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 #98

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


...THURSDAY SEPTEMBER 2 2021 1:08 AM EDT...

See Kate section below for an update on what was the tropical Depression in the open central Atlantic. 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.


REMNANTS OF KATE... While pushed northwest around the west side of the Atlantic surface ridge… Kate was making a run toward better upper winds while moving away from the western Atlantic upper vorticity string and toward mid-latitude upper ridging. Earlier in the day some thunderstorm activity increased over the center of Kate as the tropical depression found a zone of upper outflow on the south side of the upper ridging as illustrated in the above upper air chart… with the suppressive upper convergence zone between the northeast side of the upper vorticity and southeast side of the upper ridging being further east. However by afternoon the thunderstorm activity of Kate dwindled and a research plane sampling Kate and the surrounding atmosphere found Kate had spun down while no longer having a closed surface circulation. Thus at 5 PM EDT the NHC declared Kate a remnant low pressure area. It appears Kate lost the competition with the increasing surface pressures as the Atlantic surface ridge is strengthening due to the aforementioned adjacent upper convergence zone… and none of the models

redevelop Kate within the west side of the surface ridge despite the potential for low shear and upper outflow beneath the mid-latitude upper ridging. Even the NAVGEM which had the strongest initialization of Kate’s remnants has the remnants gradually fade away. This is my planned final statement on Kate as it is no longer a tropical cyclone… unless it later shows signs of making a comeback with the aid of the mid-latitude upper ridging.


TROPICAL STORM LARRY… Tropical depression twelve has surprised in two ways in the last 24 hours… first by rapidly strengthening into a high end Tropical Storm Larry already poised to become a hurricane… and also by accelerating west of previous forecasts while apparently finding stronger flow on the south side of the Atlantic surface ridge. Between 5 PM and 11 PM EDT Larry took a pause in strengthening while the core of thunderstorm bands relaxed while the strongest thunderstorm activity became concentrated in a western outer band. However very recently a strong thunderstorm burst is seen developing just southwest of the center in the core structure of the storm… and Larry has been and for some time will be aided by the low shear and upper outflow of the tropical Atlantic upper ridge… thus it seems like Larry becoming a hurricane is a sure thing. Due to the current higher intensity compared to the previous forecast… I now forecast a category 4 hurricane by day 3. I drop the intensity down to category 3 by days 4 and 5 as Larry potentially sees some western outflow blockage while approaching the upper vorticity string that has been near Kate.

The westward acceleration of Larry appears to be due to the fact the adjacent tropical wave to the east has become so weak that it cannot hold Larry back as some of the modeling… particularly the GFS… showed in days past. The latest model guidance and my updated forecast track have followed suit with a westward adjustment. 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 72 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 upper vorticity that is currently near Kate. However unlike yesterday… the GFS has not shown this upper vorticity being re-enforced by the upper trough tied to Ida’s remnants… and so I lean South of the NHC track Forecast with the tendency of the cool core upper vorticity being weak while remaining cut-off from high-latitude upper troughs/cold air… with Larry’s thunderstorm latent heat release also potentially putting a damper on the upper vorticity. On the Forecast track below… and even with the NHC track Forecast… Larry could bring coastal sea swells to the northern Lesser Antilles by day 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 Forecast (0000Z Sep 2)… 70 mph maximum sustained wind tropical storm centered at 12.5N-30W

IOH 24 Hr Forecast (0000Z Sep 3)… 90 mph maximum sustained wind hurricane centered at 13N-36W

IOH 48 Hr Forecast (0000Z Sep 4)… 115 mph maximum sustained wind major hurricane centered at 13.5N-42W

IOH 72 Hr Forecast (0000Z Sep 5)… 145 mph maximum sustained wind major hurricane centered at 14N-47W

IOH 96 Hr Forecast (0000Z Sep 6)…125 mph maximum sustained wind major hurricane centered at 16N-51W

IOH 120 Hr Forecast (0000Z Sep 7)… 125 mph maximum sustained wind major hurricane centered east-northeast of the Lesser Antilles at 18N-55W

