Sep 27, 2016

Tropical Disturbance in Atlantic - Invest 97L

A  tropical disturbance (97L) in the Atlantic Ocean located about 415 miles to the east-southeast of the island of Barbados is showing signs of becoming better organized this afternoon. Hurricane hunters are tasked to do a reconnaissance of it this afternoon.

Based on satellite wind data and minimal surface reports the system doesn't appear to have a closed cyclonic circulation at the moment. Below is a visible satellite picture of the system on the left and the satellite winds on the right.
(Visible Satellite Imagery oif 97L)

(High Level Wind Analysis over and around 97L)
(Current Divergence Analysis)
Of interest is the wind field in and around the disturbance. The wind flow is anticyclonic or diverging. Divergence increases upward motion of the air (image to right) and causes a lowering of the atmospheric pressure at the surface,

 In addition to divergence, tropical systems need warm ocean waters Sea Surface Temperatures/SST) of at least 26°C/78°F for their development or intensification. The disturbance is currently over SSTs of 28°C to 30°C (image below).

(Sea Surface Temperature analysis)
The other favorable element for potential development is for little wind shear aloft. Strong shear (of 20kts or greater) disrupts a tropical system's circulation and structure. Shear can inhibit development or even weaken a tropical system. Based on the latest wind shear analysis (image below), the shear is favorable (i.e., LOW) over 97L., but to its west the shear is strong (unfavorable).

(Current Wind shear)
However many of the forecast models are forecasting this shear to relax considerably over the next two days, that is these forecast aids are indicating little shear as 97L moves into the Eastern Caribbean Sea.

The forecast model spaghetti plots for 97L are clustered around a W to WNW heading through the end of the week. Of these models that forecast intensity of tropical systems, most if not all are forecasting 97L to become at least a tropical storm. If this did occur then 97L would be christened "Matthew".
(Forecast model ensemble tracks for 97L)
Per the NHC, "Interests in the Windward and southern Leeward Islands, Bonaire,Curacao, Aruba, and along the northern coast of South America should monitor the progress of this disturbance. Regardless of whether the system is a tropical wave or tropical cyclone, heavy rains and tropical-storm-force winds in squalls are expected to spread over the Windward Islands and portions of the southern Leeward Islands, beginning tonight and continuing through Wednesday."

Credits: First 5 images from University of Wisconsin CIMSS Tropical System web site: CIMSS