17 October 2023

Threat increasing for Leeward Islands this weekend

The easterly wave that was near Cabo Verde that I mentioned in my post last Wednesday has been creeping toward the Lesser Antilles and is now close to becoming a tropical cyclone.  It would be Tropical Depression 20 or Tropical Storm Tammy when the time comes.  The National Hurricane Center is giving this a 70% probability of becoming a tropical cyclone within 2 days and 80% within 7 days. 

The disturbance, tagged as Invest 94L, is located about 1200 miles east of the Lesser Antilles and could reach the northern islands (the Leewards) on Saturday, potentially as a hurricane.  Model guidance is still quite split on the future of this, ranging from barely a tropical storm to a Category 4 hurricane.  Although not plotted on the map of deterministic models below, the European model barely develops this beyond an open wave... not even a depression.  Although the ocean temperature ahead of it is plenty warm for development, there is some drier air and higher vertical wind shear in the coming days, and the models handle that differently.

Looking at the ensembles, I have the American and European model ensembles below.  The American model is decidedly more aggressive with the intensity forecast (the scale is central pressure, so lower values are stronger), which makes the storm more likely to follow the north end of the spread... while the European is generally weaker, which makes the storm more likely to follow the south end of the spread and hit the Leeward Islands.

Given the significant uncertainty in the intensity forecasts and the system being just four days away from the Leeward Islands, residents and tourists on those islands need to be watching this very closely and be prepared to take action if the forecasts start to solidify around the stronger scenarios.

The map below shows the tracks of the 19 storms so far this year.  Next to the names are the storms' peak intensity, minimum central pressure, and Accumulated Cyclone Energy.  Recall there are only 21 names on the regular list; the remaining ones are Tammy, Vince, and Whitney.  If the regular list gets exhausted, the supplemental list kicks in, and the first three names on that are Adria, Braylen, and Caridad.

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