High Winds From Hermine Lash Mid-Atlantic, Drive Dangerous Storm Surge

Tropical Storm Warnings were flying from northern North Carolina to New York and Connecticut late Saturday morning as Tropical Storm Hermine emerged over the waters off the coast of North Carolina. Hermine rolled off the Outer Banks of North Carolina near Nags Head around 8:00 am EDT Saturday, moving east-northeast at 15 mph, and was located about 80 miles southeast of Norfolk, VA as of the 11:00 am EDT advisory from NHC. Hermine completed the transition from a tropical storm to an extratropical storm Saturday morning, and is now officially called Post-Tropical Cyclone Hermine. A post-tropical cyclone is a storm that derives its energy from atmospheric dynamics and contrasts rather than from the heat of the ocean. In a post-tropical cyclone, the circulation is often tilted northward from the surface to upper levels, resulting in an asymmetric comma shape typical of a mid-latitude winter storm. Radar and satellite imagery shows this configuration in place for Hermine: the strongest thunderstorms on Saturday morning were more than 200 miles northeast of Hermine’s low-level center. The new version of Hermine is just as powerful as the old one, though, with top winds now back up to 65 mph. Tropical-storm-force winds extend out more than 200 miles northeast of Hermine’s center. Duck Pier, NC, recently reported a sustained wind of 58 mph gusting to 73 mph.


Figure 1. MODIS visible satellite image of Hermine taken at 12:30 pm EDT September 3, 2016. At the time, Hermine was a post-tropical cyclone with top sustained winds of 70 mph. Note the asymmetric shape of the storm, with a comma-like configuration, instead of circular. Image credit: NASA.

Some of Hermine’s top winds thus far, courtesy of weather.com:

    •    79 mph at C-Tower, south of St. George Island, south of Apalachicola at an elevation of 115 feet, with sustained winds of 61 mph late Thursday
    •    75 mph early Thursday evening near Indian Shores Beach in western Pinellas County, Florida
    •    67 mph in Keaton Beach, Florida
    •    64 mph gusts at Florida State University's football stadium in Tallahassee 
    •    62 mph at Elizabeth City, North Carolina, early Saturday morning.
    •    62 mph at St. Petersburg's Albert Whitted Airport and near Clearwater Beach on Thursday evening.
    •    59 mph near Folly Island, South Carolina on Friday evening.
    •    58 mph near Dewees Island, South Carolina with sustained winds of 40 mph on Friday afternoon.
    •    55 mph at Norfolk International Airport, Virginia, Saturday morning.
    •    54 mph at Shaw Air Force Base.
    •    53 mph in Apalachicola, Florida on Thursday evening and in Brunswick, Georgia on Friday afternoon.
    •    52 mph at Clearwater Beach, Florida later Thursday evening, and a 51 mph gust at Cedar Key, Florida.
    •    51 mph at Virginia Beach, Virginia, early Saturday morning.
    •    49 mph in Charleston, South Carolina on Friday evening.


Figure 2. Residents look at Alligator Point Road, which collapsed during the storm surge from Hurricane Hermine at Alligator Point, Florida on September 2, 2016. (Mark Wallheiser/Getty Images) 

Hermine brings heavy rains
Hermine is being fed by unusually high amounts of atmospheric water vapor, and continues to dump very heavy rains along its path. According to the 11 am Saturday NOAA Storm Summary, the top rain amount in North Carolina was 13.34” at Cedar Island; South Carolina’s highest was 9.93” at Myrtle Beach AFB, and Georgia’s highest amount was 6.37” at Alma. Hermine had already dumped 3.95” on Virginia Beach, Virginia, with more rain to come.


Figure 3. Observed rainfall for the 1-day period ending at 8 am EDT Saturday, September 3, 2016. Hermine brought 24-hour rainfall amounts of 6+ inches (pink colors) to coastal portions of South Carolina and North Carolina. The highest storm-total rainfall amounts over a 4-day period were near the Tampa Bay, Florida region; 18.89” fell at Baskin and 15.27” at Largo, just north of St. Petersburg. Hermine’s rains mostly missed Florida’s Lake Okeechobee region; on Thursday, the Army Corps of Engineers decided that the lake’s vulnerable dike is safe at the current water level, and elected not to increase the amount of polluted water released from the lake into its outflow canals that lead to the ocean. In May and June, large releases of Lake Okeechobee water to relieve pressure on the dike caused massive algae blooms and serious water quality issues along both coasts of Florida. Image credit: NWS/AHPS.

