WeatherFanatics: Hurricane Hermine Poised to Make First Hurricane Strike on Florida in 11 Years

By: Jeff Masters , 10:32PM,GMT on September 1,2016

An Air Force hurricane hunter aircraft confirmed on Thursday afternoon that Hurricane Hermine had formed in the Gulf of Mexico. With Hurricane Gaston also active in the Central Atlantic, we now have multiple hurricanes in the Atlantic for the first time since the first week of September 2012, when Hurricane Leslie and Hurricane Michael were both active. Hermine will cross the coast late tonight or early Friday morning, making it Florida’s first hurricane strike in nearly eleven years, since Hurricane Wilma hit South Florida as a Category 3 storm in October 2005. Hermine will also be the first hurricane to strike the U.S. since Hurricane Arthur hit North Carolina on July 3, 2014 as a Category 2 storm with 100 mph winds. According to hurricane expert Brian McNoldy, the last time a hurricane made landfall within 1° (~70 mi) of where Hermine will was 50 years ago: Alma 1966.

Figure 1. MODIS visible satellite image of Hermine taken at 12:30 pm EDT September 1, 2016. At the time, Hermine was a tropical storm with 75 mph winds, and would be upgraded to a hurricane an hour later. Image credit: NASA.

Figure 2. Radar image of Hurricane Hermine taken at 5:15 pm EDT September 1, 2016.

Figure 3. On Thursday afternoon between 4:31 and 4:51 pm EDT, a NOAA hurricane hunter aircraft recorded a classic pass through a Category 1 hurricane eyewall and eye at their flight level of 10,000 feet. Peak winds at flight level were around 70 knots (80 mph), and an extrapolated pressure of 985 mb was measured in an eye that was 5°C (9°F) warmer than its surroundings. Image annotated from tropicaltidbits.com.

Conditions deteriorating rapidly on the Florida Gulf Coast
Tropical storm-force winds of 39+ mph were occurring at a number of buoys along the Panhandle of Florida late Thursday afternoon. The strongest winds were at buoy 42036 112 nm WNW of Tampa, Florida: sustained at 58 mph gusting to 76 mph at 5:50 pm EDT, with a wave height that had increased from 8 feet to 24 feet over the previous 24 hours. Hermine’s storm surge is building steadily along the Florida Gulf Coast, as seen using our wundermap with the Storm Surge layer turned on, or the NOAA Tides and Currents page for Hermine. At 6 pm EDT Thursday, the storm surge was approximately 3.5’ at Cedar Key, 3.3’ at St. Petersburg, 2.7’ at Tampa and 2.5’ at Apalachicola, Florida. The surge will increase dramatically during the overnight hours, especially along the eastern side of Apalachee Bay and perhaps as far south as Tarpon Springs, according to storm surge expert Hal Needham. He adds that low tide in Cedar Key is at 9 am EDT Thursday and high tide at 3 am Friday, so it is likely that the highest water will be after midnight in some areas.

Hermine was bringing heavy rains to the coast, with rainfall rates of 0.5 - 1.0 inches per hour seen on the Tallahassee radar. Hermine had brought widespread rains of 2 - 5” to Florida, with some regions of 10+” to the Tampa area.

Figure 4. Observed rainfall for the 3-day period ending at 3 pm EDT Thursday, September 1, 2016. Hermine had brought widespread rains of 2 - 5” to Florida, with some regions of 10+” to the Tampa area. Image credit: NWS.

The forecast for Hermine
After landfall, which should be just after midnight Thursday night, Hermine will be rolling through the Southeast, either near the coast or just inland. Upper-level steering flow will keep Hermine moving on a fairly direct northeastward path through Saturday. After landfall, Hermine will become embedded in a weak 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. If Hermine’s track remains near the coast or just offshore, it will benefit more directly from the warm Gulf Stream water, and it may be able to retain more of its current intensity. The official NHC forecast as of 5 pm EDT moves Hermine within about 100 miles of the coast as a gradually weakening tropical storm.

The picture gets muddier after Hermine moves past the Outer Banks of North Carolina on Saturday. The preexisting front will weaken as upper-level ridging develops in southeastern Canada. Below the area of ridging, a weak cutoff low develops at middle levels of the atmosphere. This pattern will help slow Hermine and keep it stranded for as long as a day or two in the waters east of Delaware and New Jersey and south of New England. The 12Z runs of the GFS, ECMWF, HWRF, and GFDL models all keep Hermine moving very slowly in small loops over this general area--between roughly 36-40°N and 68-74°W--from Sunday to Tuesday before starting to move Hermine out to sea off Cape Cod around Tuesday (or even later, if the ECMWF is right).

