Roaring ashore with 90-mph winds, Hurricane Hanna did a number on the Corpus Christi area with a 7-foot storm surge and drenching rains, prompting comments like, “We didn’t think it’d get this bad.”
Even State Rep. Todd Hunter remarked: “We did not dodge a bullet. Anyone that says we dodged a bullet, we did not. It was 6 mph less than a category 2 hurricane. That is severe.” (caller.com July 28, 2020)
And Hanna’s Category 1 is in terms of wind measured by miles per hour. In terms of wind measured in kilometers/hr Hanna missed Cat. 2 by just 4 km/hr coming in at 150 km/hr!
But here is where forecasts need to improve and, if you’ve heard one of my hurricane talks, then you know I have always maintained that just looking at wind can be a misleading characterization of a hurricane’s strength (and, thus, category). After all, Hurricane Ike (2008) topped out at 110-mph winds but had a storm surge that ran as high as 21 feet! But because a 111-mph wind is the next step up, Ike was determined to be a Cat. 2, rather than a Cat. 3. I suspect folks in Bolivar beg to differ.
So what to consider? Barometric pressure. You’ve seen lows and highs on the maps, indicating lower or higher pressure, and a hurricane is basically a super-strong low. That’s important because the lower the pressure, the stronger the wind. The stronger the wind, the higher the storm surge. And as you can see above, the Saffir-Simpson category scale no longer takes into account pressure or surge, just the wind.
But in fact, the Hurricane Hunters go in specifically looking for not just the winds, but the lowest pressure (where the center of circulation is found) which can be measured with a dropsonde falling from the plane to the ocean.
Okay, so what was the pressure of Hurricane Hanna? 973 millibars. At one time, the Hurricane Center considered barometric pressure as a criterion for category, and a Cat. 2 pressure measured between 965 and 979 millibars. That would put Hanna firmly in the Cat. 2 hurricane column, despite the wind not meeting Cat. 2 criteria. Or, at least, not the winds the Hurricane Hunters could find and measure.
Why is this so important?
We WARN you according to category and the public (and government officials) absolutely need to know what to expect from a particular hurricane. If a storm is classified Cat. 1 and it’s going to knock your socks off like a Cat. 2, you need to know it. Hurricane Ike absolutely should have been classified as a Cat. 3 hurricane (because it was according to its pressure of 953 mb and storm surge of 14 to 21 feet). A Cat. 3 designation would have had more people evacuating and sooner! These are important considerations for preparations and safety.
Dr. Phil Klotzbach et al recently published a paper addressing just these concerns and declaring that Surface Pressure is a More Skillful Predictor of Normalized Hurricane Damage than Maximum Sustained Wind. In this paper, he offers a category scale based on pressure, having studied storms from the past century (starting with 1900). You can see how his scale below compares to the old scale (on the far right). The first column is wind in knots.
I encourage you to read this paper. It’s a bit scientific but absolutely makes the case that the measured wind speeds alone are not always enough to predict the damage a hurricane will bring. The National Hurricane Center should reconsider their criteria for a hurricane’s category. It’s not like they haven’t done that in the past as nothing is set in stone and because that category directly influences public perception and, ergo, public reaction, maintaining the current approach is proving inadequate.