Everyone heard it: An unsurvivable 15- to 20-foot storm surge forecast for Louisiana. Category 4 Hurricane Laura’s 150-mph winds would shove a mountain of deadly water onto the Cajun coast. Here’s the most I could find:
Those higher amounts were truly the exception with 5-feet above ground level really the more common readings. No one is complaining, but let’s dive into this surge a bit (no pun intended).
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.
So let’s look at our famous hurricanes this century for comparison: Katrina, Rita, Ike and Laura. By satellite, they all seem pretty similar in size and scope:
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:
So one big difference was just how far out the hurricane winds went from the center. The Weather Company put out this graphic, illustrating Laura being the most tightly wound:
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
A final point 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. Here’s a chart I put together:
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.
My bottom line here is that if you are wondering why Laura didn’t produce the “unsurvivable” 15- to 20-foot storm surge, the answer is that it moved faster and was smaller than other storms of similar wind strength.