Hermine’s strange journey ahead: a unusual danger for the Mid-Atlantic and Northeast coast
Despite its new classification as a post-tropical system, Hermine will continue to be tracked through advisories by the National Hurricane Center until it no longer poses a threat to land. They will have a difficult job: over the next few days, Hermine will take one of the oddest and most unsettling trajectories in memory for a named storm along the U.S. East Coast. The upper-level trough that pulled Hermine northeastward is now leaving it behind, and steering currents will become very weak. As a result, Hermine will spin for several days in the region east of the Mid-Atlantic and south of New England, gradually working its way northward. Because Hermine will be slowing to a crawl close to the north edge of the Gulf Stream on Sunday and Monday, it will be near or atop sea-surface temperatures of 26-28°C (79-82°F), which is at least 2°C above average and more than warm enough to support tropical development. Instability in the atmosphere will be enhanced by some residual cold air aloft, a fragment of the departing trough. As a result of all this, Hermine is likely to re-organize from Sunday into Monday into a more symmetric, warm-core system, perhaps embedded within the weak upper low fragment.

It’s unclear whether Hermine will again be technically classified as a tropical storm or hurricane rather than a post-tropical storm, but that point is moot in terms of impact. Computer models are near-unanimous in bringing Hermine’s top winds up to or above hurricane strength for at least a few hours on Sunday night or Monday. NHC’s official outlook at 11 am NHC outlook puts Hermine at minimal hurricane strength from Sunday night to Tuesday morning. Hermine’s peak winds are expected to remain stronger than their current 65 mph until at least Wednesday.

Our best track models--the GFS, Euro, and UKMET--agree that Hermine will remain within about 150 to 250 miles of the coast till at least the middle of next week before a more definitive move out to sea. Because steering currents are so weak, we cannot yet be certain how long Hermine will linger nearby. We can expect some erratic small-scale motion, perhaps including one or more clockwise or counterclockwise loops, especially from Sunday through Tuesday. Any of these small jogs might bring Hermine closer to, or farther from, the mid-Atlantic and Northeast coastline. Fortunately, there is no sign of any large-scale upper-level feature that would pull Hermine well inland, and eventually the polar jet stream will dip far enough south to haul Hermine out to sea.


Figure 4. Forecast from the 06Z Saturday run of the GFS model (one of our three most reliable track models) for the surface center of Hermine. From top left to lower right, the valid time periods are 00Z (8:00 pm EDT) Saturday, September 3, through 00Z Thursday, September 8. Wind speeds are shown in knots; add 15% for miles per hour. Green colors denote winds of at least tropical storm strength (34 knots or 39 mph); purple colors denote minimal hurricane strength (65 knots or 74 mph). Image credit: tropicaltidbits.com.

Major storm surge flooding expected along mid-Atlantic, Northeast coast
The next phase of Hermine could easily be more destructive than its Florida landfall. An unusually prolonged period of high surf, beach erosion, and storm-surge flooding will unfold along the coastline from Delaware to New York. In some places, the peak surge could be comparable to that experienced during Hurricane Irene in 2011, with at least some level of high water persisting for days on end. There is very 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. Multiple days of strong wind and heavy surf are likely to produce enormous amounts of damaging beach erosion.

One complicating factor with Hermine is that the amount of coastal flooding may end up far out of proportion to what local weather conditions would lead you to expect. The water will be driven into the coast mainly by the processes unfolding offshore. There will likely be a sharp cutoff to the heaviest rain and highest wind associated with Hermine (see Figure 5 below). Outside of this zone, it could be merely breezy and partly cloudy; inside it, you could experience torrential rain and tropical storm-force wind. The transition zone may wobble inland or offshore at times, making it hard to gauge exactly what to expect at any given spot. Tourists and residents should keep this nontraditional storm behavior in mind. It would be prudent to avoid flood-prone areas and roadways near the coast even if the weather doesn’t seem particularly threatening. Rip currents will add to the danger of the rough surf, making this a very good Labor Day weekend to stay off the beach. Unlike most tropical and winter storms, the storm surge and associated flooding may persist for several days in some areas.

