11 October 2023

Cabo Verde season isn't over yet... watching Tropical Storm Sean and a second disturbance

"Cabo Verde season" is the portion of Atlantic hurricane season that refers to when easterly waves trek across Africa and emerge over the Atlantic Ocean near Cabo Verde... and a small percentage of them go on to become long-lived hurricanes.  The vast majority of the most infamous hurricanes are of Cabo Verde pedigree.  This season doesn't have exact bounds, but is generally mid-August through early October.  The large-scale environment tends to be too hostile for those easterly waves to develop before and after that timeframe.  So, the point of this introduction is that it's getting to be rather late to be watching one let alone two systems out there!

I suspect some of the reason for that is the anomalously warm water still present out there. On this map below, the sea surface temperature anomaly is the background image (in °C), Tropical Storm Sean's forecast track and track forecast cone are overlaid in the center, and the easterly wave's current position and potential formation zone in the coming week are shown by the yellow X and corresponding shaded area.

Sean is the season's 19th named storm, but probably won't be around much longer.  Conditions were only marginally favorable for development in the first place, and increasing vertical wind shear should limit the storm's lifetime to just 3-5 days.  But, a couple hurricane models show it turning to the north a little sooner, and missing some of the interaction with the stronger shear... allowing it to intensify in 4-5 days.  So we can't tune this out just yet.

The easterly wave that just left Africa is given a 30% probability of development in the coming week by the National Hurricane Center.  It's not expected to move very quickly, but models generally indicate that it will gradually develop and stay in the deep tropics... potentially reaching the Lesser Antilles in about 10 days.  However, at this long lead time, it's premature to venture a guess if it will be anything of concern to the Antilles... just something to watch and be aware of. Should it become a tropical storm, the next name on the list is Tammy.  

By the end of the day, the Accumulated Cyclone Energy (ACE) will be at about 124% of the climatological average for the date, and 105% of an average full season.

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