Published 7:43 PM EDT Sep 12, 2018
Storm surge, the massive mound of water that builds up and comes ashore during a hurricane, is often the deadliest and most destructive threat from the storms.
Storm surge has accounted for about half of the deaths in hurricanes since 1970, according to the National Hurricane Center. It caused most of the 1,200 deaths in Hurricane Katrina in 2005.
As Hurricane Florence nears the U.S. East Coast, forecasters at the hurricane center are warning of a storm surge of up to 13 feet from Cape Fear to Cape Lookout, North Carolina. The destructive storm surge will occur as the hurricane's eye comes ashore from Thursday night into Friday, the Weather Channel said, and coastal flooding may persist through multiple high tide cycles into this weekend east of the center of Florence.
Overall, dangerous storm surge is possible all the way from Edisto Beach, South Carolina, to the North Carolina/Virginia border.
The National Weather Service began issuing specific weather alerts for storm surge just last year.
Storm surge watches and warnings are separated from hurricane alerts because hurricane-force winds and storm surges don't always occur at the same place or the same time, said Rick Knabb, the former director of the hurricane center, and now the hurricane expert at the Weather Channel.
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In addition, preparing for hurricane-force winds is different than for storm surge, he said. For storm surge, people should evacuate, while for wind, they can stay in place in a strong structure as long as it's away from flood-prone areas, Knabb said.
A "storm surge watch" is issued when flooding is possible, while a "warning" is issued when flooding is expected. Every coastal city along the Gulf or East Coast of the USA is at risk of storm surge, the hurricane center said.
In addition to being the deadliest threat, storm surge is also typically the most destructive part of a hurricane. In Hurricane Sandy in 2012, storm surge-induced flooding measured as high as 9 feet above ground in parts of New York and New Jersey, leading to billions of dollars in damages.
The damage occurred even though Sandy spun ashore as the equivalent of only a Category 1 hurricane, with winds of about 80 mph, and was downgraded below hurricane status shortly thereafter.
Storm surge flooding does not include floods caused by the heavy rain from the hurricane, such as what happened last year in Houston with Hurricane Harvey and in North Carolina in 2016 during Hurricane Matthew.
It also has nothing to do with tsunamis, large ocean waves generated by offshore earthquakes that are not related to weather.