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 #106

*******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 10 2021 3:34 AM EDT...

See Larry section below for more info on the only currently active tropical cyclone in the Atlantic basin. See Mindy section below for a final statement on what was a short-lived tropical storm. See area of interest sections below for areas being monitored for future tropical development in the Atlantic.

HURRICANE LARRY… Larry has continued to slowly weaken and is now a category 1 hurricane while passing just east of Bermuda. The southern side of the hurricane lacks thunderstorm activity… a potential sign of the hurricane continuing to be disrupted by dry sinking air generated by the convergence zone of the western Atlantic upper vortex to the southwest. Once again my updated intensity forecast is trimmed down.


Regarding track… Larry going forward is expected to accelerate north-northeast in the flow ahead of the approaching eastern Canada frontal low and its upper trough. The acceleration will give little time for Larry to weaken over cooler waters before it aligns with the supportive eastern divergence zone of the upper trough… with the divergence zone then helping to make the weakening rate of Larry slow after it loses tropical character. This is why in 24 hours I have Larry at category 1 hurricane force even after transitioning to a non-tropical frontal cyclone supported by the upper trough. This is expected to result in Larry crossing southeastern Newfoundland as a powerful frontal cyclone… more on that in the bulletins below:


**Coastal sea swells for Bermuda will begin to relax once the Hurricane Larry accelerates to the north-northeast and away.

**Coastal sea swells will reach the northeastern US and Atlantic Canada coasts through the next 24 hours. As mentioned above… dynamics could converge in a way that helps Larry maintain strength after losing tropical characteristics while heading toward Newfoundland by tomorrow night… which is expected to result in coastal storm surge and gusty winds with damage potential. The greatest risk for wind impact is in southeastern Newfoundland based on the latest model projections of Larry’s remnant cyclone. I have adjusted the 24 hour forecast point closer to Newfoundland due to the GFS model insisting on a slightly faster track… therefore weather conditions in southeastern Newfoundland could deteriorate as soon as tomorrow night.

** Southeastern Greenland and Iceland will see coastal sea swells from the remnant cyclone of Larry by 2 to 4 days…

with the possibility of strong winds for southeastern Greenland.

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

0 Hr Position (0000Z Sep 10)… 90 mph maximum sustained wind hurricane centered northeast of Bermuda at 34.2N-62.3W

IOH 24 Hr Forecast (0000Z Sep 11)… 75 mph maximum sustained wind non-tropical frontal cyclone centered just south of Newfoundland at 45N-57W


TROPICAL DEPRESSION MINDY (RECENTLY DOWNGRADED TO REMNANTS OF MINDY)… Mindy weakened to a tropical depression centered over southern Georgia early on Thursday due to ongoing travel on land… and after delivering heavy rains over the coastal Carolinas has zipped offshore into the western Atlantic. Mindy is being swept east-northeast by the eastern Canada frontal low and its upper trough. The upper trough is not particularly friendly to Mindy while producing southwesterly shear… and also the upper divergence zone in the southern part of the upper trough is linear without a maximum… which as of 11 PM EDT caused Mindy to elongate into a remnant low without a defined center similar to a typical surface frontal zone supported by a mid-latitude upper trough. In fact the frontal zone just west of Mindy… also supported by the linear divergence zone of the upper trough… could soon absorb Mindy’s remnants. This is my final statement on Mindy on this blog as it is no longer a tropical cyclone

and the wind shear is making conditions hostile for tropical redevelopment.


AREA OF INTEREST #1… A tropical wave of low pressure over Western Africa is nearing the eastern tropical Atlantic with a large area of strong and somewhat organized thunderstorm activity. The upper trough associated with the current north Atlantic frontal low is Forecast to amplify in the east Atlantic as the warm Central Atlantic upper ridge amplifies in what will be the warm sector of Larry’s remnant cyclone. This will result in the frontal cyclone associated with the upper trough staying toward the south… which will weaken the east side of the Atlantic surface ridge enough to bend the track of this tropical wave more west-northwest toward the Republic of Cabo Verde Islands. Once the tropical wave moves past the weaker part of the surface ridge… I forecast a more west track by 96+ hours.


The only negative factor against this wave is dry Saharan air lurking to the north… with the initial north angle in bringing the wave closer to the dry air. However with the ongoing large field of thunderstorms… chances are that the wave could wall off the dry air. In addition model support showing the wave developing remains… and so I agree with the NHC Outlook’s high 70% chance of tropical cyclone formation. I trim odds of development to 50% by days 3 and 4 as the forecast track would bring this system closer to upper vorticity to be deposited by the upper trough… resulting in some westerly shear imparted by the upper vorticity. The GFS has returned to a more south position with this upper vorticity… so shear remains highly probable. However by day 5 the cool core upper vorticity begins to fade while remaining cut-off from high-latitude cold air… and therefore I increase development odds back up to 60% by then.


With models agreeing on this wave likely developing while aiming for the Republic of Cabo Verde Islands in 2 to 3 days… interests here should be aware of this system and anticipate possible tropical storm conditions arriving this weekend.

