Invest 99L Finally Develops Into Tropical Depression 9 in the Florida Straits

After spending ten days in meteorological limbo-land frustrating forecasters as an “Invest”, 99L finally developed into Tropical Depression Nine, confirmed a NOAA hurricane hunter aircraft late Sunday afternoon. But the storm isn’t done perplexing us yet—the model predictions for the future intensity of the storm remain wildly divergent, even if we now have growing confidence that this storm will track into the coast of Florida north of Tampa on Thursday.


Figure 1. MODIS visible satellite image of Tropical Depression Nine (formerly 99L) forming in the Florida Straits on Sunday afternoon, August 28, 2016. Image credit: NASA.

Satellite images on Sunday evening showed a steady increase in the intensity and areal coverage of TD 9’s heavy thunderstorms, though Key West radar showed only a few spiral bands trying to form near the center. The depression is not likely to organize quickly, as it was dealing with wind shear that was a moderately high 15 - 20 knots. TD 9 was also struggling with dry air, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) remained favorable for development, though, near 30 - 30.5°C (86 - 87°F).


Figure 2. Total rainfall from Tropical Depression Nine as of 8 pm EDT Sunday, August 28, 2016. Rainfall amounts of 2 - 4 inches were common over Cuba.

Track forecast for TD 9
There is now model consensus among the GFS, European, HWRF and UKMET models that TD 9 will continue on its current west-northwest track through Monday, slow down and stall out in the central Gulf of Mexico on Tuesday, then get caught in the steering flow of a trough of low pressure passing to its north on Wednesday. These steering currents should bring TD 9 to a landfall on the Florida coast north of Tampa on Thursday. In their 5 pm EDT Sunday Wind Probability Forecast, the highest odds for getting tropical storm force winds of 34+ mph from TD 9 were 26%, 24%, and 22%, respectively, for Apalachicola, Panama City, and Cedar Key, Florida. Tampa was given 18% odds.

Intensity forecast for TD 9
The SHIPS model on Sunday afternoon predicted moderately favorable conditions for intensification, with wind shear falling to a moderate 10 - 15 knots, Monday through Wednesday. SSTs will be a very warm 30°C (86°F), and mid-level relative humidity was predicted to be a reasonably moist 65 - 70%. However, the usually reliable European and GFS models showed little to no development of TD 9 in their latest 12Z Sunday (8 am EDT) runs. Our best intensity model, the HWRF model, had TD 9 rapidly intensifying into a strong Category 2 hurricane just before landfall. Other intensity models like the DSHIPS and LGEM models had TD 9 as a borderline Category 1 hurricane at landfall. This storm’s history has been to under-perform, so NHC’s conservative forecast of a 50 mph tropical storm at landfall was a reasonable first guess, given the storm’s weak showing in the GFS and European models. But TD 9 is hiding its cards—still—and could easily be an intensifying hurricane at landfall. The latest 18Z Sunday forecast from the GFS model had TD 9 about 5 - 10 mph stronger than in its previous run. I support a forecast of TD 9 being a strong tropical storm near hurricane strength at landfall—70 mph winds—until the models come into better alignment.

Bob Henson had a detailed summary of the action in the rest of the tropics in his 2 pm EDT Sunday post. We’ll be back Monday morning with a fresh look at the tropics.

Jeff Masters