Disturbance in western Caribbean taking shape and threatens Gulf coast

The disturbance that I mentioned last Wednesday is indeed organizing.  It's still not a tropical cyclone, but it has been designated as Invest 97L as of Sunday evening.  That designation triggers regional hurricane models to run on it, and facilitates aircraft reconnaissance data to be assimilated into the models.  Models are in excellent agreement on it forming soon (perhaps even later today) and intensifying into the season's fifth hurricane by Tuesday night or Wednesday morning as it enters the Gulf of Mexico.  The next name on the list is Helene.

Remarkably, this is likely going to be the fourth hurricane landfall along the US northern Gulf coast this season... Beryl, Debby, and Francine were the first three.  The timing of landfall is pretty tight among models: Thursday afternoon-evening.  But of course storm surge, heavy rain, and damaging wind will arrive before that and will be a concern from Jamaica to Cuba and the Yucatan, then the west coast of the Florida peninsula, then the Florida panhandle.

These model forecast tracks all agree on the worst case: the center remaining over the ocean the entire time and passing over the very warm Loop Current in the eastern Gulf of Mexico prior to landfall in the Florida panhandle.

A significant concern is the fairly high probability of it rapidly intensification in the Gulf of Mexico.  Not only is the sea surface temperature along its path a toasty 30-31C, the warm water runs deep, producing extremely high ocean heat content... an endless source of fuel.  A warm ocean isn't everything when it comes to hurricanes, but it's a lot.

https://bmcnoldy.earth.miami.edu/tropics/sectors/

After landfall, the storm surge threat diminishes quickly, but the threat of flash flooding from heavy rain will continue inland for a few days.  This map shows the latest rainfall forecast over the next week: an enormous area covered by 3"+ totals.  This will evolve as the storm's forecast evolves, but you can find the most recent version at https://www.wpc.ncep.noaa.gov/#page=qpf.

This could become the season's 8th named storm on Tuesday (September 24), the 5th hurricane on Wednesday (September 25), and if the models are right, the season's 2nd major hurricane on Wednesday or Thursday.  The average dates for those milestones are September 9, September 28, and September 19.

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