When a hurricane threatens the United States, a lot of attention is focused on the path and the wind speed. Storm surge can be just as important to monitor, because it can quickly cause extreme destruction and death.
So what is storm surge? Simply put, it’s the amount of seawater pushed onto land by a tropical system. With major storms, the amount of water that can surge onto shore can fill buildings and cause homes to collapse. It’s one of the most dangerous parts of a hurricane and a key reason why people who live along coastlines face mandatory evacuations ahead of major storms.
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Science behind the surge
Many people understand sea level, high tide, and low tide. During normal conditions, ocean water levels rise and fall in a predictable way due to the gravitational forces of the sun and moon combined with the rotation of the earth.
Typically, a location will observe two high tides and two low tides within a day.
A storm surge is an abnormal rise in sea level accompanying a hurricane or other intense storm. Surge relates to the height difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone.
Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide.
A storm tide is the actual level of seawater resulting from the astronomic tide combined with the storm surge.
Back in 2020, KPRC 2 Chief Meteorologist Frank Billingsley explained storm surge like this:
You can relate to a car accident and it’s easy to understand what matters is the speed and size of the vehicle coming at you along with the angle it hits you (t-bone vs. side-swipe). That’s simple physics. Bigger and faster equals “badder.”
Likewise, surge increases with the size of the storm, the speed of the winds, the distance traveled over water, the angle the storm approaches the coast (head-on strikes equal higher surge). The bathymetry, or depth of the ocean, matters also, but a shallow coast is pretty even in the Gulf vs. the Eastern Seaboard.
Billingsley compared four famous hurricanes from this century for comparison: Katrina (2005), Rita (2005), Ike (2008) and Laura (2020). He noted that by satellite, they all seem pretty similar in size and scope.
Billingsley wrote:
Looks are deceiving, of course. Katrina wins for surge -- at 28 feet in southern Mississippi -- with Ike coming in with 17 feet (21 feet at Eagle’s Point where Bacliff is). Rita produced 15 feet and Laura, generally, produced 5 to 10 feet, with a few exceptions. All of them were plenty strong -- Ike at 110 mph, Laura at 150 mph, Katrina at 175 mph and Rita at 180 mph. Also, all of them pretty well t-boned the coast with similar angles of approach. Of the three, Ike had the lowest winds, BUT they spread out the farthest from the center. So clearly that has a lot to do with surge size. Like a larger ceiling fan can move more air, a larger hurricane can move more water!
Another big difference is time over water. I did some research and Katrina went into the Gulf at 5 a.m. on a Friday, then into Mississippi on Monday morning at 4 a.m. That’s 73 hours over water. Rita was in the Gulf from 2 p.m. on a Tuesday, making landfall early Saturday morning. Ike, likewise, was over water from a Tuesday to a Saturday. Laura spent half the time as the others over water: from 4 a.m. Tuesday to a landfall at 1 a.m. Thursday -- only 39 hours.
You can see the category for the storms vary, while their angles of approach to the coast did not differ widely. So, the “tells” are just how far out hurricane-force winds extend from the center and just how long those winds are over water. The longer a storm is over water, obviously means a slower storm -- Katrina, Ike and Rita all traveled at a speed of 10 to 13 mph, while Laura zipped at 15 to 17mph.
Billingsley explained that Laura’s surge wasn’t as severe because the storm moved faster and was smaller than other storms of similar wind strength.
If you live along a coastline or have family and friends who do, the National Hurricane Center has an interactive Storm Surge Risk Map online to help depict which communities are most vulnerable to storm surge. The maps show that surge can extend many miles inland as the category of a hurricane increases. You can view the map here.
Watch: How the dirty side and clean sides of a hurricane impact wind speed, storm surge, and rainfall
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Watch: How a hurricane forms
Storm Surge and how it’s measured
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