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 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #99

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


...FRIDAY SEPTEMBER 3 2021 12:09 AM EDT...

See Larry section below for more info on the only currently active tropical cyclone in the Atlantic basin. See area of interest sections below for all areas being monitored for tropical cyclone formation in the Atlantic.

Elsewhere… satellite imagery shows a tropical wave of low pressure over Western Africa whose thunderstorms bands have recently weakened tonight. Some models however agree that this tropical wave will exit Africa in about 2 days and potentially develop in the eastern tropical Atlantic beyond that time as the favorable tropical upper ridge axis in the region recovers in the wake of the current northwest Atlantic upper vorticity… once that vorticity begins to retrograde westward and away around mid-latitude upper ridging. Therefore this tropical wave may also emerge as an area of interest for tropical development in the coming days.


HURRICANE LARRY… Larry has become a hurricane as expected… but has not sharply intensified while still a category 1 hurricane with 85 mph maximum sustained winds as of the 11 PM EDT NHC advisory package. The thunderstorm activity has been limited north of Larry’s thunderstorm core… perhaps an indication of dry Saharan air affecting the hurricane. With these observations and current intensity compared to my previous forecast… I have slightly lowered my updated intensity forecast. Larry has been and for some time will be aided by the low shear and upper outflow of the tropical Atlantic upper ridge… so I still project category 4 strength (but just a slightly weaker one) by 0000Z September 5. I drop the intensity down to category 3 by days 3 and 4 as Larry potentially sees some western outflow blockage while approaching the upper vorticity currently lingering in the central Atlantic. The latent heat release of Larry is Forecast in the GFS to dissipate this cool core upper vorticity by day 5… so I ramp the intensity forecast back up to category 4 by then.


Regarding track… the current speed of Larry and near-term forecast track is faster than the typical 5W longitude per day of tropical systems in the low-latitudes for the next 48 hours. After that time the models show Larry slow down a bit as the west side of the Atlantic surface ridge weakens from the remnants of Ida. Also a more north track angle is expected as Larry will be strong/tall enough to be influenced by the lingering central Atlantic upper vorticity. Even though the upper vorticity dissipates by day 5… Larry is still expected to turn more north in track while heading toward a large surface ridge weakness to be induced by a series of frontal systems to eject from North America behind Ida’s remnants. On the Forecast… Larry could bring coastal sea swells to the northern Lesser Antilles and Bermuda by days 4 and 5 if the storm becomes the large and powerful hurricane the models have been predicting.

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

0 Hr Position (0000Z Sep 3)… 80 mph maximum sustained wind hurricane centered at 14.1N-36.2W

IOH 24 Hr Forecast (0000Z Sep 4)… 105 mph maximum sustained wind hurricane centered at 15N-42.5W

IOH 48 Hr Forecast (0000Z Sep 5)… 135 mph maximum sustained wind major hurricane centered at 16N-48W

IOH 72 Hr Forecast (0000Z Sep 6)…115 mph maximum sustained wind major hurricane centered at 18N-52W

IOH 96 Hr Forecast (0000Z Sep 7)… 115 mph maximum sustained wind major hurricane centered northeast of the Lesser Antilles at 20N-56W

IOH 120 Hr Forecast (0000Z Sep 8)… 130 mph maximum sustained wind major hurricane centered at 24N-59W


AREA OF INTEREST #1… The tropical wave of

low pressure in the central Caribbean Sea appears to have become less defined as the NHC has relocated the wave axis to the east in their TAFB surface analysis products. So the focus of this area of interest has turned toward the small surface low pressure area currently centered just inland over the Nicaragua/Honduras border… which was created by the enhanced poleward outflow channel streaming into the current west Atlantic upper vorticity string. The upper vorticity string is Forecast to weaken while remaining cut-off from high-latitude cold air. The only global models today that shows this tropical low pressure system developing are still the CMC and GFS… however they show development in the northern Gulf of Mexico and in the longer range beyond day 5. I do not forecast this system to develop before making it to the Yucatan peninsula for the following reasons: (1) the breakup of the west Atlantic upper vorticity string is expected to be slow… and some of the dissipating upper vorticity will be over the west side of the system over the next day or so which will suppress upper outflow and thunderstorms over the west half of the low pressure area (2) the thunderstorm activity with this system is currently not organized. Thus system will have to deal with a second round of upper vorticity in the Gulf of Mexico currently being left behind by the amplify upper trough tied to Ida’s remnants. The modeling shows the upper vorticity weakening to a small western Gulf vortex by days 3 to 5 while this batch of cool core upper vorticity also remains cut-off from high-latitude cold air… and so by days 4 and 5 is when I elect to raise odds of development above 0% as the upper vortex’s eastern divergence zone may aid this system. I have slightly raised development odds to 20% by day 5 as the CMC and GFS remain in agreement on possible development… and will wait to see if other models come onboard.


