Aug 25, 2016

Is the Wait-and-See Game With 99L Ending?

Dominican Republic
Jeff Masters and Bob Henson
Weather Underground
Satellite image of 99L taken at 10:39 am EDT August 25, 2016. A surface circulation center was trying to form over the southeastern Bahamas and Turks and Caicos Islands. Image: NASA, Weather Underground
Satellite image of 99L taken at 10:39 am EDT August 25, 2016. A surface circulation center was trying to form over the southeastern Bahamas and Turks and Caicos Islands. Image: NASA, Weather Underground

A high-stakes game of wait-and-see is underway with a large but disorganized tropical wave (Invest 99L) centered near the southeastern Bahama Islands on Thursday morning. The storm brought heavy rains of 3 - 5” over the past 24 hours to portions of the Dominican Republic and Puerto Rico, and was generating winds near tropical storm force in the waters to the north of Hispaniola. 99L could become a tropical depression or tropical storm at any time over the next three days as it heads west-northwest through The Bahamas. If 99L develops a well-defined surface circulation and tropical storm-force winds, it will be called Tropical Storm Hermine.

Satellite loops late Thursday morning showed the possibility that the game of wait-and-see may be ending. A well-defined surface circulation was attempting to form near the extreme southeastern portion of the Bahamas and Turks and Caicos Islands. However, 99L’s heavy thunderstorms were several hundred miles to the southeast of this surface swirl, and there was very little thunderstorm activity near the center. Development of heavy thunderstorms was being inhibited by high wind shear of 15 - 25 knots and dry air, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) were favorable for development, though: 29°C (84°F)...

Track forecast for 99L
A strong upper-level ridge now covering much of the Southeast U.S. and northern Gulf of Mexico will remain in place through Saturday, which should keep 99L on its general west-northwest track for the next three days. The storm will slow down from its current 15 mph forward speed to about 8 mph on Friday and Saturday, and reach the northwest Bahamas on Saturday and South Florida or the Florida Keys on Sunday. At that point, the models predict that the ridge of high pressure steering the storm will weaken, allowing 99L to turn more to the north. The timing of this turn and how far 99L might make it into the Gulf of Mexico is uncertain, though the Florida Gulf Coast is currently the area of the Gulf Coast considered to be most at risk by the models.

Intensity forecast for 99L
Since 99L is now pulling away from the high mountains of Hispaniola and is moving into an area of higher moisture and lower wind shear, the storm is likely to consolidate around the circulation center attempting to form in the southeast Bahamas. The storm should be able to develop into a tropical depression or tropical storm on Friday or Saturday. The SHIPS model predicts that wind shear will fall to the low range, 5 - 10 knots, on Friday through Sunday, and SSTs will increase to 30°C (86°F)—conditions that are very favorable for development. However, there will still be some dry air for 99L to contend with, and the storm is large, which will slow down development.

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 0Z Thursday (8 pm EDT Wednesday) runs. These models brought 99L across South Florida or the Florida Keys on Sunday and into the Gulf of Mexico, with a second landfall occurring on the Florida Gulf Coast on Tuesday.

Unfortunately, we don’t have much skill forecasting rapid intensification, but the possibility exists that 99L could undergo a period of rapid intensification in the 24 hours before landfall in South Florida and arrive there as a Category 1 hurricane on Sunday. However, it is more likely that South Florida will experience a tropical storm. If the storm manages to spend an extra day of two over the warm waters of the Gulf of Mexico on Monday and Tuesday, 99L will have additional time to organize and intensify, and there is a greater chance it will be a hurricane for its second potential landfall on Tuesday.

Regardless of whether or not 99L ever becomes Tropical Storm Hermine, Florida is going to get a lot of rain from this system. The 7-day precipitation outlook from NOAA’s Weather Prediction Center (WPC) calls for a large area of 5” - 8” rains across southern and western Florida over the next week. Such rains could put stress on the 80-year-old dike that protects thousands of residents near Lake Okeechobee