Shown below are NWS predictions as of midday Saturday for the potential peak storm surge levels over the next several days. 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. For the latest on potential impacts, be sure to check local statements compiled on the NHC website.

Southside Hampton Roads, VA: 5.5’ to 7.0’, with ocean waves greater than 15’ and bay waves of 6-9’
Eastern Shore of the Chesapeake Bay, VA: 5 - 5.5’, with 3-5’ waves on James River
Atlantic City, NJ:  2’ - 4’
Sandy Hook, NJ:  2’ - 4’
Coastal New York and Long Island Sound, including parts of Connecticut and New Jersey: Significant surge with moderate to major coastal flooding (details to come)
Coastal Rhode Island and Massachusetts: 1’ - 2’ feet, except 2’ - 3’ possible from Westerly to Point Judith, RI


Figure 5. Projected 5-day precipitation totals from 12Z (8:00 am EDT) Saturday, September 3, to Thursday, September 8. A huge contrast is evident between the torrential amounts expected near the center of Hermine--where more than 20” of rain could fall--and the much lesser amounts over most inland areas. A small nudge in Hermine’s notion could bring significantly heavier amounts to the coastline than shown here. Image credit: NOAA/NWS Weather Prediction Center.

What’s next in the Atlantic?
We’re two weeks into the peak part of the Atlantic hurricane season, with the half-way point coming up on September 11. Sea surface temperatures (SSTs) are at their maximum now, basin-wide wind shear is at its minimum and the African monsoon is at its peak, so we should expect near-average activity with one or two more named storms in the coming two weeks. The Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, is currently weak, and will not act to discourage Atlantic tropical storm formation, like it was doing in mid-August, when it was centered in the Western Pacific. The only major impediment for tropical storm formation would seem to be a higher than average amount of dry air coming off the coast of Africa for this time of year.


Figure 6. MODIS visible satellite image of 92L taken on Saturday morning September 3, 2016. Image credit: NASA.

Satellite images show that shower activity has increased over the past day in association with a large tropical wave located about 500 miles east of the Lesser Antilles Islands (Invest 92L). However, the tropical wave was embedded in a major area of dust and dry air from the Sahara Desert, which was slowing development and making 92L’s thunderstorms disorganized. The 8 am EDT Saturday SHIPS model forecast for 92L shows low wind shear and warm SSTs for the next four days--favorable for development--as it moves west at 15 - 20 mph, but shows the air surrounding 92L will grow even drier as it enters the eastern Caribbean, which should keep any development slow to occur. The latest 0Z Saturday 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. A strong and persistent ridge of high pressure should keep 92L on a fairly straightforward west to west-northwest path, and the storm will move through the Lesser Antilles Islands on Sunday and be near Jamaica and Haiti on Tuesday. By Wednesday, when 92L will enter the Western Caribbean, it will find a moister environment and have more potential for development. In their 2 pm EDT Saturday Tropical Weather Outlook, NHC bumped up their 2-day and 5-day development odds to 20% and 30%, respectively. The Hurricane Hunters are scheduled to investigate 92L on Monday afternoon, if necessary.

A tropical wave expected to leave the coast of Africa on Tuesday could develop into a tropical depression by Wednesday a few hundred miles west-southwest of the Cabo Verde Islands, predicted the 00Z Saturday run of the UKMET model. The 0Z Saturday run of the European model and 06Z Saturday run of the GFS model did not develop this tropical wave over the next five days, but did show development after that time. The NASA/GMAO model predicted that Sahara Desert dust and dry air machine would be moderately active during the week, and this new tropical wave will likely have its development hindered by dry air.

Lester cruising well north of Hawaii
Hawaii’s prolonged two-part threat of the past week from Hurricane Madeline and Hurricane Lester is finally winding down. As of 11 am EDT (5 am HST) Saturday, Lester was located 260 miles east of Honolulu, with top sustained winds down to 100 mph. As expected, Lester is tracking parallel to the islands on a west-northwest track, and it appears that the model consensus was on target in keeping that track far enough north of the islands (about 100-150 miles) to avoid major trouble. The Hurricane Watch for the islands has now been cancelled, but dangerous surf is still expected.

Bob Henson and Jeff Masters