Still-Disorganized 99L Remains a Threat to Southeast U.S. Coast; Gaston Growing

A large and strong tropical wave about 250 miles east of the Lesser Antilles, Invest 99L, continues to move west-northwest at about 20 mph on a course that would bring it within striking distance of the southeast U.S. coast by late in the weekend or early next week, quite possibly at hurricane strength. Venturing into 99L on Tuesday, Air Force Hurricane Hunters found that the wave had an poorly defined circulation, elongated from north to south. Often it will take one or more days for a symmetric, circular center to emerge inside a tropical wave this large. After that point, convection (showers and thunderstorms) can consolidate and the system can grow more rapidly if conditions are favorable. A substantial growth in convection may occur overnight Tuesday, as tropical systems often experience convective bursts during the nighttime hours. Late Tuesday afternoon, convection was scattered over a broad area around 99L (see Figure 1). Increasing upper-level outflow was evident, especially on the wave’s north side.

Two of the latest Tuesday (12Z) 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 its 2 pm EDT Tropical Weather Outlook, the National Hurricane Center maintained 40 percent odds that 99L will develop at least to tropical depression strength by Thursday and 60 percent odds by Sunday. Given the scenarios being painted by models, and given the location and time of year, the 60% NHC odds that we will have at least a tropical depression by Sunday are on the conservative side, as noted by Jeff Masters in this morning’s post.


Figure 1. Enhanced infrared image of Invest 99 as of 1945Z (3:45 pm EDT) Tuesday, August 23, 2016. Image credit: CIRA/RAMMB/CSU.

Track forecast: 99L likely to affect The Bahamas and Southeast U.S.
One big question for 99L where a center of circulation might develop within the north-south swath of elongation. In its Tropical Weather Discussion issued at 2 pm EDT Tuesday, NHC placed the elongation between 12°N and 20°N, with a center of low pressure near 17°N, 56°W. Late Tuesday afternoon, satellite imagery suggested that a center of circulation may be trying to form around 15-16°N, roughly 70 to 140 miles further south. If this apparent trend holds, it could raise the odds of 99L passing near Puerto Rico on Wednesday and just to the north of Hispanola on Thursday, potentially slowing any rapid short-term development of 99L. Regardless of where the center consolidates, heavy rains from 99L’s large circulation could easily pose flooding problems for islands as far west as Hispaniola. A flash flood watch was already in effect Wednesday for Puerto Rico and the U.S. Virgin Islands, where widespread 2” - 4” totals and locally higher amounts are expected.

Models are in strong agreement on 99L’s general west-northwest motion around a strong upper-level ridge in the Atlantic subtropics over the next several days, which would bring 99L into The Bahamas by Friday into the weekend. There could be a slight northwestward bend around Friday due to a potential weakness in the ridge produced by the remains of now-deceased Tropical Storm Fiona. Beyond that point, models generally agree that the ridge will restrengthen and send 99L back toward the west-northwest or west. The 12Z runs of the ECMWF and UKMET models take 99L on quite similar tracks across southern Florida on Sunday or Monday, then onward into the Gulf of Mexico, which would open the possibility of a landfall somewhere along the Gulf Coast next week. The other of the three most reliable models for tropical cyclone genesis, the GFS, has failed to develop 99L for its last several runs, keeping it as an open wave. The 12Z run of the HWRF model also failed to develop 99L--a major shift from its previous few runs.

Intensity forecast for 99L: a potentially dangerous storm for the Southeast U.S. (and perhaps the Gulf)
It may take some time for 99L to develop into what could be a formidable storm. The 2 pm EDT Tuesday run of the SHIPS model continued to show 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 from the Saharan air layer (SAL), and large-scale sinking air over the tropical Atlantic imparted by an unfavorable phase of the Madden-Julian Oscillation (MJO).

Assuming that it consolidates and develops into a tropical cyclone, as projected by the majority of reliable models, 99L has the potential to strengthen significantly toward the weekend. The region surrounding The Bahamas is a notorious breeding ground for hurricane development, especially in late August and September, when the fairly shallow waters are at their warmest. The track forecast suggests that 99L could be moving for 2-3 days across near-record warm waters around 30°C (86°F). There will be ample amounts of oceanic heat content, and upper-level wind shear is expected to remain light. In short, we have the potential for 99L--which has a large circulation--to rapidly intensify into a large major hurricane that would strike the southeast U.S. coast on Sunday or Monday, potentially moving into the Gulf of Mexico. At present, Florida appears to be at greater risk than states further north. The uncertainties are high, though, 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.


Figure X. Enhanced infrared image of Tropical Storm Gaston as of 1745Z (1:45 pm EDT) Tuesday, August 23, 2016. Image credit: NOAA/NESDIS.

Gaston is on its way to hurricane status
After an impressive growth spurt earlier on Tuesday, Tropical Storm Gaston continued to pack top sustained winds of 65 mph as of the 5 pm EDT update from NHC. An eye-like feature has intermittently appeared within Gaston during the day Tuesday. Located far out in the eastern Atlantic at 14.2°N, 35.8°W, Gaston is now projected to become a strong Category 1 hurricane before it encounters several days of strong wind shear (above 20 knots). Beyond that point, it could strengthen even further as it passes over subtropical SSTs that will be 1-2°C above average for this time of year. Fortunately, Gaston’s northwestward path will keep it many hundreds of miles from any land area for at least the next several days, and it is likely to recurve long before approaching North America or even Bermuda.

We’ll have our next update on Wednesday morning.

Bob Henson