AREA OF INTEREST #1… The tropical wave of

low pressure that a few days ago spawned Kate is now in the Central Caribbean Sea merging with a surface low pressure area offshore of Nicaragua that has formed due to the ongoing poleward upper outflow channel that streams into the current west Atlantic upper vorticity string. The upper vorticity string is Forecast to weaken while remains cut-off from high-latitude cold air. The only global models today that shows this tropical low pressure system developing are the CMC and GFS… however they show development in the western Gulf of Mexico and in the longer range beyond day 5. I now do not forecast this system to develop before making it to the Yucatan peninsula for the following reasons: (1) the GFS continues to show a slower breakup of the west Atlantic upper vorticity string and keeps some upper vorticity over the west side of this system as it moves across the western Caribbean… (2) the thunderstorm activity with this system is currently not impressive… instead it is quiet weak and scattered. I finally show 10% odds of development in the western Gulf of Mexico by day 5… after the GFS shows the west extent of Gulf of Mexico upper vorticity to be left behind by the upper trough tied to Ida’s remnants breaking off into an upper vortex whose eastern divergence zone may aid this system… but in the process perhaps giving this system a more subtropical appearance as well.


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 forecast to support 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 northern Gulf by days 4 and 5… helping to push this system into the western Gulf after crossing the Yucatan peninsula. A north turn 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 at the end of the previous paragraph.

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

IOH 24 Hr Outlook (0000Z Sep 3)…0% chance of tropical cyclone formation (central Caribbean Sea near 14.5N-80W)

IOH 48 Hr Outlook (0000Z Sep 4)…0% chance of tropical cyclone formation (western Caribbean Sea near 16.2-85W)

IOH 72 Hr Outlook (0000Z Sep 5)… 0% chance of tropical cyclone formation (Yucatan peninsula near 19N-89.5W)

IOH 96 Hr Outlook (0000Z Sep 6)… 0% chance of tropical cyclone formation (southwestern Gulf of Mexico near 21N-92.5W)

IOH 120 Hr Outlook (0000Z Sep 7)… 10% chance of tropical cyclone formation (southwestern Gulf of Mexico near 24N-92.5W)


AREA OF INTEREST #2… Ida’s remnant low and it’s amplifying supporting upper trough is forecast to drive a cold front into the west Atlantic in the coming days. 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 US to be caused by the warm sector of the next frontal system to swing across North America.


Today’s model runs have moreso hinted at a surface low pressure area coalescing from the tail end of the front… with the CMC taking the lead while presenting the most defined low pressure… and so I have raised peak development odds to 20%. The updated forecast track follows the model consensus… showing the surface low pressure area progress on a slow anticyclonic Loop turn thru 96 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 day 5 a turn to the northeast is expected in the flow ahead of the next frontal system to swing across North America. The odds are trimmed down slightly at day 5 due to possible westerly shear from the approach of the upper trough associated with the next frontal system.

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

IOH 24 Hr Outlook (0000Z Sep 3)…0% chance of tropical cyclone formation (offshore of the southeastern US near 31N-77.5W)

IOH 48 Hr Outlook (0000Z Sep 4)…10% chance of tropical cyclone formation (offshore of the southeastern US near 30N-76W)

IOH 72 Hr Outlook (0000Z Sep 5)…20% chance of tropical cyclone formation (offshore of northeast Florida near 29.5N-78W)

IOH 96 Hr Outlook (0000Z Sep 6)… 20% chance of tropical cyclone formation (offshore of the southeastern US near 30.5N-78.5W)

IOH 120 Hr Outlook (0000Z Sep 7)… 15% chance of tropical cyclone formation (offshore of North Carolina near 33N-76.5W)


...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 Tropical Storm Larry… located at 21N-51.5W at 120 hours as a hurricane

**For area of interest #1… consolidates into a tropical low in the Bay of Campeche just offshore of Veracruz near 20.5N-96W at 126 hours

**For area of interest #2… surface low consolidates near 30N-77.5W at 54 hours… while drifting north ahead of large frontal low arriving from western North America opens to a surface trough near 31N-77.5W at 114 hours.

**Strong tropical wave forecast to emerge from Western Africa at 72 hours… consolidates into a tropical low pressure over the southern Republic of Cabo Verde Islands by 120 hours.


1200Z ECMWF Model Run…

*For Tropical Storm Larry… located at 21.5N-52.5W at 120 hours as a hurricane

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

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


1800Z GFS Model Run...

**For Tropical Storm Larry… located at 21.5N-55W at 120 hours as a hurricane

**For area of interest #1… consolidates into a tropical low offshore of the Texas/Louisiana border at 168 hours… tropical cyclone formation suggested just after.

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


1200Z NAVGEM Model Run...

**For Tropical Storm Larry… located at 20N-52.5W at 120 hours as a hurricane

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

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

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