By: Bob Henson and Jeff Masters , 3:57 PM GMT on August 24, 2016 From Weather Underground

Invest 99L is already bringing winds of tropical storm force to the northern Lesser Antilles Islands, and it could become a tropical storm at any time over the next day as it heads west-northwest at 15 mph towards Puerto Rico and the Virgin Islands. At Barbuda, sustained winds at minimal tropical storm-force—39 mph—were observed at 7:18 am AST, with a wind gust of 45 mph. At 10 am AST, Princess Juliana Airport in St. Maartin recorded sustained winds (below tropical storm-force) of 32 mph, gusting to 48 mph.

An Air Force hurricane hunter aircraft was investigating 99L late Wednesday morning, and found sustained surface winds of 45 – 50 mph, and sustained winds at their 500-foot flight level of 50 – 55 mph. At 11:35 am EDT, the National Hurricane Center issued a special Tropical Weather Outlook noting that the reconnaissance mission was still ongoing. As soon as 99L develops a well-defined surface circulation, it will be called Tropical Storm Hermine.

The storm was generating heavy rains over the northern Lesser Antilles Islands on Wednesday morning, as seen on radar out of Martinique. Satellite loops on Wednesday morning showed that heavy thunderstorm activity had increased in intensity over the previous 24 hours, and had grown more organized. A well-defined surface circulation had not yet formed, though several swirls in the cloud pattern suggested that the storm may not be far from establishing one. Moderate to high wind shear of 15 – 25 knots along the north side of the storm was keeping all of 99L’s heavy thunderstorms confined to the south. Dry air from the Saharan Air Layer (SAL) was still hindering development, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) were favorable for development: 29°C (84°F), about 0.5°C above average. A flash flood watch is posted for Puerto Rico and the U.S. Virgin Islands, where 2 – 4 inches of rain are expected.

Figure 1. Latest satellite image of 99L.

Figure 2. Radar image of 99L taken at 9:45 am EDT August 24, 2016. Heavy rains were observed over the northern Lesser Antilles Islands. Image credit: Meteo France.

Track forecast: Still uncertainty beyond The Bahamas
Since 99L had not yet formed a well-defined circulation center as of early Wednesday, it has been difficult for models to agree on its future track and intensity. Moreover, it appears that upper- and lower-level circulations are not yet well aligned. As a result, there remains a good bit of spread in how models are foreseeing its potential track.

A strong upper-level ridge now covering much of the Southeast U.S. and northern Gulf of Mexico will remain in place through the weekend, keeping 99L on its general west-northwest track for the time being. Among our three most reliable models for tropical genesis and track forecasting, the ECMWF has been the most consistent over the last couple of days. The ECMWF continues to bring 99L across or near South Florida on Sunday or Monday and moves it onward into the Gulf of Mexico. More than 90% of the 50 ECMWF ensemble members from 00Z Wednesday–and all four members of the “high probability cluster” (those that performed the best on 99L during the preceding 24 hours)–bring 99L into the Gulf. The system could make landfall next week anywhere from Texas to the Florida Panhandle, according to various ECMWF ensemble members.

The UKMET model has largely agreed with the ECMWF in recent runs, bringing 99L over Florida and into the Gulf. In contrast to its two counterparts, the GFS has been skeptical that 99L will develop at all, keeping it as a weak low–although the 00Z Wednesday run of the GFS does develop the system on its way out to sea off Florida’s East Coast. The GFS ensemble from 00Z Wednesday offers a wide range of track possibilities, with some members bringing 99L into the eastern Gulf and others keeping the system well offshore of the southeast U.S. Coast. It appears that the GFS has been putting more weight on the potential effect of Tropical Storm Gaston in helping to create a potential out-to-sea route for 99L well to the northeast of Florida. Meanwhile, UKMET ensemble runs are consistent in bringing 99L across Florida and into the Gulf, though at varying latitudes. See Figure 3 below for graphics depicting the GFS and UKMET ensemble run from 00Z Wednesday.

Given the overall agreement between the UKMET and ECMWF, and the latter’s typically high performance, a track somewhere through Florida or the Florida Straits and into the Gulf appears to be the most likely outcome, though we can’t yet rule out the northward GFS possibilities shown below. The Florida Keys will need to be especially vigilant, as 99L could track over or near them as soon as Sunday if the relatively speedy ECMWF projection is correct. The Keys are extremely vulnerable to hurricane impacts, and up to 84 hours are needed for a full evacuation along the only highway that runs from Key West to the mainland.

Figure 3. Ensemble output from the GFS (red) and UKMET (white) models from 00Z Wednesday, August 24, 2016, shows a wider range of track possibilities for Invest 99L (left) than for Tropical Storm Gaston (right). Image credit: NOAA/ESRL/GSD.

Intensity forecast: Watching and waiting
Models are now in close agreement that 99L will spend roughly 2 to 3 days over the very warm waters surrounding The Bahamas, where sea-surface temperatures are running around 29-30°C (84-86°F). The latest output from the SHIPS model (12Z Wednesday) indicates that wind shear will remain light to moderate over 99L, holding around 10-20 knots for the next couple of days and then dropping to the 5-10 knot range by this weekend. These factors support intensification, which would become more probable as soon as 99L forms a more consolidated center. The longer 99L takes to organize and the further south it tracks, the better the chance of interaction with the mountains of Hispaniola, which would interfere with its development. If the track remains further north, 99L will have a better shot at intensifying sooner. The 00Z Wednesday run of the HWRF–our top intensity model in recent years–keeps 99L as a weak wave or depression until Friday, then brings it to hurricane strength just east of South Florida on Sunday. The 06Z Wednesday HWRF run is similar, though less intense and even slower.

Even our best models are not very skilled at forecasting rapid intensification. If 99L makes it into the Bahamas with a well-organized center of circulation, the possibility of a burst of strengthening prior to any encounter with Florida would need to be monitored very closely, given the prime conditions that may be present. Should 99L enter the Gulf, it could have a longer period of potential intensification before any landfall.

The bottom line: it may well be another day, or several, before 99L manages to become a tropical storm or hurricane (assuming it does). If 99L does intensify by this weekend, there is still the possibility for tropical storm or even hurricane-level impacts in Florida. We also have the chance of a potentially serious hurricane in the Gulf of Mexico at some point next week.

Already a concern: Heavy rains and flood potential
Even if it’s well below hurricane strength, a tropical system that’s as large, moist, and slow-moving as 99L can produce torrential amounts of rain. The flow around 99L is likely to produce huge rainfall amounts and the potential for flash flooding over parts of the Dominican Republic and Haiti later this week. As noted above, a flash flood watch continued in effect Wednesday for the U.S. Virgin Islands and for Puerto Rico, where the eastern half of the island is at particular risk of flash flooding.

The 7-day precipitation outlook from NOAA’s Weather Prediction Center (WPC) calls for a large area of 3” – 8” rains across Florida from Lake Okeechobee southward. Such rains could put stress on the 80-year-old dike that protects thousands of residents near Lake Okeechoobee, as discussed by Jeff Masters in a special post earlier today.

The WPC outlook also projects several inches of rain for hard-hit southeast Louisiana early next week, anticipating the potential effects of 99L’s circulation in the Gulf of Mexico.

Figure 4. Enhanced infrared image of Tropical Storm Gaston at 1445Z (10:45 am EDT) Wednesday, August 24, 2016. Image credit: NOAA/NESDIS.

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