed into a tropical cyclone as I previously projected. For days 4 to 5 I slow the intensification rate as the upper trough to approach late in the forecast period... noted in the prior paragraph... will likely impart some westerly shear. In addition the forecast track by days 4 and 5 takes this system toward the cooled sea surface temperatures caused by recent hurricanes Franklin... Idalia... and Lee .


If the current forecast track and intensity shown below holds... surf generated by this system will likely reach the shores of the northern Lesser Antilles by day 4 and the shores of Bermuda by day 5.

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

0 Hr Position (0600Z Sep 15)... Tropical low centered in the central tropical Atlantic at 13.8N-41W

IOH 24 Hr Forecast (0600Z Sep 16)... 50 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 16N-45W

IOH 48 Hr Forecast (0600Z Sep 17)... 65 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 18.5N-48W

IOH 72 Hr Forecast (0600Z Sep 18)... 80 mph maximum sustained wind hurricane centered in the central Atlantic at 21N-53W

IOH 96 Hr Forecast (0600Z Sep 19)... 100 mph maximum sustained wind hurricane centered in the central Atlantic at 23N-57.5W

IOH 120 Hr Forecast (0600Z Sep 20)... 110 mph maximum sustained wind hurricane centered southeast of Bermuda at 28N-60W

******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM EDT***************************

Formation chance through 48 hours... 90%

Formation chance through 7 days... 90%


AREA OF INTEREST #36... Satellite imagery shows a tropical wave of low pressure with some signs of organized thunderstorm activity currently moving into western Africa from central Africa and positioned near 10N-5E as of 0600Z earlier this morning. Currently upper winds in the eastern tropical Atlantic are not conducive for development as suppressing upper vorticity from the current northwestern Africa and northeastern Atlantic upper troughs have sagged southward into the region thanks to north Atlantic upper ridging bolstered by the outflows of tropical cyclones Lee and Margot. However by day 5... around the time this tropical wave is forecast to emerge from western Africa and into the eastern tropical Atlantic... the north Atlantic upper ridging is expected to continue east in the mid-latitude westerlies and hence push the suppressing upper vorticity westward and out of the way... allowing for tropical upper ridging with favorable low shear and upper outflow to recover in the eastern tropical Atlantic and over this tropical wave. Many recent global model runs develop this wave in fact very quickly after it emerges from the west coast of Africa... therefore this tropical wave has been added as an area of interest in the NHC tropical weather outlook. This marks the thirty-sixth tropical Atlantic area of interest tracked on this site this year. For day 5... I assign a low 5% odds of tropical cyclone formation when the wave will be just offshore of Africa as rapid tropical cyclone formation offshore of Africa is generally a rare event. However should this tropical wave remain organized while global models continue to show such rapid formation... I will raise development odds for what is now the day-5 timeframe in future updates. Note the Republic of Cabo Verde Islands may see heavy rainfall and gusty winds from this tropical wave just after 5 days regardless of tropical cyclone formation or not.

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

IOH 24 Hr Outlook (0600Z Sep 16)... 0% chance of tropical cyclone formation (western Africa near 10N-0W)

IOH 48 Hr Outlook (0600Z Sep 17)... 0% chance of tropical cyclone formation (western Africa near 10.5N-5W)

IOH 72 Hr Outlook (0600Z Sep 18)... 0% chance of tropical cyclone formation (western Africa near 11N-10W)

IOH 96 Hr Outlook (0600Z Sep 19)... 0% chance of tropical cyclone formation (near west coast of Africa near 11.5N-15W)

IOH 120 Hr Outlook (0600Z Sep 20)... 5% chance of tropical cyclone formation (southeast of the Republic of Cabo Verde Islands near 12N-20W)

******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM EDT***************************

Formation chance through 48 hours... 0%

Formation chance through 7 days... 20%


...COMPUTER MODEL SUMMARY...

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


1200Z (Sep 14) CMC Model Run...

**For Hurricane Lee... passes just west of Bermuda thru 12 hours... makes landfall on the southwest coast of Nova Scotia as a weakening but still strong frontal cyclone at 60 hours... center of gradually weakening frontal cyclone slides across Gulf of St Lawrence then northern Newfoundland through 90 hours... while becoming elongated east-to-west the frontal cyclone reaches 45N-29W by 120 hours

**For Hurricane Margot... circles in a clockwise loop and reaches 38N-41W by 120 hours as a weakened tropical storm

**For Area of Interest #34... tropical cyclone formation suggested near 20.5N-29W at 72 hours... reaches 25.5N-55.5W as a strengthening tropical storm by 120 hours

**For Area of Interest #36... tropical wave emerges from west coast of Africa at 144 hours... evolves into a tropical depression just east of the Republic of Cabo Verde Islands thru 168 hours

**Tropical wave currently south of the Republic of Cabo Verde Islands merges with additional wave that emerges from west coast of Africa at 84 hours... the combined system develops into a tropical low near 10N-26.5W through 120 hours... evolves into a tropical depression near 12N-32W at 168 hours


1200Z (Sep 14) ECMWF Model Run...

**For Hurricane Lee... passes just west of Bermuda thru 18 hours and makes landfall on the New Brunswick coast at 60 hours as a weakening but still strong frontal cyclone... center of gradually weakening frontal cyclone slides across Gulf of St Lawrence then northern Newfoundland through 90 hours... while becoming elongated east-to-west the frontal cyclone reaches 43.5N-24W by 120 hours.

**For Hurricane Margot... circles in a clockwise loop and reaches 37.5N-41.2W by 120 hours as a weakened tropical storm

**For Area of Interest #34... tropical cyclone formation suggested near 20N-47W at 66 hours... reaches 23.5N-56.5W as a strengthening tropical storm by 120 hours

**For Area of Interest #36... tropical wave emerges from west coast of Africa at 126 hours... tropical cyclone formation suggested near 13.5N-19.8W at 138 hours... tropical cyclone moves through the Republic of Cabo Verde Islands by 162 hours


1800Z (Sep 14)... GFS Model Run...

**For Hurricane Lee... passes just west of Bermuda thru 6 hours and makes landfall on the New Brunswick coast at 57 hours as a weakening but still strong frontal cyclone... center of gradually weakening frontal cyclone slides across Gulf of St Lawrence then southern Newfoundland through 84 hours... while becoming elongated east-to-west the frontal cyclone reaches 43.5N-46.5W by 120 hours.

**For Hurricane Margot... circles in a clockwise loop and reaches 41.8N-32.5W by 120 hours while maintaining at least strong tropical storm strength.

**For Area of Interest #34... tropical cyclone formation suggested near 16N-38W at 30 hours... reaches 27.5N-58.5W at 120 hours as a hurricane

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


1800Z (Sep 14) NAVGEM Model Run...

**For Hurricane Lee... akes landfall on the New Brunswick coast at 60 hours as a strong frontal cyclone... the gradually weakening but still stirng frontal cyclone turns northeast across the northwestern Gulf of St Lawrence... southeastern Quebec... and southren Labrador through 84 hours and by 120 hours is located offshore of Labrador

**For Hurricane Margot... circles in a clockwise loop and reaches 39.9N-38.8W by 120 hours while maintaining hurricane strength

**For Area of Interest #34... tropical cyclone formation suggested near 14N-40.5W at 12 hours... reaches 29.5N-57W by 120 hours as a hurricane

**For Area of Interest #36... tropical wave emerges from west coast of Africa at 150 hours with tropical cyclone formation suggested near 15N-20W by 168 hours

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