A new disturbance heads for Lesser Antilles

By Bob Henson From Weather Underground

A tropical wave in the central tropical Atlantic, dubbed Invest 99L and located about 750 miles east of the Lesser Antilles Islands at 8 am EDT Thursday morning, bears watching as it heads west-northwest at 15 – 20 mph. Sea surface temperatures (SSTs) in this region were near 28.5°C (83°F), and wind shear was a moderate 10 – 20 knots, conditions favorable for development. There is a large area of very dry air, associated with the Saharan Air Layer (SAL), just to the north of the wave, and this dry air will retard development. Satellite images Thursday morning showed that 99L had a moderate amount of spin, but heavy thunderstorm activity was sparse and disorganized.

The 18Z Thursday run of the SHIPS model predicted that 99L would be moving into a steadily drier environment, with the relative humidity at mid-levels of the atmosphere falling from 65% on Thursday to around 50% by late Saturday. The 0Z Thursday runs of our top three models for predicting tropical cyclone genesis, the European, GFS, and UKMET models, did not develop 99L. About 30% of the 50 members of the 0Z Thursday European model ensemble and 5% of the 20 members of the 0Z Thursday GFS model ensemble supported development, but all of these forecasts showed the system dying by Sunday in the eastern Caribbean—a region nicknamed the “hurricane graveyard” for its well-documented effects on incipient systems, especially early in the season. In its 2 pm EDT  Tropical Weather Outlook, the National Hurricane Center gave 2-day and 5-day odds of development of 20% and 20%, respectively, which implies there is very little chance of 99L becoming a depression after Day 2 (Saturday).

SOURCE: https://www.wunderground.com/cat6

Tropical Weather Outlook
NWS National Hurricane Center Miami FL

800 AM EDT Fri Aug 17 2018

A tropical wave located about 400 miles east-southeast of the Windward Islands continues to produce disorganized showers and thunderstorms. Environmental conditions are only marginally conducive, and significant development of this system is not anticipated. This disturbance is moving westward at about 15 mph and is forecast to move through the Windward Islands on Saturday, where it could bring locally heavy rainfall to portions of the Lesser Antilles during the next couple of days.

* Formation chance through 48 hours…low…10 percent.
* Formation chance through 5 days…low…10 percent.

SOURCES:

https://www.weathernerds.org/tc_guidance/storm.html?tcid=AL99

http://www.spaghettimodels.com/