Huge 99L Generating 55 mph Winds, But Remains Disorganized

By Jeff Masters
Published: 10:13 PM GMT die 24o August, anno 2016

A huge and powerful tropical wave (Invest 99L) is generating winds of tropical storm force near the Virgin Islands, and could become a tropical storm at any time over the next two days as it heads west-northwest at 15 mph towards The Bahamas. If 99L develops a well-defined surface circulation, it will be called Tropical Storm Hermine. An Air Force hurricane hunter aircraft was investigating 99L early Wednesday afternoon and found sustained surface winds of 50 - 55 mph just to the northeast of St. Croix in the Virgin Islands. The storm has brought widespread rainfall amounts of 1” to Puerto Rico and the Virgin islands as estimated by San Juan radar, with a one area of northwest Puerto Rico receiving over 3”. A flash flood watch continues for Puerto Rico and the U.S. Virgin Islands until midnight Wednesday.

Satellite loops late Wednesday afternoon showed a dramatic west-northwestward expansion of 99L’s heavy thunderstorm activity, and this sprawling tropical wave was spreading heavy rains across a 1000-mile wide stretch of ocean, from the southeast Bahama Islands to Barbados in the southern Lesser Antilles Islands. It will be difficult for such a massive storm to develop a well-defined surface circulation, and multiple swirls separated by hundreds of miles have been evident in the system during the day. By Wednesday afternoon, wind shear had increased by 10 knots in 24 hours over 99L’s northern flank, to a high 15 - 35 knots. This wind shear was prohibiting the storm from getting organized, as was dry air from the Saharan Air Layer (SAL), as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) were favorable for development: 29°C (84°F), about 1°C above average.


Figure 1. At 4:45 pm EDT Wednesday, 99L sprawled out over a area 1,000 miles across. Two swirls in the cloud pattern were evident, and 99L did not have a single well-defined circulation center. Image credit: NOAA.


Figure 2. Tropical storm-force winds of 55 mph in 99L as seen from an altitude of 600 feet during Wednesday afternoon’s hurricane hunter flight. Image credit: ARWO 1st LT Carpenter, Air Force Hurricane Hunters. Thanks go to wunderground member Saltydogbwi1 for posting this image in the blog comments.

Forecast for 99L still uncertain
Since 99L has not yet formed a well-defined circulation center, it has been difficult for models to agree on its future track and intensity. This situation will likely continue until at least Thursday afternoon, when the SHIPS model predicts that wind shear will fall to the moderate range, 10 - 20 knots. This drop in shear will potentially allow 99L to organize into a tropical storm and give the models something more substantial to chew on.

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, which should keep 99L on its general west-northwest track through at least Friday. Two of our three reliable models for predicting tropical genesis, the ECMWF and UKMET, continued to show development of 99L into a tropical storm by Friday in their latest 12Z Wednesday runs. These models brought 99L across or near South Florida on Sunday and into the Gulf of Mexico, with a second landfall occurring on the Florida Gulf Coast on Tuesday. Our other reliable tropical cyclone genesis model, the GFS, continued to insist that 99L would not develop through Sunday. In their 2 pm EDT Wednesday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 60% and 80%, respectively.

We’ll be back with a full analysis on Thursday morning. The Hurricane Hunters have flights scheduled every six hours into the storm. Wunderblogger Steve Gregory offers his take on 99L in his Wednesday afternoon post.

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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