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 

  • NCHurricane2009


*******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 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 dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.**********

...SUNDAY SEPTEMBER 5 2021 11:58 PM EDT...

Satellite image as of 1840Z. Areas of interest circled in yellow are not mentioned in the NHC tropical weather outlook. Areas circled in a green dashed line are in the NHC 5-day tropical weather outlook. Areas circled in a solid green line are in the NHC 2-day tropical weather outlook:

NHC TAFB Surface Analysis 1200Z:

GFS Model Upper-Level Winds 1200Z:

See Larry section below for more info on the only currently active tropical cyclone in the Atlantic basin. See area of interest section below for an update on the tropical disturbance that has entered the southern Gulf of Mexico from the Yucatan peninsula. Elsewhere… tropical cyclone formation in the Atlantic is not expected for the next few days.

MAJOR HURRICANE LARRY… Despite dealing with western outflow disruption from upper vorticity to the west that has reduced the intensity of the thunderstorm bands west of the eye… and even causing the western eye wall to be torn open at times on colorized infrared Satellite… Hurricane Larry has managed to maintain category 3 strength with 120 to 125 mph max sustained winds over the last 24 hours. In addition… the latent heat release of Larry is expected to weaken the cool core upper vorticity over the next 24 hours which may help the hurricane establish better upper outflow to the west. Despite this… my intensity forecast overall is the same as previous… it’s just that I have slightly increased the amount of time Larry may be a low-end category 4. This is because Larry may need some time to repair it’s western side… plus the 11 PM EDT NHC advisory package stated that some Satellite data suggests Larry may be undergoing another eye wall replacement cycle… thus at this time I do not see Larry getting stronger than my previous Forecast or current forecast shown below. By day 4 Larry is forecast to move into waters with less heat content… below 28 deg C… and so I taper down the forecast intensity back to category 3 by then.

Regarding track… my forecast points are nudged north and east due to the current position of the hurricane. Larry has seen some increase in the north angle of its track as it is strong/tall enough to be influenced by the upper vorticity mentioned in the previous paragraph. Over the next 24 hours I slightly relax the north angle as the upper vorticity is forecast to weaken… something else also noted in the prior paragraph. By 48+ hours a north turn is imminent due a large surface ridge weakness to be induced by a pair of frontal systems to eject from North America behind Ida’s remnants… the first of which is currently over eastern Canada. The north turn will also be aided by what will be left of the aforementioned upper vorticity…to be located east of the Bahamas. It should be noted the western convergence zone of the upper trough tied to the first of the ejecting frontal systems (the system currently over eastern Canada) will produce a passing surface ridge in the northwest Atlantic which will keep Larry’s track on a slight west lean thru day 3… and prevent an immediate east turn at day 4. On day 5… Larry is expected to accelerate north-northeast in the flow ahead of the second of the two frontal systems. 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 tied to the second frontal system… with the divergence zone then helping to make the weakening rate of Larry slow after it loses tropical character. This is why by day 5 I have Larry at strong category 1 Hurricane force after transitioning to a non-tropical frontal cyclone supported by the upper trough. This could result in a significant frontal cyclone that could affect Newfoundland… more on that in the bulletins below:

**Larry will bring coastal sea swells to the northern Lesser Antilles over the next couple of days.

**Coastal sea swells will reach Bermuda by 48 to 96 hours… and become vigorous as the hurricane passes just to the east. The potential for the west side of the hurricane to pass over the island is dropping as the forecast track has remained unchanged

** Coastal sea swells will reach the northeastern US and Atlantic Canada coasts in 72 to 120 hours. As mentioned above… dynamics could converge in a way that helps Larry maintain strength after losing tropical characteristics while heading toward Newfoundland just after day 5… which could result in coastal storm surge and gusty winds with damage potential. Interests in Newfoundland should monitor the progress of Larry.

