Lesser Antilles Disturbance 97L Disorganized

A tropical wave over the Lesser Antilles Islands (Invest 97L) is moving west to west-northwest at 10 mph, and is bringing sporadic heavy rain showers to the islands. Top sustained winds observed in the islands as of 10 am AST Monday were mostly below 15 mph. Heavy thunderstorm activity has decreased in organization since Sunday, and is spread out over a larger area, as seen on satellite loops. The thunderstorms are poorly organized, and there is no sign of a well-organized surface circulation. Martinique radar shows little evidence of rotation to the echoes, and no low-level spiral bands forming. Upper level winds are favorable for development, with wind shear a low 5 - 10 knots, and an upper-level anticyclone overhead. An area of dry air from the Saharan Air Layer (SAL) surrounds 97L and is interfering with development, though the atmosphere has moistened since Sunday. The 12Z Monday balloon sounding from Barbados in the southern Lesser Antilles showed less than 10 knots of wind shear and a moist atmosphere. Wind shear was in the moderate range and the atmosphere was drier in the northern Lesser Antilles at Guadeloupe.


Figure 1. Latest satellite image of Invest 97L.

Forecast for 97L
Wind shear is expected to be low for the next five days, and ocean temperatures will be warm, 28 - 29°C. The disturbance is steadily moistening the atmosphere and is moving into a moister environment, so dry air will be less of an impediment to development as the week progresses. A key factor keeping the disturbance from developing over the next two days is the fact that 97L is quite large, and is stretched out from east to west over a wide expanse. Large, elongated systems like 97L usually take several days to consolidate and spin up. Another factor that will likely retard development is the presence of strong surface trade winds over the Eastern Caribbean ahead of 97L, south of the Dominican Republic. These strong trade winds are a common feature of the Eastern Caribbean, and make the region something of a hurricane graveyard. As the surface wind flow to the west of 97L accelerates into this wind max, air will be sucked from aloft downward towards the surface, creating sinking air, interfering with the formation of thunderstorm updrafts.

The models have shifted markedly in their projected path for 97L, and now take the system more to the west-northwest, bringing it near or over Hispaniola on Wednesday. A track over the high mountains of that island would disrupt the circulation of 97L, forcing the storm to regroup on Thursday over the Southeast Bahamas and Turks and Caicos Islands. The best chance for development of 97L would appear to be on on Thursday, after it has finished its encounter with Hispaniola. This morning's 06Z run of the experimental GFDL ensemble--which produces 10 simulations with slightly varying initial conditions to create a plume of potential storm tracks--foresees that once 97L organizes into a tropical depression, it might take only two days for it to intensify into a hurricane to the north of Hispaniola. However, a number of storm tracks from the 00Z Monday GFS and ECMWF ensembles foresee a more westerly track for 97L over Eastern Cuba, which would further disrupt the storm. The wave will bring heavy rain showers to the Lesser Antilles islands on Monday and Tuesday, and this activity will likely spread to Puerto Rico, the Virgin Islands, the Dominican Republic, Haiti, and Eastern Cuba on Tuesday and Wednesday. These rains will be capable of causing life-threatening flash flooding and mudslides in mountainous regions in Puerto Rico, Hispaniola, and Cuba. Of our three reliable models for predicting genesis, the UKMET, GFS, and European models, only the UKMET model develops the disturbance, predicting it will become a tropical depression over the Southeast Bahamas on Thursday. There will be a strong trough of low pressure along the U.S. East Coast this weekend, which will be capable of turning 97L to the north before the storm can hit the U.S. In their 8 am EDT Monday Tropical Weather Outlook, NHC left the 5-day odds of formation of 97L at 50%, and dropped the 2-day odds to 20%. The Hurricane Hunters are on call to investigate 97L on Tuesday.

Have a great Labor Day, everyone!

Jeff Masters