Weathering Storms

The National Hurricane Center helps the nation ready itself for worsening storm surges.

By Craig Collins

If you lived in the New York metro area in 2012, you’ll never forget Sandy. The 900-mile-wide storm raked the Atlantic coast from Florida to Massachusetts in October of 2012. When it struck the New York/New Jersey region on October 29, it drove a massive surge of water that obliterated beaches and boardwalks, filled subway tunnels, and buried some parts of New York City under nine feet of water. 51 square miles of the city, about 17 percent of its land mass, were flooded by Sandy.

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If you’re a meteorologist, you might remember something else about Sandy: By the time it arrived in New York Harbor, it barely qualified as a hurricane. It’s usually referred to as “Superstorm Sandy.”

Weather experts have been debating whether the traditional way of classifying hurricanes – a rotating low-pressure tropical weather system, or cyclone, with sustained winds of at least 74 miles per hour – by their maximum sustained wind speed, might be incomplete. The National Weather Service’s National Hurricane Center (NHC) has been among the leading voices arguing that classifying the severity of hurricanes on a scale of Category 1 to Category 5, increasing with wind speed, isn’t enough to communicate a hurricane’s potential for damage, since the category only reflects the wind damage potential.

This image of Hurricane Katrina was taken by the NOAA-16 AVHRR instrument on Aug. 28, 2005. Katrina was a reminder that 21st-century hurricanes tend to be bigger than in the past, bringing with them more rainfall and more powerful storm surges.

The NHC, one of the National Weather Service’s National Centers for Environmental Prediction, is located on the campus of Florida International University in Miami, and relies on several expert units to meet its mission to save lives and reduce economic losses with watches, warnings, forecasts and analyses of hazardous tropical weather. The NHC recently released a report clarifying the dangers of hurricanes: over the past 50 years, storm surge has been directly responsible for nearly half the deaths in the U.S. attributable to Atlantic storms, far more than rainfall or winds. During this half-century, aided by higher-resolution satellite imagery and better numerical models, NHC products and services have become increasingly accurate and timely. At the same time, there has been a growing sense that an equally important element of NHC’s mission – to increase public under- standing of these hazards – needed updating, to match the changing nature of tropical storms.

“Going all the way back to storms like Isabel, in 2003 – which had a tremendous storm surge impact in places like Chesapeake Bay,” said Michael Brennan, branch chief of the NHC’s Hurricane Specialist Unit, “there was a push within the Weather Service to focus more on the storm surge hazard itself, because up to that time, that risk was just bundled in with the wind hazard . . . and the thing to remember is that storm surge is the main reason evacuations are done in this country for hurricanes.”

The NHC developed and rolled out a separate warning mechanism – the storm surge watch/warning – in time for the 2017 hurricane season. “The watch is issued generally about 48 hours before we expect either the water to start to rise or tropical storm-force winds to start,” said Brennan, “either of which would result in basically the end of your preparation time. You have to be evacuated by then.” A warning means there is the danger of life-threatening surge within the warning area, and is issued within 36 hours of the hazard.

2017 was the costliest year in U.S. history in terms of weather-related damage, with hurricanes Harvey, Irma and Maria contributing to more than $306 billion in losses.

A mobile home park on the north side of U.S. Highway 98 in Mexico Beach, Florida, washed away by the storm surge and wave impacts of Hurricane Michael, Nov. 2, 2018.

With the storm surge watch/warning in place, Brennan pointed out, “we’ve seen a relatively low number of storm surge fatalities, going back to 2017, compared to what we might have historically expected from this many substantial hurricanes hitting the United States.”

The NHC will begin providing an experimental graphic in 2020 that will depict the expected storm surge inundation values for the United States Gulf and Atlantic coasts, Puerto Rico, and the U.S. Virgin Islands that are provided in the tropical cyclone public advisory (TCP), and will be available on the NHC website. These values represent the peak height the water could reach above normally dry ground within the specified areas. This sample graphic depicts values provided in Hurricane Dorian Tropical Cyclone Public Advisory 46.

The NHC continues to refine its abilities to predict and communicate tropical weather risks. As it developed its storm surge watch/warning advisories, the center also began issuing watches, warnings and advisories for “potential tropical cyclones.” Before 2017, Brennan explained, the NHC had no mechanism for alerting the public to an offshore storm system that hadn’t yet met the definition of a tropical depression or tropical storm, even if its experts believed the system would strengthen. “On average, this gives us about 17 or 18 hours of additional lead-time on the watches or warnings,” Brennan said.

In 2020 the NHC developed an experimental graphic showing the peak storm surge forecast to accompany its tropical cyclone public advisories to allow users to better visualize the areas at risk from dangerous storm surge.