Hermine to Linger Offshore after Soaking Southeast; Dangerous Surge Still Possible

The storm now known as Post-Tropical Cyclone Hermine continues to spin east of the mid-Atlantic coast with top sustained winds of 70 mph, as of the 11 am EDT Sunday advisory from the National Hurricane Center. Hermine was located about 295 miles south-southeast of the east end of Long Island, NY, and about 310 miles east-southeast of Ocean City, MD. Tropical Storm Warnings remained in effect at 11 am EDT Sunday from Cape Charles Light, Virginia, to west of Watch Hill, Rhode Island, and for locations along Delaware Bay. A Tropical Storm Watch is in effect from Watch Hill, RI, to Sagamore Beach, MA, as well as Block Island, Martha’s Vineyard, and Nantucket.

Hermine moved further east than expected on Saturday, leaving the weather surprisingly mellow on the Mid-Atlantic and New York coastline--but it is still possible that dangerous storm surge could arrive over the next couple of days (see below).


Figure 1. Visible satellite image of Hermine as of 1515Z (11:15 am EDT) Sunday, September 4, 2016. The solid white areas at top coreespond to Hermine’s intense showers and thunderstorms, which are located well north of the low-level center of circulation. Image credit: NOAA/NESDIS.

On Saturday, Hermine’s strong winds drove a large storm surge to the coasts of Virginia and northern North Carolina, causing extensive street flooding and beach erosion. The highest water level was observed at Cape Hatteras, North Carolina--a storm tide of 4.68 feet (height of the water above mean sea level), which was the highest water level observed at that station since it was established in 2010 (previous record: 4.02’ during Hurricane Sandy on October 29, 2012.) The high surge in the North Carolina Outer Banks flooded the only road into these vulnerable barrier islands, NC 12. While there was some sand deposited on the road, no major damage was reported by the North Carolina DOT, and the road was open on Sunday. Both bridges connecting NC 12 to the mainland were closed by high winds for a period on Saturday; the high winds caused a fatal crash of a semi-truck on the US-64 bridge, bringing Hermine’s death toll to two people.

High storm surges were also observed on Saturday along the southern Virginia coast at the entrance to Chesapeake Bay, where three tide gages recorded a storm surge between 3.5 and 4.0 feet. Duck, NC received a 3’ storm surge. On Sunday morning, Hermine’s storm surge had abated along the coast, and was less than two feet everywhere, as seen using our wundermap with the Storm Surge layer turned on, or the NOAA Tides and Currents page for Hermine.


Figure 2. Cars drive on the flooded NC Highway 12 in Hatteras, N.C., Saturday, Sept. 3, 2016 after Tropical Storm Hermine passed the Outer Banks. (AP Photo/Tom Copeland) 

Hermine brings heavy rains
Hermine’s rains had mostly ended along the U.S. coast on Sunday, but the storm had left very heavy rains in its wake. Inland flooding was relatively minimal, thanks in large part to the dry conditions that have prevailed across much of the Southeast over the last few weeks in this drought-stricken summer. According to the 11 am Sunday NOAA Storm Summary, the top rain amounts by state were:

Florida, 18.89” at Baskin
North Carolina, 13.34” at Cedar Island
South Carolina, 9.93” at Myrtle Beach AFB
Georgia, 6.37” at Alma
Virginia, 6.50” at Holiday Island


Figure 3. Observed rainfall for the 7-day period ending at 8 am EDT Sunday, September 4, 2016. Hermine brought 7-day rainfall amounts of 10+ inches (pink colors) to coastal portions of Florida, South Carolina and North Carolina. The highest storm-total rainfall amounts over a 7-day period were near the Tampa Bay, Florida region; 18.89” fell at Baskin and 15.27” at Largo, just north of St. Petersburg. Image credit: NWS/AHPS.

The forecast for Hermine
Thankfully, it appears that Hermine’s impacts along the Mid-Atlantic and Northeast coast will be considerably less than feared just a day ago. Hermine is still expected to track very slowly north from Sunday through Tuesday, as it becomes entangled with a weak upper-level trough to its west. Our three best track models--the Euro, GFS, and UKMET--now agree that Hermine should remain at least 150-200 miles off the Mid-Atlantic and Northeast coasts. This is a bigger margin of safety than Saturday’s model runs had implied. Because Hermine scooted farther east than expected on Saturday, it will now be harder for any potential westward wobble to make much of a difference for conditions along the coast. The official NHC track as of 11 am Sunday reflects the model consensus for a slow northwestward arc in Hermine’s path that would bring it as far west as about 70°W on Monday into Tuesday. Models vary on how quickly Hermine begins shifting east of Cape Cod and out to sea for good, with the ECMWF now calling for this to happen on Tuesday, the GFS on Wednesday, and the UKMET on Thursday. The slower solutions keep Hermine a bit closer to the coast before its departure.

