Tropical Storm Bertha Hits the Lesser Antilles

The center of Tropical Storm Bertha was passing between the Lesser Antilles islands of Martinique and Dominica near 4 pm EDT on Friday, but has brought little in the way of strong winds or heavy rain to the Lesser Antilles Islands so far. As of 4 pm, Martinique had received 0.35" of rain, with top winds of 22 mph. Dominica had a wind gust of 43 mph at 4 pm EDT, and had picked up 0.08" of rain. The storm's top winds of 50 mph were located about 100 - 150 miles east-northeast of the center, and this portion of the storm will affect the northernmost Leeward Islands Friday night as Bertha speeds west-northwest at 22 mph. Visible satellite loops on Friday afternoon showed that although Bertha's surface circulation was exposed to view due to wind shear, the storm was growing more organized. Bertha had a modest area of heavy thunderstorms on the east side of the circulation, but heavy thunderstorms were beginning to fire up near the center of circulation. Martinique radar also showed increased organization, with more spiral bands forming and growing more intense near the center. This modest increase in organization may be due to the fact that wind shear due to strong upper-level winds out of the west had dropped by 5 knots since Friday morning, and was a moderate 15 knots on Friday afternoon. These winds were still driving dry air to the west of Bertha into the circulation, limiting heavy thunderstorms on the west side of the storm. An Air Force C-130 hurricane hunter aircraft was investigating Bertha early Friday afternoon, and found that the storm's central pressure had fallen 3 mb since Friday morning, to 1007 mb. Top surface winds measured by the plane were about 50 mph.


Figure 1. Radar image of Tropical Storm Bertha as the center passed between Martinique and Dominica at 4 pm EDT August 1, 2014. Image credit: Meteo France.


Figure 2. True-color MODIS image of Tropical Storm Bertha from approximately 1:15 pm EDT August 1, 2014. At the time, Bertha had top winds of 50 mph. A long, near-surface arc-shaped cloud racing away from Bertha to the north and west is seen. These arc clouds are the sign of a tropical cyclone struggling with dry air, and form when dry air at mid-levels is ingested into a tropical cyclone's thunderstorms, creating strong downdrafts that rob the storm of moisture. When the downdrafts hit the surface, the air spreads out in an arc-shaped pattern, creating clouds at the edge of the outflow boundary. The low level center of Bertha was fully exposed to view, the sign of a tropical storm struggling with wind shear. Image credit: NASA.

Forecast for Bertha
Moderate wind shear of 15 knots is expected to affect Bertha through Saturday morning, according to the 18 UTC Friday forecast from the SHIPS model. With the atmosphere around Bertha quite dry, the shear will be able to drive dry air into Bertha's circulation, keeping intensification slow through Saturday morning. Given that Bertha was beginning to fire up heavy thunderstorms on the west side of its circulation Friday afternoon, in defiance of the shear and dry air, makes it more likely that the storm can have sustained winds as strong as 65 mph when it makes its closest approach to the Virgin Islands and Puerto Rico on Saturday. Wind shear is forecast to fall to the light range, 5 - 10 knots, by Saturday afternoon. When Bertha leaves Puerto Rico and heads towards the Southeast Bahamas on Saturday night, wind shear is expected to remain low. However, passage over the rough terrain of Puerto Rico and the eastern Dominican Republic may disrupt the storm. The 12Z Friday runs of the GFS, European, and GFDL models showed Bertha's core tracking over the eastern Dominican Republic Saturday night, and this track would significantly weaken the storm. Bertha will still be capable of dumping heavy rains on the Southeast Bahamas on Sunday and Monday, though, as the storm turns 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.


Figure 3. Latest satellite image of Iselle in the Eastern Pacific.

Hawaii keeping an eye on Tropical Storm Iselle
In the Eastern Pacific, Tropical Storm Iselle is close to hurricane strength, and could affect Hawaii late next week. Satellite images show that Iselle has developed a large and ragged eye, and Iselle has a large area of heavy thunderstorms that are improving in organization. With wind shear a moderate 10 - 15 knots and SSTs near 28°C, Iselle is likely become a hurricane today and maintain hurricane status over the weekend. Early next week, the storm will encounter cooler waters of 26°C, which are marginal for maintaining a hurricane. The GFS and European models predict that Iselle will pass close to Hawaii next Friday, but the storm should be weakening and may be close to dissipation by then. It's been a very active hurricane season in the Eastern Pacific, which has seen 9 named storms, 3 hurricanes, and 2 intense hurricanes so far in 2014. On average, we expect to see 6 named storms, 3 hurricanes, and 1 intense hurricane by August 1 in the Eastern Pacific.

Tropical Storm Halong a threat to Japan
In the Western Pacific, Tropical Storm Halong is intensifying, and satellite strength estimates put Halong at Category 1 typhoon strength on Friday afternoon. The storm is expected to head northwards and affect Japan's Ryukyu Islands on Wednesday.

Jeff Masters