30 August 2023

Idalia rapidly intensifies and makes landfall, Franklin weakening

I unfortunately did not have time to write a post on Tuesday, but a lot happened.  On Tuesday morning, Idalia had just become a Category 1 hurricane.  By Tuesday night it was a Category 2 hurricane and by Wednesday morning it was a Category 4 hurricane.  The peak wind speed increased by 58 mph in 24 hours: rapid intensification.  The storm took full advantage of the hyper-hot water temperatures in the eastern Gulf and minimal wind shear.

It made landfall in the Big Bend region of Florida, and the last time that part of the state experienced anything like this was 1896.  The peak winds dropped slightly as it was making landfall so it was technically a Category 3, but at that point it was too little too late.  That part of the state is extremely surge-prone because of the concave shape of the coastline, though it will be a while until we know the official peak storm surge values.

It's not over of course, as the storm is tracking over Georgia, then South Carolina and North Carolina, creating rainfall flooding and coastal flooding issues along the way.  The full suite of NHC products is available at https://www.nhc.noaa.gov/graphics_at5.shtml?start#contents, but this map shows the probability and timing of tropical storm winds.

Franklin has passed west of Bermuda and is now weakening as it heads into the north-central Atlantic. It will become a strong extratropical cyclone this weekend over the cold ocean there.

But wait, there's more!  Tropical Depression 11 formed on Tuesday morning, though it's not expected to be around long.  Ex-Gert is actually still lingering out there and also has a dim future.  Then there's a new easterly wave that just left Africa yesterday, Invest 94L.  That is strongly favored to develop by the models, but almost certainly turn north into the open ocean.  I won't go into any detail now, but the wave behind that one looks like potential trouble in model-land.  That should leave the continent on Sunday or so...

Not surprisingly, the ACE has spiked due to these two major hurricanes and is now at 138% of average for the date.  If the activity magically switched off right now, the ACE would slip back to the average value in just one week: September 5.

As far as named storm counts go, here's the latest status.  Any way you slice it, this season is well ahead of average for the date.

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