Intensifying Tropical Storm Hermine Beginning to Close Off an Eye

Tropical Storm Hermine is gathering strength as it steams north-northeastwards at 14 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. An Air Force hurricane hunter aircraft was in Hermine early Thursday afternoon, and found top surface winds of 65 mph to the east of the center, and a central pressure that had fallen to 991 mb. The aircraft recorded a distinct double wind maximum on either side of the center, evidence that Hermine was not far from from closing off an eye. This process was also apparent on satellite images, which showed a band of intense thunderstorms rotating around the north side of Hermine’s center, forming the northern portion of an eyewall. At this rate, Hermine should have a fully closed eye by late afternoon, and could be a Category 1 hurricane by Thursday evening. It’s a good thing Hermine didn’t get its act together a day earlier, or we might be looking a a major hurricane in the Gulf of Mexico today. Wind shear continued to be a moderate 10 - 20 knots on Thursday morning, but the direction of the shear was from the west-southwest, where the upstream air is not quite as dry, as seen on water vapor satellite imagery. Sea surface temperatures (SSTs) near the center remained favorable for development, near 30°C (86°F). 


Figure 1. Radar composite of Tropical Storm Hermine taken on Thursday morning, September 1, 2016 by NOAA hurricane hunter aircraft N43RF (flight path shown in red, with the flight path of the simultaneous flight by the NASA Global Hawk aircraft shown in yellow.) Image credit: NOAA/AOML/HRD Twitter feed.


Figure 2. Latest satellite image of Hermine.

Hermine expected to become a Category 1 hurricane
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 11 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 84%, 83%, and 71%, respectively, for St. Marks, Cedar Key and Apalachicola, Florida. The SHIPS model on Thursday morning predicted moderately favorable conditions for intensification, with wind shear staying a moderate 10 - 20 knots through landfall on Thursday night. SSTs will be a very warm 30°C (86°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 late Thursday morning on a landfall intensity of 70 - 75 mph--a borderline strong tropical storm or minimal Category 1 hurricane.

The Gulf Coast of Florida can receive 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 - 8’ 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 potentially 1-3 feet along a 150-mile stretch of the Georgia and southern South Carolina coast. Early Thursday afternoon, Hermine continued to create storm surge heights over 1’ along the entire Gulf Coast from New Orleans, Louisiana to Naples, Florida. According to the NOAA Tides and Currents page for Hermine, the maximum storm surge early Thursday afternoon was approximately 2.0’ at Cedar Key and Apalachicola, Florida.

Extremely rich moisture available to Hermine
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. At 8 am EDT Thursday, the upper-air balloon sounding at Jacksonville, Florida measured 2.41” 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. This value ranked in the upper 1% of all TPW measurements taken at the site since 1948. According to the National Weather Service, Jacksonville’s all-time greatest precipitable water sounding was 2.82” on July 20, 1993. Near-record TPW values around 2.5” were analyzed by satellite over the Gulf of Mexico on Thursday morning, and the NAM model predicted a large surge of this rich moisture will accompany Hermine along its track up the Southeast U.S. coast into North Carolina.


Figure 3. Precipitable water (blue = high values, in millimeters) is running as high as 65-75 mm (2.6” - 3.0”) across the eastern Gulf of Mexico in association with Tropical Storm Hermine. This moisture will swing northeast as Hermine makes landfall and moves across the Southeast U.S. Image credit: tropicaltidbits.com.

Heavy rains expected across Southeast, Mid-Atlantic
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 11 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 Virginia. With Hermine now expected to track near or just inland from the Southeast coast, we can expect copious rainfall of 4” - 8”, with locally higher amounts, in a swath extending about 200 miles inland from the coast (see Figure 4) from late Thursday into Friday. Parts of the Southeast have been dealing with a parched, hot summer, so some moisture will be welcome, although these amounts could be excessive in places.

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. Rains of 1” - 4” should sweep along 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 may emerge in parts of the eastern Delmarva and/or southern New Jersey, with the location hinging on how close ex-Hermine gets to the Mid-Atlantic coast as it slows down.

Flash flood watches are in effect from Georgia to North Carolina 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. 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.


Figure 4. NWS precipitation outlook for the 5-day period from 12Z (8:00 am EDT) Thursday, September 1, 2016, through 12Z Tuesday, September 6. Image credit: NOAA/NWS Weather Prediction Center.

Links
Links
Storm surge from NOAA Tides and Currents page for Hermine
Storm surge expert Dr. Hal Needham's new post, New Coastal Flooding Products Help Us Track Hermine's Storm Surge
NWS Local Statements on Hermine
NWS Excessive Rainfall Forecasts
Radar loops from Brian McNoldy
Loews Don CeSar webcam from St. Pete’s Beach (thanks to WU member LuckySD for this link)
Blue Parrot Cafe webcam in Apalalachicola (thanks go to WU member captainhunter for this link)

Madeline angles south of Hawaii; hurricane watch for Lester
Tropical Storm Madeline, a mighty Category 4 hurricane just two days ago, limped across the Pacific just south of Hawaii’s Big Island on Thursday morning, producing much less impact than feared. Madeline was located 180 miles south of South Point, HI, as of the 11:00 am EST (5:00 am HST) advisory from the Central Pacific Hurricane Center (CPHC). Madeline’s top winds were holding steady at 50 mph, but the storm is expected be a depression by Friday as it continues moving west over open water, eventually arcing to the west-northwest.

