LIttle Change to African Tropical Wave 94L Headed Towards Lesser Antilles

A tropical wave (Invest 94L) that moved off the coast of Africa on Saturday was located near 12°N, 26°W on Monday morning, and could potentially be a tropical storm when it arrives in the Lesser Antilles Islands on Saturday. Satellite loops show the wave has a modest amount of spin and respectable amount of heavy thunderstorms. Water vapor satellite images and the Saharan Air Layer analysis show that 94L is located in a fairly moist environment, with the dry air coming off or Africa located well to the north and west of the disturbance. Wind shear was a high 25 knots, but the 8 am EDT Monday run of the SHIPS model predicted that wind shear would fall to the low range, 5 - 10 knots, by Wednesday. Sea Surface Temperatures beneath 94L were 27°C on Monday, but were predicted to fall to 25.5°C by Wednesday, limiting the potential for development through Wednesday. The wave is headed west at 15 - 20 mph, and by Friday, will move over warmer waters of 27°C, as the storm nears the Lesser Antilles Islands. However, 94L will gain some latitude as it approaches the islands, and move into increasingly dry air to the northwest. These conditions are similar to what Bertha encountered as it approached the islands. If 94L does develop, the odds are that it will be a storm similar to Bertha--struggling against dry air, never reaching hurricane strength in the islands. Arrival in the islands should occur on Saturday, according to the Monday morning runs of the GFS and European models. None of the reliable computer models for predicting tropical storm formation (GFS, European, and UKMET) develop 94L, but about 1/3 of the 20 members of the GFS model ensemble show development late this week (the GFS ensemble is a set of 20 runs of the GFS model done at lower resolution with slightly different initial conditions to generate an uncertainty "plume" of model runs.) None of these ensemble forecasts showed 94L reaching hurricane strength. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 94L 2-day and 5-day development odds of 0% and 30%, respectively.


Figure 1. Satellite analysis of the Saharan Air Layer (SAL) done at 8 am EDT August 11, 2014, showing Invest 94L lying just south of a large area of dry air that covered much of the Atlantic. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Jeff Masters