17 September 2024

Watching western Caribbean for activity next week

There is a growing signal among long-range global model ensembles that the next storm could start brewing in the western Caribbean in the coming days.  There is nothing to look at yet, but a broad low pressure system could take shape on Friday-Saturday east of Honduras then consolidate and track north from there.  The next name on the list is Helene.


This animation below shows trackable low pressure systems evolving from today through next Thursday from the most recent run of the American global model (GFS) ensemble.  Definitely don't pay attention to details, but you get the general idea of the formation area, and the approximate spread of possibilities as it heads north.  Based on this, the spread is roughly centered on the Florida peninsula with some members out over the Bahamas and out over the Gulf of Mexico by the middle of next week.  The European model's ensemble is similar but everything is shifted slightly west.


The key at this point is to monitor the various ensembles and look for inter-run and inter-model consistency and trends.  But for Cuba, Bahamas, Florida, and the northern Gulf coast, it's time to pay attention every day.

The water temperatures in the western Caribbean are extremely warm... record-breaking.  The figure below shows four panels: the left side is sea surface temperature (actual value on top, anomaly on bottom) and the right side is ocean heat content (actual value on top, anomaly on bottom).  All of this means that should anything form in the western Caribbean soon, the ocean will provide endless high-octane fuel for it.



The deep tropics continue to be much less active than normal, and certainly less active than anticipated this season.  Although the typical large-scale environmental factors are conducive for development, there is something odd happening over Africa.

From mid-August through early October, Africa tends to be a major source of tropical cyclone seedlings.  African Easterly Waves come off the continent every few days, and a small percentage of them usually become hurricanes.  This summer, the area of convergence that runs east-to-west across the continent has been displaced quite far north.  Instead of those waves coming off at the normal 10-15°N, they've been coming off at 15-20°N and immediately encountering cool water and dry air.  This also places frequent thunderstorms and heavy rain over the Sahara Desert resulting in tremendous flooding in central and western desert countries.


As such, the season's Accumulated Cyclone Energy (ACE) is down to about 83% of average for the date and slipping roughly 2% more behind each day without an active named storm.  That means if nothing forms through September 25, this season's ACE will be down to 70% of average.


The next chart shows how 2024 stacks up against previous years (again, through September 17).  It's nothing too notable... some recent years were much higher and some were much lower by now.


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