Tropical storm warnings for Madeline were cancelled across Hawaii at 2:00 am HST (8:00 am EDT) Thursday. No major damage or injury had yet been reported, though we’ll have to wait till after daybreak Hawaii time to know for sure. Hilo racked up more than 4” of rain during Hermine’s approach on Tuesday and Wednesday, and rainfall was no doubt heavier along the east slopes of the Big Island. Tropical storm force winds were observed as far away from Madeline as Hawaii’s northwest coast, where a mesonet station at the Kohala Ranch Resort reported sustained winds of 44 mph at 2:38 pm HST Wednesday and a gust to 60 mph at 11:03 am HST.


Figure 5. Infrared image of Tropical Storm Madeline (left) and Hurricane Lester (right) at 1200Z (8:00 am EDT and 2:00 am HST) Thursday, September 1, 2016. Image credit: NASA/MSFC Earth Science Office.

Next in Hawaii’s unusual hurricane lineup is Hurricane Lester, located about 750 miles east of Hilo as of the 11 am EDT advisory from CPHC. A Hurricane Watch has been issued for Hawaii’s Big Island, Maui, Molokai, Lanai, and Kahoolawe. Although Lester retains a distinct eye in infrared satellite imagery, its top sustained winds were down to 105 mph, making it a Category 2 storm. Wind shear along Lester’s path remains quite low at 5-10 knots, but dry air at mid-levels of the atmosphere (relative humidities only around 35-40%) is feeding into Lester’s shield of thunderstorms, which has become less intense in satellite imagery. Also, Lester is passing over a corridor of waters churned up by Madeline, which may have pushed SSTs below the 26°C benchmark for tropical development, as suggested in daily SST mapping. The SHIPS model predicts SSTs in the 26-27°C range for the next several days, with little change in wind shear or relative humidity, so Lester may weaken only gradually.

Lester’s potential path is worrisome enough to require vigilance. Computer models agree that Lester will angle west-northwest on a path that parallels the Hawaiian island chain, but they disagree on how close that path will be to the islands. The 00Z and 06Z runs of the HWRF model take Lester about 100 miles north of the islands as a Category 1 hurricane, with the GFDL runs considerably closer and equally strong. The 00Z and 06Z GFS runs are similar to the HWRF but with Lester as a strong tropical storm, and the European model also keeps Lester about as far north. Some members of the 00Z Euro and GFS ensembles are considerably further south: the GFS ensemble average (GEFS) moves Lester across Hawaii northwest of the Big Island, and the Euro average is similar, though a shade further north. These conflicting messages from the models underscore the need for continued caution. The official 11 am EDT track from CPHC moves Lester just north of the islands from Saturday into early Sunday as a minimal Category 1 hurricane, bringing Lester to about 100 miles north of Oahu.


Figure 6. Official track for Hurricane Lester as of 11:00 am EDT (5:00 am HST) Thursday.

Weakening Gaston heads toward the Azores
A Tropical Storm Warning is up for the islands of the western and Central Azores as Hurricane Gaston continues to speed east-northeast at 23 mph towards the islands, where the storm is expected to arrive on Friday morning. On Wednesday morning, Gaston peaked as an impressive Category 3 hurricane with 120 mph winds. Since then, higher wind shear and a traverse over cooler waters near 26.5°C (80°F) has caused weakening, and Gaston was a Category 1 hurricane with 80 mph winds at 11 am EDT Thursday. This weakening trend should continue, resulting in Gaston arriving in the western Azores on Friday morning as a tropical storm with 65 mph winds. The Azores only averages one hurricane strike per decade, and has already seen one this year: 2016’s first Atlantic storm, Hurricane Alex, which struck the island of Terceira in the central Azores on January 15 as a bizarrely out-of-season tropical storm in January. Alex did minimal damage and caused no direct deaths.


Figure 7. MODIS visible satellite image of Hurricane Gaston on Wednesday afternoon, August 31, 2016. At the time, Gaston was a Category 3 storm with 115 mph winds. Image credit: NASA.

92L in the central tropical Atlantic embedded in dry air
A large tropical wave with plenty of spin but no heavy thunderstorm activity is located midway between Africa and the Lesser Antilles islands. Originally designated Invest 92L, this wave was no longer deemed worthy by NHC as an “Invest”, and they are no longer issuing their suite of model forecasts for the system. The tropical wave was embedded in a major area of dust and dry air from the Sahara Desert, and this dry air will greatly interfere with development over the coming days as 92L heads west at 15 - 20 mph across the tropical Atlantic. The latest 0Z Thursday runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models--had one of the three, the UKMET, showing development of the system over the next five days. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will likely move through the Lesser Antilles Islands on Sunday night, and be near Hispaniola on Tuesday. In their 8 am EDT Thursday Tropical Weather Outlook, NHC reduced their 2-day and 5-day development odds to 0% and 20%, respectively.

We’ll be back with a new post later today.

Jeff Masters and Bob Henson