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

0 Hr Position (0000Z Sep 6)…125 mph maximum sustained wind major hurricane centered at 20.7N-51W

IOH 24 Hr Forecast (0000Z Sep 7)… 130 mph maximum sustained wind major hurricane centered at 22.2N-55.5W

IOH 48 Hr Forecast (0000Z Sep 8)… 135 mph maximum sustained wind major hurricane centered at 26N-57.5W

IOH 72 Hr Forecast (0000Z Sep 9)… 135 mph maximum sustained wind major hurricane centered east-southeast of Bermuda at 30.5N-59.5W

IOH 96 Hr Forecast (0000Z Sep 10)… 120 mph maximum sustained wind major hurricane centered east-northeast of Bermuda at 35.5N-60.5W

IOH 120 Hr Forecast (0000Z Sep 11)… 95 mph maximum sustained wind non-tropical frontal cyclone centered south of Newfoundland at 44N-57W

AREA OF INTEREST #1… The surface trough of low pressure over the Yucatan Peninsula has reached the peninsula’s northwest coast. The future evolution of this system lies with the upper vorticity currently in the Gulf of Mexico. The cool core upper vorticity is already in the process of weakening to a western Gulf vortex while it remains cut-off from high-latitude cold air… and an area of showers and thunderstorms over the northwestern parts of the peninsula and offshore waters indicates this system is reforming northward while transitioning into a system aided by the eastern divergence zone of the developing upper vortex. By 2+ days the modeling agrees on shifting this system east-northeast toward the Florida panhandle in the flow ahead of the pair of frontal systems to cross North America in ex-Ida’s wake (the first system of the pair is already currently crossing eastern Canada). On the one hand… model support showing development has declined with the CMC dropping development and the ECMWF and GFS showing a weaker tropical cyclone. On the other hand… satellite animation reveals a cyclonic turning in the low clouds and the aforementioned area of concentrated showers and thunderstorms. Therefore I have opted not to raise or lower odds of development… thus keeping them at a peak of 40%. However I have lowered short-term development odds as this system does not have enough organization to suggest tropical cyclone formation in the next 24 hours. Odds are dropped to 0% by day 3 due to the inland location of my forecast track during that timeframe.

Models have increasingly suggested that this system could re-emerge into water at a location offshore of the southeast US by days 4 and 5 and make a second attempt at developing… therefore I have extended my updated outlook to the 5 day window. This development would be facilitated by the eastern divergence zone of the upper trough associated with the second of the two frontal systems to traverse North America in the wake of ex-Ida. However in the event the upper trough is not amplified enough to keep westerly shear levels down… I have opted for lower-than-40% development odds in that timeframe (only reaching 25% by day 5).

**While it is concerning that this system in 48 hours will be near southeast Louisiana where Hurricane Ida recovery efforts are ongoing… it appears this system has lower development potential than Ida did as it will have to negotiate Gulf of Mexico upper vorticity as discussed above… followed by possible westerly shear on the South side of the North American frontal systems’ upper troughs while nearing the US Gulf coast. Also the steering flow suggests a track that turns this system east away from the area such that landfall is unlikely here

**There is a medium chance of tropical storm conditions (gusty winds… some coastal sea swells… and heavy rains with flash flooding potential) for the Alabama and Florida panhandle coastal regions by Wednesday morning. The heavy rainfall potential may spread across southern Georgia… northeast Florida… and southern South Carolina through Thursday.

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

IOH 24 Hr Outlook (0000Z Sep 7)… 5% chance of tropical cyclone formation (Gulf of Mexico near 25N-91W)

IOH 48 Hr Outlook (0000Z Sep 8)… 40% chance of tropical cyclone formation (offshore of southeast Louisiana near 27.8N-89.5W)

IOH 72 Hr Outlook (0000Z Sep 9)… 0% chance of tropical cyclone formation (southern Alabama/Georgia border at 31N-85W)

IOH 96 Hr Outlook (0000Z Sep 10)… 20% chance of tropical cyclone formation (just offshore of southern South Carolina near 32N-80.2W

IOH 120 Hr Outlook (0000Z Sep 11)… 25% chance of tropical cyclone formation (south of Cape Hatteras North Carolina near 34.2N-75.5W)

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