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Is the National Hurricane Center Underestimating Irma’s Storm Surge?

In 2015, ten full years after Hurricane Katrina, the National Hurricane Center (NHC) finallybegan making projections in advance regarding potential storm surge. This even though “[s]torm surge is often the greatest threat to life and property from a hurricane,” according to NHC [pdf]. In fact, the “fatalities that we saw in Sandy [117] and in Katrina [1,800+] were mostly, not exclusively but mostly, seawater from storm surge.” Currently, NHC is projecting that many areas along the coast in southern Florida, including in and around Miami, have a 10% or greater chance of a strong storm surge. The worst areas, particularly just north of the Everglades and in Homestead, have a chance to hit the top category (red) of 9+ feet. But is 9+ feet a reasonable high end category?

Hurricane Andrew (1992) saw storm surges in southern Florida of about 4-7 feet in the worst places with one recording (at Burger King Headquarters) of nearly 17 feet, but the worst projections of a massive, deadly surge did not materialize.

In this article Jeffrey Masters, co-founder of The Weather Underground and a former“Hurricane Hunter” with NOAA, helpfully discusses the key factors that determine the magnitude of a particular storm surge during a hurricane. The critical factor is size of the hurricane, rather than the two more well-known factors of wind speed and barometric pressure. Masters compares Hurricane Katrina (a category 3 hurricane upon land fall) and the much stronger Hurricane Camille (category 5, also a land fall in Louisiana, 1969). Even though two categories less intense on the Saffir-Simpson Scale, “Katrina’s radius of maximum winds was about 30 miles, double Camille’s,” and “Katrina set in motion a volume of water about four times greater than Camille did.”

So how does Hurricane Irma compare to Hurricane Andrew in terms of size? Quite frighteningly so, actually. While other factors (such as high tide or low tide at land fall, where direct land fall actually occurs, and the level of barometric pressure when it hits) will contribute to the storm surge levels, it is the massive difference between Andrew (left in this graphic) versus Irma (right) that is giving me a terrible feeling in the pit of my stomach. (I had been scheduled to officiate a wedding in Miami this Saturday.)
Officials in Florida have used the storm surge projections to determine where to suggest or mandate evacuation. If the surge is like Sandy’s (10-12 feet in the worst spots), the NHC projections of 9+ feet at the upper end of the scale may seem quite justified. If, however, the monster size of Irma means a surge more like Katrina’s, nearly 28 feet at it’s highest as discussed by Masters, we may be looking a scale of devastation and loss of life that will raise very serious questions about whether adequate and accurate information was provided in advance to decision makers in Florida.

As of Tuesday, according to Masters, Irma’s radius of maximum winds was 35 miles across, five miles wider than Katrina’s, and growing. This could very well mean a storm surge of 15-20 feet or more for Florida, potentially more than double the 9 feet that begins NHC’s highest category.

At this point, we may only be able to hope and pray for the best. However, if you are reading this in southern Florida ahead of Irma’s land fall and can still make a choice to evacuate, it is highly advisable.