10 September 2020

A very busy period lies ahead across the entire Atlantic

Not surprisingly, the second week of September is shaping up to be quite active.  There are still only two named storms lurking out there, Paulette and Rene, but there are also four other areas of interest in the coming days scattered from the Gulf of Mexico to Africa.
Tropical weather outlook over the next five days. The track and "cone of uncertainty" are shown for Paulette and Rene. The other four areas have Xs at their current location and shading where development is possible within the next five days. (NHC)

Aside from Tropical Storms Paulette and Rene in the central Atlantic, there are two weak waves in the western Atlantic -- one approaching the Bahamas and heading west, and another west of the Florida peninsula and heading west. They don't have very high chances of becoming tropical cyclones, but it's not out of the question either. Then there are two much stronger waves in the far eastern Atlantic -- one just exiting the African coast today, and one about three days behind it over continental Africa. These are far more likely to become named storms.

The animation below shows the output from the European model's 50-member ensemble (each area of interest could have 50 trackable lows) over the next ten days. Don't pay attention to any details, but it's easy to see the general patterns and tracks.  The tracks are colored by intensity... and note that weaker storms are steered by different layers of the atmosphere than stronger storms.
Rene is forecast to track toward the west-northwest and encounter lower wind shear, allowing it to strengthen a bit more, but by early next week, its future becomes very hazy. Paulette is expected to reach hurricane intensity in a few days and reach the Bermuda area on Monday night into Tuesday morning. The two storms are close enough to each other that the larger one (Paulette) can influence the environment that Rene moves into... that's where the haziness comes in.
The next two names on deck are Sally and Teddy.  After that, only two more names remain before we switch to the Greek alphabet. The 2005 season reached the Greek alphabet on October 22 and went on to use six names from that alphabet.

In terms of ACE (Accumulated Cyclone Energy), this season is just about average -- 102% of average for this date.  It will likely slip below average on September 11 for the first time this season.
And speaking of the peak of the season, the first two weeks of September are historically it, though the actual peak depends on which period one uses as the climatology!  In the time series below, I plot the daily average ACE (with a centered seven-day smoothing) from four different time periods.  There are humps, bumps, dips, and wiggles, and if you really hunt for the absolute maxima, you get September 11 (purple), September 12 (green), September 13 (blue), or September 14 (orange). But it's clear from looking at these that those exact dates don't really mean much.


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