A tropical wave (Invest 99L) near 9°N 41°W, halfway between the Lesser Antilles Islands and the coast of Africa, has the potential to develop into a tropical depression later this week as it moves westward at 10 - 15 mph. Visible satellite loops show that the disturbance now has a moderate amount of poorly-organized heavy thunderstorms that continue to slowly increase in intensity and areal coverage. There is no surface circulation, but some counter-clockwise rotation of the large-scale cloud pattern is evident. Water vapor satellite loops show that 99L has a reasonably moist environment. The latest Saharan air layer analysis shows that the dry air from the Sahara lies to the north of 99L and is currently not affecting the storm. WInd shear over the disturbance is a light 5 - 10 knots, and ocean temperatures are 28°C, (82°F) which is well above the 26.5°C (80°F) threshold typically needed to allow formation of a tropical depression.
Figure 1. Morning satellite image of Invest 99L.
Forecast for 99L
Wind shear is expected to remain light through Friday, and ocean temperatures will remain near 28°C, according to the 8 am EDT run of the SHIPS model. However, a band of high wind shear of 20 - 40 knots associated with the subtropical jet stream lies just to the north of 99L, and it would not be a surprise to see 99L experience some higher shear conditions than are currently forecast. The farther north 99L gets, the higher the shear it will experience, and the SHIPS model is predicting shear in the moderate range, 10 - 20 knots, for Saturday - Sunday, as the storm works its way to 15°N. The disturbance is at 9°N, which is close enough to the Equator that the storm will have some difficultly getting spinning. Most of the models are showing some slow development of 99L. There are some major differences in the predicted forward speed of 99L, with the ECMWF and UKMET models predicting the storm will reach the Lesser Antilles on Friday, and the GFS predicting a later arrival, on Saturday. At 8 am Tuesday, NHC gave 99L a 20% chance of developing into a tropical depression by Thursday morning. I expect the storm will begin having trouble with tendrils of dry air reaching down from the north at times this week, but give 99L a 50% chance of eventually developing into Tropical Storm Ernesto sometime in the next ten days. Residents and visitors to the Lesser Antilles Islands should anticipate heavy rains and strong winds from 99L beginning to affect the islands as early as Friday morning. The long-range fate of 99L next week is uncertain. A track west to west-northwest through the Caribbean, or to the northwest towards the U.S. East Coast are both possible. The storm is less likely to survive if it heads northwest towards the U.S....MORE