Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 2:14 PM GMT on August 20, 2014
A tropical wave (96L) located near 11°N 53°W, several hundred miles east of the Lesser Antilles Islands, is headed west-northwestwards at about 10 - 15 mph. Satellite loops on Wednesday morning showed the wave had a broad, elongated surface circulation and a modest amount of heavy thunderstorm activity that was steadily increasing in areal coverage and intensity. Decent upper-level outflow channels were present on the storm's west and south sides. The storm was poorly organized, though, with a clumpy appearance and just a few small low-level spiral bands. An 8:57 pm EDT Tuesday pass from the ASCAT satellite showed top surface winds near 35 mph in a clump of thunderstorms a few hundred miles to the east of center of 96L. Wind shear is moderate, 10 - 20 knots, and water vapor satellite images and the Saharan Air Layer analysis show that while the wave has plenty of dry air to its north to contend with, it has managed to moisten its environment considerably since Monday. Sea Surface Temperatures (SSTs) are near 28°C, which favors development. The outermost thunderstorms of 96L had appeared on Barbados radar by Wednesday morning.
Figure 1. Latest satellite image of 96L.
Forecast for 96L
The wave should continue to organize over the next two days, and pass through the Lesser Antilles Islands Thursday night and Friday morning, bringing heavy rain showers and strong winds--particularly to the southern islands in the chain. The wave will then track west-northwest through the Caribbean a few hundred miles south of Puerto Rico. The 0Z Wednesday runs of our three most reliable models for predicting tropical cyclone genesis, the UKMET, GFS, and European models, had one model, the UKMET, predicting development into a tropical depression south of Puerto Rico. All three models show that on Saturday, 96L will pass over or just south of the island of Hispaniola, whose rugged terrain would likely disrupt the storm. The 8 am EDT Wednesday run of the SHIPS model predicted that wind shear would stay in the moderate range, 10 - 20 knots, for the next five days. With dry air expected to be in the Caribbean, the moderate levels of wind shear would likely be able to drive the dry air into the circulation of 96L, keeping any development slow. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day development odds of 30% and 50%, respectively. The Hurricane Hunters are on call to investigate the disturbance on Thursday afternoon, if necessary. If 96L does develop, it would likely be similar to Tropical Storm Bertha of early August while in the Caribbean--a disorganized system that struggles against dry air. The most likely day for development into a tropical depression is Friday, when the storm will be south of Puerto Rico.
A second disturbance near 14°N, 46°W, about 1000 miles east of the Lesser Antilles Islands, has a small area of disorganized heavy thunderstorms with some modest rotation. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 10% and 10%, respectively. None of our three reliable models for predicting tropical cyclone genesis predict that this disturbance will develop over the next five days as it heads west-northwest at about 10 mph.
Active in the Eastern Pacific
In the Eastern Pacific, we have Tropical Storm Lowell and Tropical Storm Karina , which are expected to become entangled with each other early next week and die in the cool waters well to the west of Baja Mexico. The models have been consistently predicting that a another named storm (Marie) will form late this week from a tropical wave (Invest 92E) that crossed Central America on Monday and will move parallel to the Mexican coast a few hundred miles offshore. This storm, which NHC is giving 5-day odds of development of 80%, is something residents of the Baja Peninsula should monitor next week, though current model runs show the storm staying well offshore. Ocean temperatures in the waters just west of the Baja Peninsula are unusually warm---30°C (86°F), which is about 3°C (5°F) above average--so Marie will have plenty of heat energy available to power it. Satellite images are showing the the disturbance already has a pronounced spin to it and a growing area of heavy thunderstorms.
The Western Pacific remains mercifully quiet, with no new named storms expected to develop over the next five days.
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