Florida’s First Hurricane Warning in 4 Years as Hermine Intensifies to 65 mph

Tropical Storm Hermine is gathering strength as it steams north-northeastwards at 10 mph towards the Florida Panhandle, and appears poised to give Florida its first hurricane strike in nearly eleven years when it crosses the coast late tonight or early Friday morning. A NOAA hurricane hunter aircraft was in Hermine early Thursday morning, and found top surface winds of 65 mph to the east of the center, and a central pressure that had fallen to 992 mb. Buoy 42003, located about 65 miles to the east-southeast of Hermine’s center, had sustained winds of 50 mph gusting to 60 mph at 5:50 am EDT Thursday. Wave heights at the buoy had built from 8 feet to 19 feet over the previous 24 hours. Strong winds from Hermine continued to create storm surge heights over 1’ along the entire Gulf Coast from New Orleans, Louisiana to Naples, Florida on Thursday morning. The maximum surge so far from the storm was about 2.5’ at Cedar Key, Florida on Wednesday evening. Satellite images on Thursday morning showed a steadily organizing storm, with heavy thunderstorms building near the storm’s center and some significant low-level spiral bands forming. It’s a good thing Hermine didn’t get its act together a day earlier, or we’d be looking a a major hurricane in the Gulf of Mexico today. Wind shear continued to be a moderate 10 - 15 knots on Thursday morning, but the direction of the shear had switched to the west-southwest. This switch allowed Hermine to begin intensifying more rapidly, since the upstream air was not quite as dry to the west-southwest, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) near TD 9’s center remained favorable for development, near 30.5°C (87°F).


Figure 1. Blended visible/IR satellite image of Tropical Storm Hermine at dawn, 7:45 am EDT September 1, 2016. Image credit: Navy Research Lab NexSat page.


Figure 2. The Hurricane Warning for Hermine is Florida’s first one since August 25, 2012, when Hurricane Isaac threatened South Florida and the Panhandle. During the period 2006 - 2015, only three storms provoked a Hurricane Warning in Florida. Image credit: Iowa State University.

Intensity forecast: Hermine expected to become a Category 1 hurricane
The SHIPS model on Thursday morning predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 15 knots through landfall on Thursday evening. SSTs will be a very warm 30 - 30.5°C (86 - 87°F), and mid-level relative humidity was predicted to be a reasonably moist 65%. Our three best intensity models--the HWRF, DSHIPS and LGEM models--were in agreement with their latest runs available early Thursday morning on a landfall intensity of 75 mph—minimum Category 1 hurricane strength. The Gulf Coast of Florida is highly vulnerable to large storm surges, due to the extensive stretch of shallow continental shelf waters offshore that extend up to 90 miles from the coast, and NHC has increased their maximum storm surge forecast to 5 - 7’ above ground along a stretch of the Florida coast to the right of where the center is expected to make landfall. Hermine is a large storm, with tropical storm-force winds that extend out up to 140 miles east and southeast of the center, and will likely deliver a storm surge of at least 3 feet to a 150-mile stretch of the Florida Gulf Coast and a 150-mile stretch of the Georgia and southern South Carolina coast.


Figure 3. Radar estimated rainfall between August 29 - and 7:44 am EDT September 1, 2016 from the Tampa radar. Swaths of 2 - 4” of rain (yellow colors) were common over Florida, with some areas of 4 - 6” near Tampa and Melbourne. Heavy rain will be the main threat from Hermine, with rainfall amounts of 5 - 10" expected along its track through northern Florida and southern Georgia.


Figure 4. Projected 7-day rainfall from 12Z (8:00 am EDT) Thursday, September 1, through 12Z September 8, 2016. Rainfall amounts of 5 - 10” are expected along Hermine’s path across Florida and along the Southeast U.S. coast. Image credit: NOAA/NWS Weather Prediction Center.

Track forecast for Hermine: a Florida Gulf Coast landfall, followed by a run up the Southeast coast
The latest Thursday morning runs of our top models are in solid agreement that Hermine will make landfall along the Florida Big Bend coast on Thursday evening near midnight. In their 5 am EDT Thursday Wind Probability Forecast, NHC’s highest odds for getting tropical storm force winds of 34+ mph from Hermine along the Gulf Coast of Florida were 74%, 70%, and 61%, respectively, for Apalachicola, St. Marks and Cedar Key, Florida. After landfall, Hermine will become embedded in a cold front as it sweeps through Georgia, and the storm will begin transitioning to a powerful extratropical storm, deriving energy from atmospheric dynamics rather than from the heat energy of the ocean. This extra energy source should allow Hermine to maintain tropical storm intensity as it speeds to the northeast along the Southeast U.S. coast. In their 5 am EDT Thursday Wind Probability Forecast, NHC gave odds of at least 30% for tropical storm-force winds to affect the entire U.S. coast from northern Florida to Cape Hatteras, North Carolina. Hermine is expected to become fully extratropical by Saturday night, when it will be offshore from North Carolina. On Sunday and Monday, ex-Hermine is expected to stall out off the Mid-Atlantic coast, as the storm becomes entangled with an upper-level trough of low pressure. The storm won’t be named Hermine anymore, but perhaps we should call it “Her-Mean”, since this very large and wet storm will bring high winds and heavy rains to much of the Mid-Atlantic and southern New England coasts during the Sunday and Monday portion of the Labor Day holiday weekend.

Bob and I will have a full update on the tropics, including the latest on Hermine, around noon EDT today.

Jeff Masters