95L Near Tropical Depression Status, Headed Towards Lesser Antilles

A strong and unusually well-organized tropical wave for so early in the season (95L) is speeding westwards at 20 - 25 mph over the Central Atlantic. At noon EDT on Sunday, 95L was near 9.5°N, 43°W, about 1200 miles east of the Lesser Antilles Islands. Satellite loops show that 95L has a well-developed surface circulation center that is exposed to view, with a modest clump of heavy thunderstorms on the south side of the center. The system has several well-developed low-level spiral bands, but is fighting dry air associated with the Saharan Air Layer (SAL). July African tropical waves typically have considerable trouble getting organized in the very dry air of the SAL, and the only factor keeping 95L from currently being classified as a tropical depression is the relative lack of persistent heavy thunderstorm activity near the center of the storm due to dry air. This dry air is readily apparent on water vapor satellite loops. However, the latest SAL analysis (Figure 2) from the University of Wisconsin's SSEC group shows that 95L has moistened the air around it sufficiently to keep the SAL somewhat at bay. An 11:12 am EDT pass by the ASCAT satellite showed top winds of 45 mph on the west side of 95L's center of circulation. ASCAT did not show a closed circulation at the time. Wind shear is light to moderate, 5 - 15 knots, and ocean temperatures are warm, 27.5 - 28°C. An Air Force hurricane hunter aircraft is on call to investigate 95L on Monday afternoon.


Figure 1. MODIS image of 95L taken at approximately 10 am EDT Sunday, July 7, 2013. Image credit: NASA.


Figure 2. Analysis of the Saharan Air Layer (SAL) from 11 am EDT Sunday, July 7, 2013, shows that 95L has walled itself off from the western edge of the SAL. A new tropical wave that has emerged from the coast of Africa is visible on the right of the image. This wave will need to be watched for development late this week, but is farther north than 95L, and may have trouble with dry air as it moves westward across the Atlantic. Image credit: University of Wisconsin SSEC.

Forecast for 95L
The 8 am EDT Sunday forecast from the SHIPS model predicted that 95L would experience low to moderate shear through Tuesday morning as it headed west to west-northwest at 20 - 25 mph. The disturbance should arrive in the Lesser Antilles Islands on Tuesday, and affect the Dominican Republic by Wednesday night. A band a strong upper-level winds associated with the subtropical jet stream is expected to be over the northern islands this week, and as soon as 95L penetrates as far north as 15°N latitude, it will encounter high wind shear of 20 - 30 knots. But if 95L stays farther to the south, wind shear should be lower, giving the storm a better chance of development. None of the reliable forecast models predict that 95L will develop. In their 8 am EDT July 7 Tropical Weather Outlook, NHC gave the disturbance a 40% chance of developing into a tropical depression or tropical storm by Tuesday. I put these odds higher, at 70%. Climatology argues against 95L becoming a tropical depression east of the Lesser Antilles Islands; there have been only 20 July tropical depressions that have formed east of the Lesser Antilles since 1851, an average of one tropical cyclone every eight years.


Figure 3. MODIS image of Hurricane Erick (right) and Tropical Depression Dalila (left) taken at approximately 5 pm EDT Saturday, July 6, 2013. At the time, Erick was a Category 1 hurricane with 80 mph winds. Image credit: NASA.

Hurricane Erick brushes Mexico
In the Eastern Pacific, Tropical Storm Erick weakened from a 80 mph Category 1 hurricane on Saturday to a 70 mph tropical storm on Sunday. Erick is moving away from the southwestern coast of Mexico, and is expected to brush Baja on Monday and bring 2 - 4" of rain as the storm weakens and heads out to sea. The 11 am EDT Sunday wind probability forecast from NHC gave Cabo San Lucas on the tip of Baja a 41% chance of experiencing tropical storm-force winds on Sunday night or Monday.

Jeff Masters