Tropical Storm Bertha Nearing Lesser Antilles Islands

Tropical Storm Warnings are flying Friday morning in most of the Lesser Antilles Islands as Tropical Storm Bertha, with top winds of 45 mph, cruises west-northwest at 20 mph towards the islands. Bertha was born last night at 03 UTC August 1, when tropical disturbance 93L was finally able to produce enough heavy thunderstorms to be classified as a tropical storm. The formation of Bertha on August 1 was right on schedule, according to climatology from 1966 - 2009: the Atlantic's 2nd named storm has historically formed on August 1 during this period. Visible satellite loops on Friday morning showed that Bertha was not well-organized; the surface circulation was partially exposed to view, with a modest area of heavy thunderstorms confined to the east side of the circulation. The storm was fighting moderate to high wind shear of 15 - 20 knots, due to strong upper-level winds out of the west. These winds were driving dry air to the west of Bertha into the circulation, keeping heavy thunderstorms from forming on the west side. An Air Force C-130 hurricane hunter aircraft was investigating Bertha Friday morning, and found the storm's central pressure was a rather high 1010 mb at 11:48 UTC (7:48 am EDT). Top surface winds measured by the plane were about 45 mph, so Bertha is showing no signs of intensifying. Barbados radar on Friday morning showed that Bertha was generating some respectable rain bands, which will bring heavy rains to the Lesser Antilles beginning late Friday morning. Water vapor satellite loops and the Saharan Air Layer analysis showed that a large amount of dry air lay to the north and west of Bertha. Ocean temperatures were about 28°C, which is 2°C warmer than the typical 26°C threshold for development.


Figure 1. Latest satellite image of Bertha.


Figure 2. Short-term drought conditions in the Caribbean as of July 2014, as measured by the one-month Standardized Precipitation Index (SPI). Bertha's rains will cause welcome drought relief in some areas, though may also cause flash flooding. Image credit: NOAA's Global Drought Portal.

Forecast for Bertha
Moderate to high wind shear of 15 - 20 knots is expected to affect Bertha through Saturday morning, according to the 12 UTC Friday forecast from the SHIPS model. With the atmosphere around Bertha quite dry, the storm will have to work hard to insulate itself from disruptive dry air incursions, and only slow intensification is likely through Saturday morning. I don't see Bertha being stronger than a 55 mph tropical storm during this period. By Saturday afternoon, wind shear is forecast to fall to the moderate range, 10 - 15 knots, and this may allow Bertha to intensify as it moves over Puerto Rico. Passage over the high mountains of Puerto Rico may disrupt the storm some, counteracting the decrease in wind shear. If the system takes a more southwesterly track over the eastern Dominican Republic like the European model is suggesting, this would also disrupt Bertha. I don't see Bertha being stronger than a 60 mph tropical storm as it affects Puerto RIco and the Dominican Republic. Two of our best intensity models, the GFDL and HWRF, predicted in their 06Z Friday runs that Bertha would have winds of about 40 mph as it passed over Puerto Rico on Saturday.

The GFS and European models continue to agree on the long-range fate of Bertha. The storm is expected to clip the southeastern Bahamas and Turks and Caicos Islands on Sunday, then turn north in response to a strong trough of low pressure over the Eastern United States. This trough should be strong enough to recurve Bertha to the northeast without the storm hitting the mainland U.S. coast.

I'll have a new post this afternoon.

Jeff Masters