Wednesday, August 24, 2016

Hurricane Watch: Invest 99L Poised to Become Tropical Storm Hermine

As noted yesterday:
For readers new to this stuff the order of development is: Invest; Tropical Depression; Tropical Storm; Hurricane, categories 1-5....
From Wunderblog:
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....

...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
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.
Hurricane Watch: Mid-Atlantic Structure Has Development Potential