96L Approaching Lesser Antilles a Threat to Develop

By: Dr. Jeff Masters , 7:58PM,GMT on August 19,2014

A tropical wave located in the Central Atlantic near 10°N 50.5°W, about 800 miles east of the Lesser Antilles Islands, was designated Invest 96L by NHC on Tuesday afternoon, and is headed westwards to west-northwestwards at about 10 - 15 mph. Satellite loops show the wave has a broad, elongated surface circulation and modest amount of heavy thunderstorm activity that is poorly organized. Thunderstorm activity was beginning to increase slightly in areal coverage early Tuesday afternoon, though, and a solid outflow channel to the north had developed on 96L's west side. An 8:12 am EDT Tuesday pass from the ASCAT satellite showed top surface winds near 35 mph. Wind shear is moderate, 10 - 20 knots, and water vapor satellite images and the Saharan Air Layer analysis show that the wave has plenty of dry air to contend with. Sea Surface Temperatures (SSTs) have warmed, and are now near 28°C, which favors development.


Figure 1. MODIS true-color image of Invest 96L northeast of the coast of South America, at approximately 10:30 am EDT August 19, 2014. Image credit: NASA.

Forecast for 96L
The wave should pass through the Lesser Antilles Islands Thursday night or Friday morning, then track west to west-northwest through the Caribbean a few hundred miles south of Puerto Rico. The 12Z Tuesday run of the GFS and European models showed 96L tracking through the Central Caribbean early next week, and arriving in the Western Caribbean on Tuesday. The UKMET model showed a more northerly component of motion, with a path over Hispaniola. Two of our three reliable models for predicting tropical cyclone genesis, the UKMET and GFS models, predicted that 96L would develop into a tropical storm after passing through the Lesser Antilles. When both of these models show development, the odds increase that development will occur. The UKMET model showed 96L developing on Saturday, and the GFS showed it developing on Friday. Given the presence of so much dry air near the disturbance, the risk of development is low through Wednesday, but development odds will increase on Thursday as ocean temperatures warm and the atmosphere becomes moister. The 2 pm EDT Tuesday run of the SHIPS model predicted that wind shear would stay in the moderate range, 10 - 20 knots, for the next five days. In their 2 pm EDT Tuesday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day development odds of 30% and 40%, 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 it is in the Caribbean--a weak and disorganized system that struggles against dry air.

A second disturbance near 13°N, 39°W, midway between Africa and the Lesser Antilles Islands, also has a small area of disorganized heavy thunderstorms with some modest rotation. In their 2 pm EDT Tuesday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 10% and 20%, 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.


Figure 2. Tropical Storm Lowell as seen at 11 am EDT Tuesday, August 19, 2014. At the time, Lowell had top winds of 50 mph. Image taken from a super-rapid scan mode loop from the NOAA/RAMMB website.

The Eastern Pacific heating up
In the Eastern Pacific, we have a new named storm, Tropical Storm Lowell, which formed at 03 UTC on Tuesday. The GOES-West satellite is in super-rapid scan mode over Lowell today, and you can access some very impressive one-minute time resolution loops of Lowell at the NOAA/RAMMB website. Lowell's formation gives the Eastern Pacific 12 named storms, 7 hurricanes, and 5 intense hurricane so far this season. On average, the Eastern Pacific sees 8 named storms, 4 hurricanes, and 1 intense hurricane by August 18, so it has been a very active year in the basin. Tropical Storm Karina is also spinning away in the Eastern Pacific today, and Karina and Lowell 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 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 70%, is something residents of the Baja Peninsula should monitor next week. 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.

The Western Pacific remains mercifully quiet, with no new named storms expected to develop over the next five days.

Jeff Masters