Hermine may start to transition into an extratropical system as it rolls through the Southeast U.S., especially if its track runs inland. Later on, there is a distinct chance it could become more symmetric and perhaps regain tropical characteristics late in the weekend. Bolstering this idea: Hermine will likely be positioned near the north edge of the Gulf Stream, with SSTs that are well above average: at least 2°C higher than normal in the “becalmed” zone mentioned above, and even further above normal near the New Jersey and New York coast. On Thursday, the Texas Tower Buoy at 39.6°N, 72.6°W (station 44066) reported a water temperature of 77.9°F, and a buoy just off the VA/NC coast (36.6°N, 74.8°W; station 44014) reported 81.7°F (see these and other stations at the National Data Buoy Center website). Hermine will be spending time over water largely around 26-28°C (79-82°F), much closer to the rule-of-thumb threshold for tropical development than one usually sees this far north. The models above agree on briefly re-intensifying Hermine’s winds to near hurricane strength on Sunday or Monday well offshore, and this appears to be a plausible scenario regardless of its degree of tropical development.

Figure 5. Sea surface temperatures for the last week of August 2016 (left) and departures from normal for the time of year (right), both in degrees C. Image credit: NOAA/NHC.

The official NHC outlook does not reflect the small cyclonic loops depicted by some models, but it does include the modest intensification of Hermine as a post-tropical storm from Saturday into Sunday (top winds increasing from 50 to 60 mph), together with the very slow movement from Saturday through Tuesday. The NHC forecast discussion notes the possibility of Hermine regaining some tropical characteristics by Monday or Tuesday.

Fortunately, there is no sign of any major feature that would drive Hermine back onshore, such as the strong upper trough that pulled Hurricane Sandy into New Jersey and Pennsylvania in 2012. After Hermine dawdles offshore during the holiday weekend, models agree that it will begin moving out to sea by around Tuesday, although the ridging to its north may keep that seaward motion fairly gradual.

The “Sandy rule” to be invoked for Hermine
Some confusion emerged when Hurricane Sandy was declared post-tropical just an hour before it moved onto the New Jersey coast. Post-tropical cyclones are handled through a different part of the National Weather Service (the Weather Prediction Center, rather than NHC). After reviewing the Sandy situation, NHC decided to revise its policy and allow post-tropical storms to be covered through NHC bulletins that include the tropical cyclone name. The policy means that we can look to NHC advisories to continue for as long as Hermine remains a significant threat to land, regardless of whether it's classified as tropical or post-tropical.

Heavy rains expected across Southeast, mid-Atlantic
Near record-warm ocean temperatures in the Gulf of Mexico are evaporating near-record amounts of water vapor into the atmosphere for Hermine to feed off of. A special upper-air ballon sounding near Tampa at 2 pm EDT Thursday measured 2.85” of total precipitable water (TPW)--the amount of water that would result if one condensed all the water vapor in a column above and precipitated it out. According to the National Weather Service, Tampa’s all-time greatest precipitable water sounding was 2.85” on September 6, 2004, when Hurricane Frances was crossing Florida (though SPC lists one higher value around 3.1”, year unknown).

Figure 6. The swath of heavy rain expected from Hermine is evident in the 5-day rainfall outlook from 8:00 pm EDT Thursday, September 1, to 8:00 pm Tuesday, September 6. Image credit: NOAA/NWS Weather Prediction Center.

As Hermine tracks near or just inland from the Southeast coast, we can expect copious rainfall of 4” - 8”, with locally higher amounts, in a swath extending from north and central Florida along the Atlantic coast to around 200 miles inland (see Figure 6) from late Thursday into Friday. Flash flood watches are in effect from Georgia to Virginia along a swath northwest of tropical storm warnings. Recent weeks have been relatively dry for much of the Southeast and mid-Atlantic, which will help reduce the risk of major flooding, but an event like Hermine can easily outweigh that factor. Rains of 1” - 5” and gusty winds will work their way up the Delaware, New Jersey, and New York coastlines from Saturday into Sunday and perhaps Monday--a very unwelcome prospect for Labor Day beach goers. A zone of even heavier rain (perhasp 5” to 10”) and stronger wind could emerge in parts of the eastern Delmarva and/or southern New Jersey on Sunday and Monday, with the location hinging on how close ex-Hermine gets to the Mid-Atlantic coast when it slows down.

As it appears now, Hermine is not likely to be a billion-dollar hurricane. However, the huge amounts of rain it will unleash on the U.S. coast could set the stage for a follow-up hurricane to be an extremely serious flood disaster, should another hurricane visit the region in September.