Taking Its Time, 99L Remains a Potential Threat for Florida, Gulf

By Jeff Masters and Bob Henson
Published: 10:20 PM GMT on August 25, 2016

The watching and waiting continues for Invest 99L as it rolls toward The Bahamas. 99L remained a very large but very disorganized tropical wave on Thursday afternoon. Visible satellite imagery shows one circulation near the extreme southeastern portion of the Bahamas and Turks and Caicos Islands. This circulation is largely naked, with little cloudiness around it, although some filling-in was evident late Thursday afternoon. Air Force Hurricane Hunters found winds of tropical-storm force in 99L early Thursday, but since the circulation was not fully closed, it could not yet be classified as a tropical depression or tropical storm. On their second mission of the day, Air Force Hurricane Hunters found that winds to the east of this not-quite-closed circulation had decreased by early afternoon Thursday to just below tropical storm force.

Well south of the storm-free circulation center of 99L, very intense thunderstorms were developing late Thursday afternoon along the north coast of Haiti and the Dominican Republic, while decreasing on the south side of the island. Most provinces of the Dominican Republic were on flood alert Thursday evening, and rivers were already surging according to an Associated Press report.

Based on the Thursday afternoon flight data, NHC’s Tropical Weather Outlook issued at 2:00 pm EDT Thursday reduced the odds that 99L will develop into at least a tropical depression to 40% by Saturday and 70% by Tuesday. The slow pace of development thus far isn’t a total shock, as a number of computer models suggested as far back as Monday night that 99L would be no more a mid-strength tropical storm at best by this point. However, the lack of development at this point will keep 99L from growing as intense as it otherwise might have been by this weekend.

Figure 1. Visible image of Invest 99L as of 2045Z (4:45 pm EDT) Thursday, August 25, 2016. Image credit: NOAA/NESDIS.

Figure 2. Enhanced infrared image of Invest 99L as of 2045Z (4:45 pm EDT) Thursday, August 25, 2016. Image credit: NOAA/NESDIS.

99L remains a potential threat for Florida and other Gulf Coast states
It’s too soon to completely write off 99L as a threat to Florida, as it still has at least 48 hours to organize before reaching the area. The wave remains a potentially significant threat to the Gulf of Mexico coast next week. Up to now, a substantial amount of vertical wind shear has helped to displace the upper- and lower-level portions of 99L, while also driving large amounts of dry air into the wave. The University of Wisconsin-CIMSS wind shear analysis from Thursday afternoon was still showing a high 25 knots of wind shear over the core of 99L, which likely accounts for the inability of the storm to develop any heavy thunderstorm activity at its center of circulation. This state of affairs may change in the near future, though. Wind shear as analyzed by the SHIPS model is forecast to fall below 15 knots by Friday afternoon and then to remain in the low to moderate range, 5 - 15 knots, through Tuesday.

Along with the reduced wind shear on Friday and Saturday, 99L will be traveling over very warm waters (around 30°C or 86°F). Since tropical waves often develop convection overnight, we’ll have to see if 99L can form a core of showers and thunderstorms around its low-level center during this usual noctural peak. If it does, the chances of a substantial tropical storm reaching Florida will rise significantly.

Figure 3. Vertical wind shear (shown here as the difference between winds at the 200- and 850-mb heights, or between about 40,00 and 5000 feet) at 18Z (2:00 pm EDT) Thursday, August 25, 2016. Northerly shear of greater than 20 knots has prevailed over 99L, but the wave will be moving into an area of lower wind shear this weekend. Image credit: tropicaltidbits.com.

99L is likely heading toward the Gulf of Mexico
The lack of a well-defined center makes it more difficult for computer models to predict the track and intensity of 99L. If a center does consolidate, it appears likely to move in a general west-northwest direction that would bring it near the southern Florida peninsula late Saturday or Sunday. A slight bend toward the west, as suggested by the ECMWF model, could allow 99L to miss the peninsula entirely, perhaps crossing the Florida Straits. In either case, 99L appears likely to enter the eastern Gulf of Mexico by the start of next week. Conditions in the eastern Gulf will favor development, and the ECMWF, UKMET, and HWRF models have been largely consistent in showing 99L intensifying across the Gulf, perhaps well beyond hurricane strength. The GFS is stubbornly in the other camp, continuing to insist that 99L will not develop significantly.

NOAA/RAMMB has this excellent 1-minute resolution loop of 99L. You can really see how an upper level anticyclone centered to the east of 99L is bringing strong southerly upper-level winds to the south of 99L, blowing the tops of the thunderstorms from south to north over Cuba and Hispaniola.

A multi-day animation from the Navy is excellent, too.

We’ll be back with an update by midday Friday.

Bob Henson and Jeff Masters

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About The Author
Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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