Threat to Southeast U.S. Growing From 99L; Gaston Forms in Eastern Atlantic

The Hurricane Hunters are in the air, investigating Invest 99L, a steadily organizing tropical wave that was located about 300 miles east of the northern Lesser Antilles late Tuesday morning. This storm will bring heavy rains and gusty winds to the northern Lesser Antilles Islands Tuesday evening through Wednesday, and is likely to develop into a tropical storm or hurricane that will affect the Bahamas and the Southeast U.S. coast late this week or early next week.

Satellite loops on Tuesday morning showed that heavy thunderstorm activity associated with 99L had slowly increased over the the previous 24 hours. The storm was still poorly organized, though, with no surface circulation center apparent and few low-level spiral bands. Upper-level cirrus clouds streaming away from the northeast side of the storm gave evidence that an upper-level outflow channel was trying to develop, however. Dry air from the Saharan Air Layer (SAL) that has been interfering with development over the past few days had decreased, as seen on water vapor satellite imagery. Wind shear was marginally favorable for development, at 10 - 15 knots, and sea surface temperatures (SSTs) were favorable for development: 28.5 - 29°C (83 - 84°F) (about 0.5°C above average.)


Figure 1. Latest satellite image of 99L.

Track forecast: 99L likely to affect the Bahamas and Southeast U.S.
A strong ridge of high pressure will keep 99L headed west-northwest over the next few days, and the storm will traverse the northern Lesser Antilles Tuesday night through Wednesday, track close to or just north of Puerto Rico on Wednesday night, then reach the Southeastern Bahamas by Thursday. At this time, the models keep 99L far enough north of the island of Hispaniola to prevent the high mountains there from significantly disrupting 99L. However, we cannot have confidence in this forecast until 99L develops a well-defined center that the models can track. Due to its large size, 99L will be capable of bringing torrential rains and flash flooding and mudslides to the Dominican Republic and Haiti. The uncertainty about the track increases greatly on Friday and into the weekend, due to a potential weakness in the ridge of high pressure steering the storm caused by a trough of low pressure passing to the north of 99L. The storm should slow its forward motion to 5 - 10 mph Friday through Saturday, in response to this trough, and may turn to the northwest or north near the central Bahamas. Over 80% of the ensemble members from the 00Z Tuesday runs of the European and GFS models show 99L hitting the Southeast U.S. between Florida and South Carolina sometime Sunday or later; very few show the storm missing the U.S. entirely. The track and intensity of 99L may be affected by the remnants of Tropical Storm Fiona, which NHC stopped issuing advisories on at 11 am EDT Tuesday. The remains of Fiona will be a few hundred miles to the north or northeast of 99L this weekend. Hurricane Gaston will be too far from 99L to exert a steering influence on it.


Figure 2. The 00Z Tuesday morning run of the operational European model (run at high resolution, shown in red) and its ensemble members (50 runs with slightly different initial conditions done at lower resolution, shown in black) came up with a variety of solutions for the future track and intensity of 99L. The operational model run—which is usually the best forecast, since the model runs at the highest resolution with the proper initial conditions—showed a track for 99L across Florida and into the Gulf of Mexico, with the storm peaking as a Category 1 hurricane. The track and intensity forecasts of the four members of the European ensemble that have done the best job tracking the progress of 99L over the previous 24 hours (called the “high probability cluster”) are also plotted here. They show that a more northeasterly track may be likely. Note that while the European model is our best model for predicting hurricane tracks, it does a poor job forecasting intensity and is generally disregarded by NHC for making intensity forecasts. Image taken from a custom software package used by Weather Underground.

Intensity forecast for 99L: a potentially dangerous storm for the Southeast U.S.
Heavy rains from 99L will likely cause flash flooding problems in the Lesser Antilles, Virgin Islands, and Puerto Rico. Wind damage should not be an issue, since 99L will likely be, at worst, a moderate-strength tropical storm with 55 mph winds once it leaves the islands. The 8 am EDT Tuesday run of the SHIPS model showed moderately favorable conditions for development through Friday. Wind shear will be in the light to moderate range, 5 - 15 knots, and SSTs will increase from 28.5°C (83°F) to 30°C (86°F), accompanied by an increase in the total heat content of the ocean. Working against development of 99L will be the large size of the storm, dry air of the SAL, and large-scale sinking air over the tropical Atlantic imparted by an unfavorable phase of the Madden-Julian Oscillation (MJO). Two of the Tuesday morning (00Z) operational runs of our three reliable models for predicting tropical cyclone genesis--the European, and UKMET models--showed development of 99L into a tropical storm over the next five days. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 50% and 60%, respectively. I think the 5-day odds should be higher, at 70%.

This storm has the potential to be a dangerous one for the Bahamas and the Southeast United States. The models are predicting a favorable environment for development near the Bahamas, and the storm will be moving quite slowly during its closest approach to the islands, potentially allowing for some very high rainfall totals in the Bahamas. While the 00Z Tuesday run of the GFS model did not show development of 99L into a tropical depression, the European model ensemble forecast had 1/3 of its members predicting that 99L would eventually become a hurricane. The storm will likely spend 2 - 3 days over a region of ocean with SSTs that are near record-warm: 29 - 30°C (84 - 86°F). Warm waters extend to great depth, resulting in an unusually high total ocean heat content. With some models forecasting that wind shear will be in the light to moderate range at this time, we have the potential for 99L--which has a large circulation--to rapidly intensify into a large major hurricane that will hit the Southeast U.S. coast on Sunday or Monday. The uncertainties are high, and we will have to wait for the storm to develop into a tropical depression before we can have more confidence in what the models are saying.

Gaston forms in the Eastern Atlantic
Tropical Storm Gaston, the seventh named storm of the 2016 Atlantic hurricane season, formed early Wednesday morning in the eastern Atlantic, a few hundred miles west of the Cabo Verde Islands. Gaston’s formation date of August 23 comes more than three weeks earlier than the usual September 16 formation date of the season’s seventh storm. Gaston is headed northwestwards into a area of ocean where it is highly unlikely to be a threat to any land areas.

We’ll have a new post late this afternoon.

Jeff Masters