Weather Nerd

Three depressions?

As expected, Tropical Storm Nana has weakened to a depression. Meanwhile, “Invest 98L” in the Eastern Caribbean has been designated Tropical Depression 15. And now there’s a new disturbance, “99L,” in the Western Caribbean, which spurred a Special Tropical Disturbance Statement at 11:30 AM EDT, and which could potentially become T.D. 16 “during the next couple of days.”

If either Fifteen or proto-Sixteen earns a name, it would be Omar, the first time that name has ever been used. The current list in the sexennial storm-name rotation has been used five times: in 1984, 1990, 1996, 2002 and 2008. Three of those seasons (’90, ’96 and ’08) have produced a “Marco,” and two (’90 and ’08) have produced a “Nana,” but we’ve never reached “Omar” — still less “Paloma,” which is next on the list. This fact is of no particular significance, but sometimes I like to geek out about storm-name history.

Anyway… the biggest concern right now is T.D. 15, which is expected to track northeastward toward Puerto Rico, and which could — could — become a hurricane. But the intensity forecast is uncertain, and Dr. Jeff Masters is skeptical that Fifteen will get its act together quite so quickly:

Wind shear is expected to fall to the moderate 10-20 knot range over the next three days, and waters will remain warm, 29°C. This should allow [TD 15] to intensify into a tropical storm by Tuesday. The HWRF and GFDL models both intensify TD 15 into a hurricane before it hits Puerto Rico on Wednesday. This seems overly aggressive, given the moderate 10-20 knots of wind shear expected. The SHIPS model forecast and official NHC forecast of a strong tropical storm hitting the island on Wednesday is more reasonable.

Regardless of its intensity, the main threat from T.D. 15 will be heavy rain:

Up to five inches of rain has already fallen over the Virgin Islands and eastern Puerto Rico, and additional heavy rains of 5-10 inches are likely over these islands through tonight. Heavy rains of 5-10 inches per day will likely spread to the eastern Dominican Republic and the northern Lesser Antilles Islands by Tuesday morning, and continue through Wednesday night. Over Puerto Rico, isolated rain amounts in excess of 20 inches are possible before the storm clears the islands by Thursday.

Thankfully, “it currently appears that Haiti will only get 1-3 inches of rain from TD 15.” Haiti has already suffered more than its share of misery this hurricane season, mostly due to flooding from torrential rains.