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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 

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*******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 12:30


Satellite image as of 2350Z. 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 sections below for all areas being monitored for tropical cyclone formation in the Atlantic.

Elsewhere… satellite imagery shows a tropical wave of low pressure exiting Western Africa while also entangled with a surge of dry Saharan air… and model support showing this wave developing has abated. This tropical wave is therefore not being considered an area of interest for tropical development.

MAJOR HURRICANE LARRY… Larry has been prevented from reaching category 4 strength for two reasons… (1) an eye wall replacement cycle which has ended as indicated by a much larger eye on satellite pictures over the last several hours (2) some southwesterly shear and western outflow blockage induced by the lingering central Atlantic upper vorticity as evidenced by less thunderstorm activity southwest of the large eye. Despite these developments… Larry did reach a still formidable peak of 125 mph max sustained winds before recently weakening to 120 mph as of 11 PM EDT due to the increase in shear. My updated intensity forecast is nudged downward… and brings Larry down to category 2 by 24 hours due to the shear. After that time… the latent heat release of Larry is Forecast to dissipate the shearing cool core upper vorticity… and so I project Larry making a run for low-end category 4 status by days 3 and 4. By day 5 Larry is forecast to move into waters with less heat content… below 28 deg C… and so I taper down the intensity to category 3 by then… with faster weakening likely after that time once Larry crosses the 26 deg C line which is the usual threshold for tropical development.

Regarding track… my short-term forecast points are nudged north from 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 lingering central Atlantic upper vorticity currently shearing the hurricane. I relax the north angle a bit by 48 hours as the upper vorticity is forecast to dissipate as noted in the prior paragraph… but by 72+ 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 north turn will also be aided by the current upper vorticity located northeast 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 central Canada) will produce a passing surface ridge in the northwest Atlantic which could keep Larry’s track on a slight west lean thru day 4… and prevent an immediate east turn at day 5. On the current forecast track… Larry will bring coastal sea swells to the northern Lesser Antilles and Bermuda by days 2 to 5. Interests in Bermuda should be aware the current forecast track brings Larry near Bermuda by days 4 and 5… and that any west shift in track could expose the island to tropical storm conditions under the west side of the hurricane.

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

0 Hr Position (0000Z Sep 5)… 120 mph maximum sustained wind major hurricane centered at 18N-48W

IOH 24 Hr Forecast (0000Z Sep 6)…110 mph maximum sustained wind hurricane centered at 20N-52W

IOH 48 Hr Forecast (0000Z Sep 7)… 125 mph maximum sustained wind major hurricane centered at 21.5N-57W

IOH 72 Hr Forecast (0000Z Sep 8)… 130 mph maximum sustained wind major hurricane centered at 25N-59W

IOH 96 Hr Forecast (0000Z Sep 9)… 135 mph maximum sustained wind major hurricane centered east-southeast of Bermuda at 30N-61W

IOH 120 Hr Forecast (0000Z Sep 10)… 120 mph maximum sustained wind major hurricane centered east-northeast of Bermuda at 35N-61W

AREA OF INTEREST #1The surface trough of low pressure over the Yucatan Peninsula over the last 24 hours had featured an increase in thunderstorms on its east side over Belize and the eastern half of the peninsula. As of now that activity has dissipated and been replaced by strong activity to the south over Guatemala where flash flooding rains cannot be ruled out. Also cyclonic turning in the low clouds over the Peninsula is visible in satellite animations. The future evolution of this system lies with upper vorticity in the Gulf of Mexico. The modeling still shows the cool core upper vorticity weakening to a small western Gulf vortex by 24+ hours while it remains cut-off from high-latitude cold air. The GFS… ECMWF… and CMC as of late show this disturbance reform northward while transitioning into a system aided by the eastern divergence zone of the forecast upper vortex. By 3+ 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 central Canada). Due to the growing model consensus that shows tropical cyclone formation as this system moves east-northeast toward the Florida panhandle… I have raised odds of development to 40%. Odds are dropped to 0% by day 4 due to the inland location of my forecast track during that timeframe.

While it is concerning that this system by day 3 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 for now would suggest a track toward the northeast US Gulf coast… to the east of the Hurricane Ida impact region.

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

IOH 24 Hr Outlook (0000Z Sep 6)… 0% chance of tropical cyclone formation (just offshore of the northwestern Yucatan peninsula near 21.2N-90.2W)

IOH 48 Hr Outlook (0000Z Sep 7)… 20% chance of tropical cyclone formation (Gulf of Mexico near 25N-91W)

IOH 72 Hr Outlook (0000Z Sep 8)… 40% chance of tropical cyclone formation (offshore of southeast Louisiana near 27.8N-89.5W)

IOH 96 Hr Outlook (0000Z Sep 9)… 0% chance of tropical cyclone formation (southern Alabama/Georgia border at 31N-85W)

AREA OF INTEREST #2No defined low pressure area with thunderstorms has formed offshore of the southeast US along the tail end of the front that extends from Ida’s remnant low… as the Gulf of Mexico upper vorticity has stayed further south and has not been able to generate an area of upper divergence over the tail end of the front. This also leaves the tail end of the surface front beneath suppressing upper convergence between the north side of the upper vorticity and southeast side of the upper ridge currently over eastern North America. This is my final statement on this area of interest on this blog as there is no development potential here.

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