Tropical Depression Seven Forms Near Puerto Rico

Tropical Storm Warnings are flying in Puerto Rico, as Tropical Depression Seven steadily organizes and moves north-northwest at 2 mph towards the south coast of the island. TD 7 had been moving to the northwest most of Wednesday, and this new NNW motion may be due to the influence of a strong tropical disturbance located about 300 miles to the east-northeast of TD 7. This disturbance to the ENE of TD 7 has greater areal extent and more intense thunderstorms than TD 7, and is a threat to become a tropical depression on its own--NHC gave it a 10% chance of development in their 8 pm EDT Tropical Weather Outlook. If this happens, the disturbance may be able to steal moisture from TD 7 and keep it weaker than it would otherwise become. It is more probable that TD 7 will absorb a portion of the disturbance, stealing some of its moisture and spin, and becoming larger. Heavy thunderstorms from TD 7 have dumped over an inch of rain in many locations in the Virgin Islands and Northern Puerto Rico as of 8 pm AST Wednesday, but TD 7 has contracted, keeping its heaviest rains focused offshore near the center of circulation. These heavy rains will move ashore Thursday morning, bringing three-day rain totals of 5 - 10" to areas along the south coast. Satellite loops show that TD 7 is developing some respectable heavy thunderstorms with very cold cloud tops near its center, and there are good upper-level outflow channels to the north and south. Radar out of San Juan, Puerto Rico shows good low-level spiral bands on the storm's southeast side, and TD 7 is not far from becoming Tropical Storm Gabrielle. Upper level winds are favorable for development, with wind shear a low 5 - 10 knots.


Figure 1. True-color MODIS satellite image of TD 7 in its formative stages, taken at 1:30 pm EDT on September 4, 2013. Image credit: NASA.

I'll have a full update in the morning.

Jeff Masters