27 August 2023

Tropical Storm Idalia has eyes on Florida as Franklin strengthens south of Bermuda

On Sunday morning, Tropical Depression 10 was upgraded to Tropical Storm Idalia, the 10th named storm of the season.  Climatologically, the 10th named storm forms on September 22, so this is definitely ahead of that pace.  The storm is stalling between the Yucatan peninsula and Cuba as expected, and should start moving toward the Gulf of Mexico on Monday morning.  Hurricane watches are in effect for parts of Florida.

Sunday afternoon's model runs had the benefit of aircraft reconnaissance data.  Unfortunately, the guidance has trended stronger and it's now looking very likely that somewhere in the northern Gulf coast of Florida will experience a hurricane landfall, perhaps a major hurricane, on Wednesday.  Tropical storm force winds could begin Tuesday morning in parts of southwest Florida and then Tuesday evening in northwest Florida.  That means just two days of preparation time as of Sunday evening there.

The global model ensembles show a track toward Florida's Big Bend area and landfall centered around Wednesday morning.  However, it's important to note the spread to see the current realm of potential outcomes based on these models.  And then remember that a hurricane is not a point or a line, it's hundreds of miles across.  So as always, impacts such as flooding rain, storm surge, and strong winds occur well away from the center. 

We can also look at the latest suite of deterministic model tracks, and they include a couple that are on the far southern periphery of the ensemble spread shown above.

Then we have the official NHC forecast and the track forecast cone.  For this, remember that it's a deterministic forecast and the accompanying cone represents a 2/3 likelihood of the path of the center of the storm (using the error statistics from the previous five years).  So 1) there's still a 1/3 chance the center of the storm tracks outside of the cone and 2) the cone tells you nothing about where impacts will be experienced.  (see 2023 "Cone of Uncertainty" Update & Refresher).  Keep yourself updated on the latest forecast, watches, and warnings related to Idalia at https://www.nhc.noaa.gov/refresh/graphics_at5+shtml/174602.shtml?cone#contents.

After its encounter with Florida, Georgia, South Carolina, and North Carolina will have to contend with it, and one factor this coming week is an exaggerated risk of coastal flooding because of abnormally high tides around the Full Moon.

Franklin is still in the open ocean between Hispaniola and Bermuda.  It should avoid a direct hit on Bermuda, but the island now has a rather high chance of experiencing tropical storm conditions on Tuesday night.  It is forecast to become the season's first major hurricane tomorrow, which is just four days ahead of climatology.

With a tropical storm and a hurricane churning simultaneously, the Accumulated Cyclone Energy (ACE) is accruing at a pace just in line with climatology.  The ACE is at 93% of average for the date.

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