19 August 2020

Closely watching three waves in deep tropics

Although there are no active tropical cyclones at the moment, that is about change.  The notable African easterly waves are making the trek across the deep tropical Atlantic. One is now in the central Caribbean (Invest 97L), one is about 1000 miles east of the Lesser Antilles (Invest 98L), and the third is just about to emerge from the African continent (could be Invest 99L).  The first two are very close to developing, while the third will need some time and luck.

"x" marks the current location of each area of interest, and the shading indicates the area of possible formation within the next five days. The probabilities are 80%, 90%, and 20% going west to east.

While it is not at all uncommon to have a string of active waves in late August, it is uncommon that we have Laura, Marco, and Nana as the next three names. The record earliest "L" (12th) storm formation is August 29th (Luis in 1995), and the record earliest "M" (13th) storm formation is September 2nd.  Recall that 2020 has already claimed the record earliest C, E, F, G, H, I, J, and K storms!!  In true 2020 fashion, this is extraordinary.

Now, on to each disturbance...

Invest 97L is centered south of Hispaniola and appears to be experiencing less wind shear than it has been. Neither the European nor American global models -- or the vast majority of their ensemble members -- develop this.  However, the HWRF regional hurricane model certainly does, and the HMON regional hurricane model does as well. 

Of the models that develop it, they indicate a sharp turn to the north on Friday, then intensifying it in the Yucatan/Cuba area before heading into the Gulf of Mexico. Once in the Gulf, the recent HMON run maintains just tropical storm strength (crosses over land), while HWRF heads due north as a Category 1-2 hurricane (stays over water). It has a long way to go, but those summarize the spread of realistic scenarios as of now. For anyone along the Gulf coast, this is worth paying attention to for potential impacts on Monday-Wednesday next week.

Shifting our attention east, Invest 98L is very close to becoming a tropical cyclone, and environmental conditions should be a green light. Some models show it moving into/near drier air in a few days, and that explains the intensity spread... along with interaction with an upper-level trough and potential interaction with Puerto Rico and Hispaniola. In other words, it's an obstacle course, but if the storm can navigate the course, it could be trouble.

The forecast track spread is remarkably low right now. Tracks are clustered tightly over the northern Windward Islands early Saturday, near Hispaniola on Sunday, then Cuba/Florida/Bahamas on Monday into Tuesday. The Americal global model's ensemble (GEFS) is also tightly clustered. Many of these runs show it at hurricane intensity by Monday. It is worth noting that the European global model deterministic and ensemble runs do not develop this disturbance at all. That must be weighed when looking at the remainder of the models. South Florida should be on high alert with this... if it should happen to develop and intensify, it is five days away (Monday evening-ish). As an aside, South Floridians always keep August 24th in their collective memory -- that was Category 5 Hurricane Andrew's landfall date in 1992.

And finally, the wave that is still over western Africa has some fairly big environmental hurdles ahead of it (such as a huge Saharan Air Layer plume to its north and west), so given the amount of activity further west, it's worth an occasional glance, but not much else.

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