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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 forecast 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.**********

...MONDAY AUGUST 28 2023 12:35 PM EDT...

Satellite image as of 1550Z showing Hurricane Franklin and Tropical Storm Idalia as both storms have strengthened more than prior forecasts:

This special update is due to the faster than expected intensification of Tropical Storm Idalia and Hurricane Franklin ahead of my next full update scheduled for late today. Due to the large swath of land areas in Idalia's path... I have provided a full update for Idalia in this post... see Tropical Storm Idalia section below for more information.

Hurricane Franklin positioned between the Bahamas and Bermuda became the season's first major (category 3+) hurricane while achieving 115 mph maximum sustained winds as of 5 AM EDT. Aircraft reconnaissance this morning caught Franklin during a rapid intensification episode while measuring category 4 130 mph maximum sustained winds and 942 mb central pressure as of 7:35 AM EDT... and 145 mph maximum sustained winds and 937 mb central pressure as of 9 AM EDT. Conditions are favorable for Franklin to further strengthen and make a run for category 5 status over the next 24 hours... before the hurricane encounters increasing shear from an upper trough to approach from the Great Lakes region of North America. Franklin is expected to recurve northeastward with this upper trough after 24 hours and still pass north of Bermuda instead of over the island. The primary impact to land from the hurricane is still expected to be wide reaching coastal surf reaching Bermuda... the US mid-Atlantic and northeastern shores... and Atlantic Canada (Newfoundland and Nova Scotia) during this week... see the following link for more information on Franklin...

Refer to the following link for information on the open central Atlantic tropical low pressure designated as area of interest #23 in previous full update #64... or Invest 92-L by the National Hurricane Center...

Refer to the following link for information on the tropical wave of low pressure now heading for the eastern tropical Atlantic from western Africa... designated as area of interest #27 in previous full update #64...

Refer to special update #64A at the following link for information on the low-latitude central Atlantic tropical wave of low pressure currently heading for the Lesser Anilles and Caribbean...

TROPICAL STORM IDALIA... Over the last 36 hours tropical depression ten centered in the northwestern corner of the Caribbean Sea intensified into Tropical Storm Idalia. The thunderstorms of the new tropical storm have tended to be lopsided to the south as upper vorticity positioned along the US Gulf coast has been restricting the storm's northern outflow. Despite this... a particularly strong thunderstorm burst has been ongoing near the center of rotation since 0000Z which has allowed Idalia to strengthen more than previous forecasts... and my updated intensity forecast below as well as the NHC's have notably increased over the last 12 hours. Idalia has been waiting for the current US Gulf coast upper vorticity to merge with an upper trough to dive into the Great Lakes region of North America... with the merger resulting in a lengthy upper trough that pulls this system north-northeast. The northward motion has recently begun with the center advancing from about 20N to 20.5N latitude this morning... suggesting the US Gulf coast upper vorticity has begun pulling the storm even though the lengthy upper trough has not materialized yet. Noting that during the forecast period the north part of the lengthy upper trough continues east toward Hurricane Franklin while the south part stays behind while pinned in place by an amplifying warm core western US upper ridge induced by warm southerly flow ahead of a western US frontal system (the western US surface frontal system will materialize over the next 48 hours while induced by the divergence zone of the current northeastern Pacific upper vortex slated to make landfall). A recent worrying model trend shows wind shear across Idalia dropping while it moves north-northeast across the eastern Gulf of Mexico in the 24 to 48 hour window... as the western US upper ridge is now shown to be amplified enough to cause the southern upper trough fragment to slighlty retrograde westward and more out of the way. My updated intensity forecast below for the next 24 hours shows Idalia strengthening further despite ongoing light shear and ouflow disruption imparted by the US Gulf coast upper vorticity as it has already been able to strenghten in this environment... with a faster intensification rate between 24 and 48 hours in the decreasing shear into a major category 3 hurricane... all in all similar to the NHC forecast from 11 AM EDT. After landfall in the southeastern United States through the northwestern Florida peninsula... Idalia should transition into a less tropical feature supported by the eastern divergence zone of the southern upper trough fragment... therefore this system is expected to only gradually weaken despite land interaction. By day 4... the non-tropical remnants are likely to turn eastward and offshore from the North Carolina coast in the westerly flow between the northern and southern upper trough fragments... and with the westerly flow being neutral (no convergence but also lacking supportive divergence) the remnants are likely to also weaken. Regeneration back to a tropical system in the day 4 window would also be challenged by the shear caused by the upper westerlies.

Regarding impact to land areas:

(1) Heavy rainfall and gusty winds have ended for the northeastern Yucatan peninsula... however coastal surf will be a concern until Idalia moves northward and away after 24 hours.

(2) Current satellite imagery shows the Cayman Islands are not embedded in heavy rainfall... however the return of heavy rainbands later today cannot be ruled out. Coastal surf will remain a concern until Idalia pulls northward and away after 24 hours.

(3) For western Cuba... although coastal surf has likely increased heavy rainfall and gusty winds have not arrived yet due to the storm structure where the heavy weather has been lopsided to the south as noted above. However conditions will deteriorate later today with coastal surf and winds likely reaching category 1 hurricane force near where the center passes because Idalia is expected to strengthen further... as such final preparations here should be rushed to completion.

(4) I recommend interests across the Florida Keys... the Florida peninsula... the eastern panhandle of Florida... and southeastern Georgia finish preparations for Idalia by tomorrow morning as gusty winds... coastal surf... and heavy rainfall will be overspreading the region by Wednesday. Due to the increase in the intensity forecast... the wind and coastal storm surge threat for the northwestern part of the Florida peninsula and far eastern part of the Florida panhandle has increased to life threatening levels... evacuate if ordered or recommended to do so by news media and your local government. Notable wind damage will likely spread inland across the northeastern part of the peninsula and southeastern Georgia.

(5) I recommend interests across South Carolina... eastern and central North Carolina... southeastern Virginia... and southeast Maryland begin preparing today for possible gusty winds... coastal surf... and heavy rain to arrive by Thursday.

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

0 Hr Position (1200Z Aug 28)... 65 mph maximum sustained wind tropical storm centered just offshore of the northeastern Yucatan peninsula at 20.6N-85.2W

IOH 24 Hr Forecast (1200Z Aug 29)... 90 mph maximum sustained wind hurricane centered just north of western Cuba at 22.5N-84.5W

IOH 48 Hr Forecast (1200Z Aug 30)... 120 mph maximum sustained wind major hurricane centered just offshore of the northwest Florida peninsula coast at 29N-83.2W

IOH 72 Hr Forecast (1200Z Aug 31)... Frontal cyclone centered over eastern North Carolina at 35N-77.5W

******National Hurricane Center ( official forecast as of 11 AM EDT*****************************

Peak Strength (1200Z Aug 30)... 115 mph maximum sustained wind major hurricane centered just offshore of the northwest Florida peninsula coast at 29.2N-83.5W

5-day position (1200Z Sep 2)... 60 mph maximum sustained wind tropical storm centered northwest of Bermuda at 34.5N-67W

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