Bertha Spreading Rain to Puerto Rico and Virgin Islands, but is not Intensifying

By: Dr. Jeff Masters , 2:06PM,GMT on August 2,2014

Tropical Storm Bertha was racing west-northwest across the Northeast Caribbean at 21 mph on Saturday morning, spreading gusty winds and heavy rain showers across the Virgin Islands, Puerto Rico, and islands of the Northern Lesser Antilles. A personal weather station at an elevation of 325 feet on the east end of St. Croix in the Virgin Islands recorded sustained winds of 47 mph, gusting to 56 mph on Saturday morning, along with 0.5" of rain. However, these winds were likely amplified by the surrounding terrain, and Bertha has not been generating sustained tropical storm-force winds (39+ mph) at any other land stations. A few peak wind gusts and rainfall amounts from the passage of Bertha through the islands:

Dominica: 43 mph wind gust, 1.38" of rain
Guadaloupe: 43 mph wind gust, 0.38" of rain

Visible satellite loops on Saturday morning showed that although Bertha was more organized than on Friday, the storm had only a limited amount of heavy thunderstorms. The misshapen structure of the storm suggested that dry air and wind shear continued to be a problem for it, and Bertha's lack of organization was also apparent on Puerto Rico radar, where very little low-level spiral banding was apparent. Wind shear due to strong upper-level winds out of the southwest was a high 20 knots on Saturday morning. These winds were driving dry air to the west of Bertha into the circulation, limiting heavy thunderstorm formation. An Air Force C-130 hurricane hunter aircraft is scheduled to investigate Bertha Saturday evening.


Figure 1. Radar image of Tropical Storm Bertha at 9:52 am EDT Saturday August 2, 2014, from the San Juan, Puerto RIco radar.

Forecast for Bertha
Moderate wind shear of 10 - 20 knots is expected to affect Bertha through Monday morning, according to the 12 UTC Saturday 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 any intensification slow. Passage over the rough terrain of the eastern Dominican Republic Saturday night and Sunday will disrupt the storm, and it is possible that Bertha will be downgraded to a tropical wave on Sunday. However, Bertha will still be capable of dumping heavy rains on the Southeast Bahamas on Sunday and Monday, 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. Wind shear will be lower and the atmosphere will be moister as the storm heads northwards, potentially allowing Bertha to intensify into a Category 1 hurricane over the open ocean.


Figure 2. Latest satellite image of Iselle in the Eastern Pacific.

Hawaii keeping an eye on Hurricane Iselle
In the Eastern Pacific, Category 1 Hurricane Iselle is headed westwards towards Hawaii, and could affect Hawaiin Islands by Thursday night. Satellite images show that Iselle has developed an eye, and the hurricane 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 to remain a hurricane over the weekend. By Tuesday, the storm will encounter higher wind shear, drier air, and cooler waters of 26°C, which will induce weakening. The GFS and European models predict that Iselle will pass close to Hawaii Thursday night and Friday, but the storm 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, 4 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.


Figure 3. True-color MODIS image of Typhoon Halong from approximately 02 UTC August 2, 2014. At the time, Halong was a Category 3 storm with top winds of 115 mph. Image credit: NASA.

Typhoon Halong a threat to Japan
In the Western Pacific, Typhoon Halong has intensified into a dangerous Category 4 storm with 135 mph winds. Satellite loops show an impressively organized storm with a large eye surrounded by eyewall thunderstorms with very cold cloud tops. The storm is expected to head northwards and affect Japan's Ryukyu Islands on Wednesday night.

Jeff Masters