Winds: Even though Hermine is classified as a post-tropical cyclone, it may regain some tropical characteristics over the next day or two as it hovers near the boundary of the Gulf Stream, where sea-surface temperatures of 26-28°C (79-82°F) are at least 2°C above average. Hermine’s peak winds are predicted by NHC to increase to minimal hurricane force (65 knots, or 74 mph) from Monday into Tuesday. Any hurricane-strength winds would be focused in a small area near Hermine’s center. However, tropical storm force winds already extend up to 205 miles from the center, and it is possible that some coastal areas will experience sustained tropical storm force winds (39 mph or greater) as Hermine wobbles its way slowly northward. Such winds are most likely on eastern Long Island, Cape Cod, and nearby islands, including Nantucket and Martha’s Vineyard. Note that Hermine could still be classified as a post-tropical cyclone even if its top winds regain hurricane force.


Figure 4. WU depiction of the official NHC track for Hermine as of 1500Z (11 am EDT). NHC may continue to keep Hermine classified as a post-tropical cyclone even if its peak winds regain hurricane-force strength, as depicted here.

Rainfall: Hermine’s low-level center was devoid of showers and thunderstorms on Sunday morning. We can expect some redevelopment around the north and west sides of the center by Sunday night and Monday, as Hermine shifts toward a more subtropical-cyclone structure. Extensive heavy rains now appear unlikely along the Mid-Atlantic and Northeast coast, given Hermine’s expected distance and the large amount of dry air being entrained on Hermine’s northwest side. Scattered showers and thunderstorms around Hermine’s periphery could drop 1” or 2” at some coastal locations. If Hermine’s precipitation shield grows bigger than expected on Monday, those rains could edge a bit farther inland.

Storm surge: The threat of a severe storm surge has gone down since Saturday due to Hermine’s more eastward location, but storm surge remains a significant threat that could produce major flooding in some locations. There is still high confidence on the long duration of this event, which raises the odds of back-bay flooding that could intensify over several days as water is continually pushed inland.

Shown below are NWS predictions as of midday Sunday for the potential peak storm surge levels at selected locations. The total water height above sea level, called the storm tide, will vary up or down from these numbers by several feet depending on whether the peak falls at high vs. low tide. Access to barrier islands of New Jersey and New York may be compromised by surge-related flooding. For the latest on potential impacts, be sure to check local statements compiled on the NHC website.

Maryland beaches: 2’ or more
Delaware beaches:  3’ or more
New Jersey beaches:  3’ or more
NY Harbor, Long Island (south shore and east bays): 2’ - 4’
Eastern Long Island Sound, Eastern South Shore Bays of Long Island: 2’ - 4’


Figure 5. Latest satellite image of 92L.

92L growing more organized
Satellite images on Sunday morning showed that the moderate level of heavy thunderstorm activity associated with a large tropical wave passing through the Lesser Antilles Islands on Sunday (Invest 92L) had increased in intensity and organization since Saturday. The storm was bringing winds near tropical storm-force to the Lesser Antilles on Sunday morning. La Desirade (Guadeloupe) reported sustained winds of 38 mph, gusting to 57mph at 9 am Sunday in a strong rain band that dumped 0.14” (3.6 mm) of rain; at 11 am, winds had decreased to a sustained 27 mph at the site. Some rotation of the storm’s echoes was apparent on Martinique radar, so we have to watch this tropical wave today to monitor it for further signs of development.

A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward westward path, and the storm will move through the Lesser Antilles Islands on Sunday and be near Jamaica by Wednesday. If 92L does develop, it probably won’t last long. The 8 am EDT Sunday SHIPS model forecast for 92L showed moderate wind shear of 15 - 20 knots through through Tuesday, rising to the high range, 20 - 35 knots, for the latter part of the week. The latest 0Z Sunday operational runs of our three top models for forecasting tropical cyclone genesis--the GFS, European and UKMET models--did not show development of the system over the next five days. More than 40% of the 70 ensemble members of the 0Z Sunday runs of the GFS and European model did show 92L developing into a tropical depression or weak tropical storm by Monday, but all of them showed the storm being destroyed by high wind shear and dry air before reaching Jamaica on Wednesday. In their 8 am EDT Sunday Tropical Weather Outlook, NHC kept their 2-day and 5-day development odds at 20% and 30%, respectively.

A tropical wave expected to leave the coast of Africa on Tuesday could develop into a tropical depression by next weekend, a few hundred miles west-southwest of the Cabo Verde Islands, predicted the 00Z Sunday runs of the UKMET and European models. The NASA/GMAO model predicted that Sahara Desert dust and dry air machine would be moderately active during the week, and development of this new tropical wave will likely be hindered by dry air.

90E in the Eastern Pacific a threat to Mexico’s Pacific coast
A large and well-organized tropical disturbance (Invest 90E) is already bringing very heavy rains and gusty winds to southwestern Mexico, as seen on satellite loops. These conditions are likely to continue for the next few days, and NHC is predicting total rainfall amounts of 5 - 10”, with isolated totals up to 15”, across the southern Mexico states of Oaxaca, Guerrero, Michoacan, Colima, Jalisco, and Nayarit. The European and UKMET models predict that 90E will develop into a tropical storm that will hit Mexico’s Baja Peninsula late this week. In their 8 am EDT Tropical Weather Outlook NHC gave 90E 2-day and 5-day odds of development of 80% and 90%, respectively. The Hurricane Hunters are on call to investigate this system on Monday.

We'll be back with our next update around midday Monday.

Jeff Masters and Bob Henson