This system is forecast to have some north angle in track due to the surface ridge weakness created by Ida’s remnants… but a sharp north turn is not anticipated either due to the eastern US surface ridge forecast to be west of Ida’s remnants and induced by the western convergence zone of the amplified upper trough associated with Ida’s remnants. The northwest convergence zone of the Gulf upper vorticity to be left behind by this upper trough will likely keep the surface ridge going over the southeast or northeastern Gulf by days 3 to 5… helping to push this system into the western Gulf after crossing the Yucatan peninsula. A north turn by days 3 and 4 is likely once this system makes it to the western Gulf while rounding the west side of the surface ridge… and also due to possible reformation toward the upper divergence zone of the upper vortex as mentioned towards the end of the previous paragraph. By day 5 this system is Forecast to then make a sharp east turn in the northern Gulf of Mexico in the flow ahead of the pair of frontal systems to cross North America in ex-Ida’s wake.


While it is concerning that this system by day 5 will be near southeast Louisiana where Hurricane Ida recovery efforts are ongoing… for now it appears this system has lower development potential than Ida did as it will have to battle upper vorticity as discussed above. Also the steering flow for now would suggest a track toward the northeast US Gulf coast… to the east of the Hurricane Ida impact region.

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

IOH 24 Hr Outlook (0000Z Sep 4)…0% chance of tropical cyclone formation (just offshore of northern Belize near 17N-87.8W)

IOH 48 Hr Outlook (0000Z Sep 5)… 0% chance of tropical cyclone formation (just offshore of the western Yucatan peninsula near 19.5N-91W)

IOH 72 Hr Outlook (0000Z Sep 6)… 0% chance of tropical cyclone formation (southwestern Gulf of Mexico near 22N-92W)

IOH 96 Hr Outlook (0000Z Sep 7)… 10% chance of tropical cyclone formation (southwestern Gulf of Mexico near 24N-92W)

IOH 120 Hr Outlook (0000Z Sep 8)… 20% chance of tropical cyclone formation (offshore of southeast Louisiana near 27.8N-89.5W)


AREA OF INTEREST #2… Ida’s remnant low and it’s amplifying supporting upper trough has driven a cold front into the west Atlantic.

Will watch to see if the tail end of the front evolves into a subtropical or tropical disturbance to be enhanced by reducing shear and increased upper divergence on the east side of amplifying upper vorticity to be cut-off from the remainder of the upper trough. This cut-off event is expected due to adjacent amplification of warm upper ridging over the central US being caused by the warm sector of the developing western to North America frontal system


Today’s model runs still hint at a surface low pressure area coalescing from the tail end of the front… with the CMC still taking the lead while presenting the most defined low pressure… and so I maintain 20% development odds. However my short-term (next 24 hour) odds are lowered as there is currently no organized thunderstorm activity along the front. The forecast track follows the model consensus… showing the surface low pressure area progress on a slow anticyclonic Loop turn thru 72 hours while pushed around by the South side of the approaching Central Canada surface ridge. The slow motion is expected from the Atlantic surface ridge to the east which will counter the force from the approaching Canadian surface ridge. By days 4 and 5 a turn to the northeast is expected in the flow ahead of the frontal system to approach from western North America. The odds are lowered at days 4 and 5 due to the consistent forecasting in the models of westerly shear being induced by the approach of the upper trough associated with the incoming frontal system from western North America.

