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Capital Weather Gang | Perspective

Why the storm surge forecast for Irma wasn't so bad, just incomplete

By Jason Samenow

September 12, 2017 at 3:29 PM

A mobile home community is flooded in the aftermath of Hurricane Irma in Everglades City, Fla., Monday, Sept. 11, 2017. (AP Photo/David Goldman)

The scariest part of the forecast for Irma was a prediction that seas would rise 10 to 15 feet above normally dry land in the Keys and Southwest Florida. The National Hurricane Center said the resulting inundation of homes and businesses would be "catastrophic."

This "storm surge," describing how hurricanes and their winds can push an enormous volume of water into the coast, turned out to be less than half that in most locations.

Naples had a storm surge of about five feet. Key West's surge was about three feet.

Related: [Storm surges are the worst part of a hurricane — and will get even more destructive]

The New York Times did a whole story on the surge being less than expected.

Did the National Hurricane Center overstate the threat?

No, according to the Hurricane Center's Jamie Rhome, who leads the storm surge unit. He said the big surge came as expected, just not where a lot of people live.

"All indications are that a 10-plus-foot surge did occur from Marco Island eastward and around Everglades City," he said.

Data from this region is not yet available to confirm Rhome's assertion, but photos from Everglades City show severe inundation even many hours after the water had begun receding.

A child walks through water at a flooded gas station in the heavily damaged town of Everglades City on Sept. 11, 2017, the day after Hurricane Irma swept through the area. (Photo by Spencer Platt/Getty Images)

Rhome explained that the Hurricane Center storm surge forecast conveys the maximum water rise expected in a given zone, and that "not everyone is going to get that."

The question then arises as to whether the Hurricane Center and media effectively communicated that it might not be so bad everywhere.

Rhome contended that these forecasts shouldn't be qualified. Much like the National Weather Service issues a tornado warning for a region substantially larger than the size of a tornado, he said the Hurricane Center needs to have wiggle room in its storm surge forecasts.

A tornado warning "is a perfect analogy," he said. "We can't predict these little wobbles [in the hurricane track]. You have no choice but to say it will occur from here to here."

In short, the limitations of the science in predicting storm surges demands that more people be prepared for the threat than will actually be affected.

A child wield as broom at his family’s flooded gas station in Everglades City on Sept. 11, 2017, the day after Hurricane Irma struck.  (Photo by Spencer Platt/Getty Images)

If the storm had moved just 10 to 15 miles farther north in the Gulf of Mexico before making landfall, the devastating surge that hit farther south would have focused on the more densely populated Naples and Fort Myers area, and no one would be questioning the storm surge forecast.

Hal Needham, a storm surge expert at Louisana State University, said he understood the rationale for the Hurricane Center forecast. "I don't think their forecast was inappropriate," he said. "If it had stayed offshore longer, it would have changed the surge big time. Naples lucked out."

Related: [Why Hurricane Irma wasn’t far worse, and how close it came to catastrophe]

But one does wonder whether the Hurricane Center could add some language in its advisories to fully explain its forecast, just for the sake of completeness and transparency. Media outlets and forecasters rely on the Hurricane Center and take its forecasts as gospel.

If a given surge forecast is the maximum over a given region and many areas will not experience such a surge, it could just say: "A maximum storm surge of 10 to 15 feet is expected in this zone. Prepare for this amount of inundation, but local levels will vary."

The Hurricane Center can still issue a very serious "better safe than sorry" forecast and a forecast that isn't omitting information at the same time.

More stories about Irma

16 million people without power and 142-mph winds: Hurricane Irma, by the numbers

Hurricane Irma drained the water from Florida's largest bays — but it wasn't gone for long

Irma's track forecast was adequate, but there's significant room for improvement


Jason is the Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science, and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association.

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