29 June 2021

Monitoring two waves in the deep tropics

With Tropical Storm Danny out of the picture (if you blinked you missed it), our attention is now solidly on two African easterly waves in the deep tropics.  Invest 95L, the one I wrote about last Thursday, is centered about 800 miles east of the Lesser Antilles, and Invest 97L just left the African coast on Sunday and is about 1800 miles east of the Lesser Antilles. 

In the satellite image above, I superimposed the centers of the two waves from NHC (x) as well as the potential formation area within the next five days (shaded, orange is 40% and corresponds to 95L and yellow is 20% and corresponds to 97L).  Note that this is NOT the "cone of uncertainty" for track because they are not yet tropical cyclones.

On their current trajectories, and regardless of development, 95L will reach the Lesser Antilles by late Wednesday, and 97L will by late Saturday.  At the very least, affected islands there can expect gusty and stormy conditions, and as of now, there is not much indication that either of these waves will be able to intensify too much by then.

The two waves should follow approximately the same track into the Caribbean, but the exact placement will matter.  The central Caribbean Sea is climatologically dominated by a powerful low-level jet, or wind streak, in June and July, so most waves or tropical cyclones that venture there never recover.  If one or both stays further north, environmental conditions would be more favorable for development.

There are too many unknowns this far out to speculate about if and when these waves could reach the US mainland, but for interests in the southeast US, just pay attention (it's that time of year when you want to want to be aware of activity in the tropics every day).

As I mentioned yesterday, the next name on the list is Elsa.  The earliest date of fifth named storm formation is July 6 (Edouard in 2020), and second place goes to July 11 (Emily in 2005).  We would typically be watching two easterly waves and talking about a potential fifth named storm in late August, not late June!

I know it's early, but it is still interesting to keep tabs on where the season stands in terms of ACE, or Accumulated Cyclone Energy.  As a refresher, ACE is the sum of the squares of each named storm's intensity from each advisory... so it is a metric that combines intensity and duration; it is not dependent on the number of storms at all.  For the average, I use the past fifty years (1971-2020).  The 2021 season is about 171% of average for the date.

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