Expect a Storm Surge of 15 - 20 Feet from Hurricane Florence

Expect a Storm Surge of 15 - 20 Feet in a Landfalling Category 4 Storm in the Carolinas

Wunderground,

10 September, 2018

Landfalling Category 4 hurricanes are rare in the mainland U.S., with just 24 such landfalls since 1851—an average of one every seven years. (Category 5 landfalls are rarer still, with just three on record). All but three of these 27 landfalls by Cat4s and Cat5s have occurred south of South Carolina’s latitude; thus, Florence will be in very select company if it manages to make landfall at Category 4 strength in North or South Carolina.

If Florence hits the coast of North or South Carolina as a Category 3 or stronger hurricane, we should expect to see record storm surge heights, with a 15 – 20’ surge very possible, according to two experts I communicated with today. Dr. Robert Young, Professor of Coastal Geology at Western Carolina University, says that “the track of Hurricane Florence, combined with its expected size and strength at landfall and the unique coastal geomorphology of the region, is likely to result in a record storm surge along portions of the warning area.” And according to storm surge expert Dr. Hal Needham, “we could definitely see a 20+ foot storm surge/storm tide in the Carolinas. Even if Florence weakens a bit in the time right before landfall, the surge heights correlate better with the pre-landfall winds than the winds at landfall.”

It’s a good thing that landfalls by such strong hurricanes are rare along the South Carolina and North Carolina coast, since this coastline is extremely vulnerable to high storm surges. Two of these three historical Carolina Category 4 hurricanes generated a storm tide of 18 - 20 feet: Hugo of 1989 and Hazel of 1954. The other storm--Gracie of 1959--did not (it hit at low tide, significantly reducing the coastal flooding). The storm tide is the combination of the storm surge and the normal lunar tide, measured in height above sea level. The National Hurricane Center uses the terminology “height above ground level” when discussing the storm tide, meaning the height the surge plus tide gets above the normal high tide mark.

The high vulnerability of this coastline is because the continental shelf extends out more than 50 miles from shore, creating a large region of shallow water less than 150 feet deep just offshore that forces storm surge waters to pile up to staggering heights. (See our storm surge basics page for more information).

Figure 1. Maximum of the "Maximum Envelope of Waters" (MOM) storm tide image for a composite maximum surge for a large suite of possible mid-strength Category 3 hurricanes (sustained winds of 120 mph) hitting at high tide (a tide level of 2.3) near the North Carolina/South Carolina border. What’s plotted here is the storm tide--the height above ground of the storm surge, plus an additional rise in case the storm hits at high tide. Empty brownish grid cells with no coloration show where no inundation is computed to occur. Inundation of 15 -22’ can occur in a worst-case scenario along most of the coast. Note that not all sections of the coast will experience this surge level simultaneously; the peak values would occur near and to the right of the storm's center where it makes landfall. The image was created using the National Hurricane Center’s Sea, Lake, and Overland Surge from Hurricanes (SLOSH) model. See our storm surge inundation maps for the U.S. coast for more information.

WU's storm surge inundation maps for the U.S. coast, computed using NOAA’s SLOSH model, tell a frightening story. Depending on where its center makes landfall, a mid-strength Category 4 hurricane with 145 mph winds hitting at high tide, in a worst-case scenario, can generate a storm tide in excess of twenty feet above ground level along the entire coast of South Carolina, and along most of the coast of southern North Carolina from the South Carolina border to Morehead City. Many locations could see a higher surge, of up to 27 feet. And a Category 5 storm is much worse: a theoretical peak storm tide of 33 feet is predicted by the SLOSH model for the Intracoastal Waterway north of Myrtle Beach, South Carolina. These peak surges occur over a 10 - 40 mile stretch of coast where the right eyewall makes landfall. If Florence were to make landfall near Wilmington, NC, for example, the highest surges would extend northeastward to around Jacksonville, NC.

According to HURDAT, the official Atlantic hurricane database, these are the three Category 4 hurricanes that have hit the U.S. coast north of Georgia, since record keeping began in 1851:

Hurricane Hugo

Hurricane Hugo made landfall near Charleston, South Carolina on September 22, 1989, as a Category 4 storm with 140 mph winds and a 934 mb pressure. The highest storm surge was 20.2 feet at Awendaw and 20.0 feet in Bull’s Bay, just north of Charleston (see storm surge animation here). Hugo’s surge caused widespread destruction, representing a large portion of the $18.7 billion in damage (2018 dollars) inflicted by the storm. At that time, Hugo was the costliest hurricane on record.

Figure 2. Track of Hurricane Hugo of 1989.

Figure 3. Maximum storm tide from Hurricane Hugo of 1989, as simulated using NOAA’s SLOSH model.

Hurricane Gracie

 

Hurricane Gracie made landfall near Edisto Beach, South Carolina on September 29, 1959, as a Category 4 hurricane with 130 mph winds and a 951 mb pressure. (Note that hurricanes with 130-mph winds were originally ranked as top-end Category 3 storms until the Saffir-Simpson scale was tweaked in 2012.) Storm surge flooding was not highly destructive, due to the storm's landfall near the time of low tide (see animation here). However, Charleston still recorded their highest storm tide since 1940. Along the coast of southern South Carolina, the storm tide was measured up to 11.9 feet above mean sea level.

Figure 4. Track of Hurricane Gracie of 1959.

Figure 5. Maximum storm tide from Hurricane Gracie of 1959, as simulated using NOAA’s SLOSH model.

Hurricane Hazel

 

Hurricane Hazel made landfall near the North Carolina/South Carolina border on October 15, 1954 as a Category 4 hurricane with 130 mph winds, a 938 mb pressure, and 40-mile wide eye. A 30-mile stretch of the North Carolina coast adjoining the South Carolina border received a highly destructive storm surge in excess of ten feet, with a peak storm tide of eighteen feet measured at Sunset Beach (see storm surge animation here). Damage was greater since the hurricane coincided with the highest lunar tide of the year. Hazel’s surge brought massive destruction to the beaches of New Hanover and Brunswick counties, with only 5 of 357 buildings left standing in Long Beach, North Carolina. The official report from the Weather Bureau in Raleigh, North Carolina stated that as a result of Hazel, "all traces of civilization on the immediate waterfront between the state line and Cape Fear were practically annihilated." According to NOAA, "every pier in a distance of 170 miles of coastline was demolished".

Figure 6. Track of Hurricane Hazel of 1954.

Figure 7. Maximum storm tide from Hurricane Hazel of 1954, as simulated using NOAA’s SLOSH model.

Even a Category 3 hurricane can generate a massive storm surge in North Carolina

 

As sobering as these numbers are, consider that it doesn’t take a Category 4 hurricane to generate a massive and destructive storm surge in North Carolina. Hurricane Fran of 1996, which hit the state as a low-end Category 3 storm with 115 mph winds and a pressure of 954 mb, drove a 14.6-foot storm tide to the coast at Topsail Island.

Bottom line: if you are told to evacuate for Florence's storm surge, get out! And don't wait until the last minute, or it may be too late to save your life.

My related post from Friday: Increased Storm Surge Damage From Florence Due to the Moon’s Phase