06 September 2023

Lee forms and expected to become major hurricane by the weekend

A well-organized easterly wave left the African continent last Friday (Sept 1) and was upgraded to Tropical Depression 13 and then again to Tropical Storm Lee on Tuesday (Sept 5).  As of midday Wednesday it is nearly a hurricane and is expected to become the season's 4th hurricane later today.

Lee is centered about 1200 miles east of the Lesser Antilles and is moving toward the west-northwest at 14 mph.  As of now, the model guidance and the NHC forecast keep the center of the storm north of the Leeward Islands this weekend.  Within 3-4 days, there is model guidance indicating that Lee could become a Category 5 hurricane, an intensity only reached by 2-3% of all tropical cyclones in the Atlantic.

In the longer term (beyond five days), there is more spread in the guidance... as one would expect.  The majority of ensemble members turn Lee to the north by the time it reaches the longitude of Puerto Rico or Hispaniola.  But some, primarily from the UKMET ensemble, miss the turn and keep it tracking toward the west-northwest.  This split would occur late next week, so there is plenty of time to see how these evolve and what land areas should be on higher alert.  It's also worth mentioning that the ensemble members that miss the north turn tend to have weaker storms that are not as likely to be steered by the approaching trough that will come from the eastern U.S.

Elsewhere in the basin, another easterly wave just south of Cabo Verde has high chances of development in the coming week, and is tagged as Invest 96L.  The next name on the list is Margot, a replacement for Maria which was retired after the 2017 season.

The seasonal track map through today is shown below, with the storms' peak intensity, minimum central pressure, and ACE (Accumulated Cyclone Energy) contribution listed on the right side.  Perhaps because of El NiƱo's suppressing influence on the tropical Atlantic, it's interesting to note that only 30% of the season's ACE has actually come from the tropics (equatorward of 23.5°N) -- more than 2/3 of the activity has occurred in the subtropics. 

On the topic of ACE, it's at about 115% of average for the date, but there's no doubt that Lee will provide a huge surge to the tally in the coming 10+ days.

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