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

IOH 24 Hr Outlook (0000Z Sep 11)… 0% chance of tropical cyclone formation (inland near west coast of Africa in vicinity of 13.5N-15W)

IOH 48 Hr Outlook (0000Z Sep 12)… 70% chance of tropical cyclone formation (just east-southeast of the Republic of Cabo Verde Islands near 15N-21W)

IOH 72 Hr Outlook (0000Z Sep 13)… 50% chance of tropical cyclone formation (west of the Republic of Cabo Verde Islands near 16.5N-27W)

IOH 96 Hr Outlook (0000Z Sep 14)… 50% chance of tropical cyclone formation (eastern tropical Atlantic near 17N-32W)

IOH 120 Hr Outlook (0000Z Sep 15)… 60% chance of tropical cyclone formation (eastern tropical Atlantic near 17.5N-37W)


AREA OF INTEREST #2…Scattered thunderstorm activity across the Gulf of Mexico and western Caribbean Sea is being generated by the eastern divergence zone of western Gulf upper vorticity. As the cool core vorticity weakens while remaining cut-off from high-latitude cold air… the upper vorticity is forecast to become replaced by expanding upper ridging to the south that will provide a lower shear and upper outflow environment. Modeling has been mixed on the end result… with some runs showing a well-defined tropical low pressure forming in the Bay of Campeche as the upper vorticity dissipates… or showing a broad lowering of surface pressure across the Gulf of Mexico due to the upper air pattern (initial eastern divergence zone of the upper vorticity… followed by the outflow of expanding upper ridging). The south and west side of the currently approaching central US surface ridge is forecast to push whatever low pressure area develops to the west and north in the western Gulf of Mexico… with the approach of a frontal system from western North America helping to aid the north turn by day 5.


I increase odds of development above 0% during the timeframe the shearing western Gulf upper vorticity is dissipated. However I am not quiet as high as the NHC outlook odds at this time as the thunderstorm activity in the region is scattered and unorganized… and model support showing explicit tropical cyclone formation is scarce. Plus in the event a more broad low pressure area does form in the Gulf… it would require time to consolidate into a tropical cyclone and may run out of time to do so before making landfall in Mexico or Texas. The odds of development are lowered at day 5 due to landfall and also possible westerly shear as this system arrives to the north side of the forecast upper ridging. The positions in the outlook below are a comprise between the more southern Bay of Campeche compact tropical low solutions and more northern Gulf of Mexico broad tropical low pressure solutions. Because it is hard to know exactly where a tropical low pressure might consolidate… interests across Veracruz and Tamaulipas provinces in Mexico as well as the south Texas coast should be aware of this forecast situation… and check back in for updates in the coming days.

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

IOH 24 Hr Outlook (0000Z Sep 11)… 0% chance of tropical cyclone formation (east coast of the Yucatan peninsula near 18N-87.5W)

IOH 48 Hr Outlook (0000Z Sep 12)… 0% chance of tropical cyclone formation (northwestern Yucatan peninsula near Yucatan peninsula near 20N-89W)

IOH 72 Hr Outlook (0000Z Sep 13)… 15% chance of tropical cyclone formation (southwestern Gulf of Mexico near 22.5N-92.5W)

IOH 96 Hr Outlook (0000Z Sep 14)… 30% chance of tropical cyclone formation (offshore of the Texas/Mexico border near 26N-95W)

IOH 120 Hr Outlook (0000Z Sep 15)… 10% chance of tropical cyclone formation (coastal Texas near 28.2N-96.5W)

...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 Hurricane Larry… remnant frontal cyclone quickly passes over southeast Newfoundland at 42 hours…approaching Canadian frontal cyclone to the west of Larry’s remnant becomes the dominant and absorbs Larry by 54 hours.

**For Tropical Depression Mindy…weakens to remnant low west-northwest of Bermuda at 42 hours… remnant low dissipates near 39N-61W at 90 hours

**For area of interest #1… forecast to emerge from Africa at 30 hours… passes just north of the Republic of Cabo Verde Islands as a well-defined tropical low at 90 hours… tropical low weakens while reaching 20N-26W at 120 hours

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


1200Z ECMWF Model Run…

**For Hurricane Larry… remnant frontal cyclone quickly passes over southeast Newfoundland between 24 and 48 hours… centered between Greenland and Iceland at 120 hours.

**For Tropical Depression Mindy…weakens to a remnant trough northwest of Bermuda at 48 hours

**For area of interest #1… forecast to emerge from Africa at between 24 and 48 hours…tropical cyclone formation suggested just northeast of the Republic of Cabo Verde Islands at 72 hours… weakens to a tropical depression that turns north and reaches 23N-25.5W at 120 hours

**For area of interest #2… tropical cyclone formation shown in the western Bay of Campeche at 20N-95.5W at 72 hours… moves north along the northeast Mexico and south Texas coast from 96 thru 120 hours.


1800Z GFS Model Run...

**For Hurricane Larry…remnant frontal cyclone quickly passes over southeast Newfoundland at 33 hours…approaching Canadian frontal cyclone to the west of Larry’s remnant becomes the dominant and absorbs Larry by 48 hours.

**For Tropical Depression Mindy… weakens into a remnant low west-northwest of Bermuda at 33 hours… remnant low dissipates near 37N-61W at 81 hours

** For area of interest #1… forecast to emerge from Africa at 24 hours… passes over the Republic of Cabo Verde Islands at 63 hours as a broad low pressure spin… broad low pressure reaches 14N-32W at 120 hours

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


1800Z NAVGEM Model Run...

**For Hurricane Larry…remnant frontal cyclone quickly passes over southeast Newfoundland at 42 hours… located just offshore of southeastern Greenland by 120 hours.

**For Tropical Depression Mindy…initialized as a surface trough which dissipates northeast of Bermuda at 60 hours

** For area of interest #1… forecast to emerge from Africa at 24 hours… tropical cyclone formation shown northeast of the Republic of Cabo Verde Islands at 78 hours… tropical cyclone is short-lived with remnant low located just north of the islands at 120 hours.

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

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