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

IOH 24 Hr Outlook (0000Z Sep 4)…0% chance of tropical cyclone formation (offshore of the southeastern US near 30N-76W)

IOH 48 Hr Outlook (0000Z Sep 5)…20% chance of tropical cyclone formation (offshore of northeast Florida near 29.5N-78W)

IOH 72 Hr Outlook (0000Z Sep 6)… 20% chance of tropical cyclone formation (offshore of the southeastern US near 30.5N-78.5W)

IOH 96 Hr Outlook (0000Z Sep 7)… 10% chance of tropical cyclone formation (offshore of North Carolina near 33N-76.5W)

IOH 120 Hr Outlook (0000Z Sep 8)… 5% chance of tropical cyclone formation (offshore of the eastern US near 37.5N-72W)

AREA OF INTEREST #3… The tropical wave of low pressure in the eastern tropical Atlantic has seen a decline in thunderstorm activity since special update #98A… it appears somewhat vigorous easterly flow on the south side of the tropical upper ridge has pushed the thunderstorms to the southwest of center of surface spin (which I estimate to be near 13.9N-22W) with no new activity near the center thus far. This tropical wave will likely slow down and turn more west-northwest in track while caught in a developing surface ridge weakness to the north and northeast to be induced by the divergence zone of upper vorticity being deposited by the current northeast Atlantic upper trough. While pushed around by mid-latitude upper ridging...this upper vorticity by 48+ hours is also expected to retrograde southwest toward this tropical wave which will likely increase the shear and also suppress the upper outflow of the wave as the upper vorticity shifts directly over the wave. However for the next 24 hours and before the upper vorticity arrives...there remains some development risk… but I have lowered my odds of development and now agree with the NHC outlook which has 30% odds as of this writing. Interests in the Republic of Cabo Verde Islands may see gusty winds and heavy rains over the next 24 hours...in particular toward the southern islands…. only if the tropical wave redevelops thunderstorms late tonight or in the early morning. Odds of development are trimmed down to 0% by 48 hours to reflect the approach of the unfavorable upper vorticity.

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

IOH 24 Hr Outlook (0000Z Sep 4)…30% chance of tropical cyclone formation (just southwest of the Republic of Cabo Verde Islands near 15N-25.5W)

IOH 48 Hr Outlook (1800Z Sep 4)...0% chance of tropical cyclone formation (west of the Republic of Cabo Verde Islands near 16N-28W)


...COMPUTER MODEL SUMMARY...

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


1200Z CMC Model Run...

**For Hurricane Larry… located at 23N-55W at 120 hours

**For area of interest #1…evolves into broad western Gulf of Mexico tropical low by 120 hours… northeast part of the tropical low evolves into a tropical cyclone in the longer range.

**For area of interest #2… surface low consolidates near 29.5N-79.5W at 42 hours… while drifting north ahead of large frontal low arriving from western North America opens to a surface trough near 30N-78W at 78 hours

**For area of interest #3… no development shown

**Strong tropical wave forecast to emerge from Western Africa at 48 hours… tropical cyclone formation suggested at 108 hours near 15N-28W… located at 15N-31W at 120 hours


1200Z ECMWF Model Run…

*For Hurricane Larry… located at 22.5N-54W at 120 hours

**For area of interest #1… no development shown

** For area of interest #2… no development shown

**For area of interest #3… no development shown

**Strong tropical wave forecast to emerge from Western Africa at 48 hours… tropical cyclone formation suggested just southeast of the Republic of Cabo Verde Islands at 96 hours… weakens to a remnant low just west-southwest of the islands at 120 hours


1800Z GFS Model Run...

**For Hurricane Larry… located at 25N-59W at 120 hours

**For area of interest #1… evolves into broad western Gulf of Mexico tropical low by 120 hours… northeast part of the tropical low evolves into a compact circulation just offshore of the Florida panhandle at 144 hours

**For area of interest #2… no development shown

**For area of interest #3… no development shown


1200Z NAVGEM Model Run...

**For Hurricane Larry… located at 23.5N-54W at 120 hours

**For area of interest #1… no development shown

** For area of interest #2… no development shown

**For area of interest #3… no development shown

**Strong tropical wave forecast to emerge from Western Africa by 72 hours… consolidates into tropical low just south of the Republic of Cabo Verde Islands at 120 hours

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