Irma’s Surge Poses Big Risk to Coast

Oh boy, this one's a doozy.

At WSJ, "Hurricane Irma’s Surge Poses Major Risk to Florida":

The size of this monster...holy hell. pic.twitter.com/naM5tQpXZC

— Alexis In NH (@AlexisinNH) September 9, 2017

Hurricane Irma, the most powerful storm to take aim at Florida in decades, is on a path that presents the worst-case scenario for deadly storm surges and powerful winds when it strikes the state Sunday, threatening millions of homes and businesses.

Irma is a massive storm, covering an area more than double the size of Florida, and generating sustained winds of more than 150 miles an hour. It has already killed more than 20 people after flattening the Caribbean islands of St. Martin and Barbuda as it arced north toward Florida. The hurricane’s impact could reach as far north as Indiana and Illinois, forecasters say, affecting about 50 million people.

Long lines of cars clogged Florida’s highways after authorities and forecasters implored the state’s 20.6 million people to leave low-lying coastal lands expected to be inundated by hurricane-driven seawater.

Storm surges, one of the most deadly threats of Hurricane Irma, are forecast to be 9 feet to 20 feet high, depending on whether the storm hits the peninsula from the Atlantic on the east or the shallower Gulf of Mexico to the west.

“If it comes in from the Gulf side, Tampa Bay could just get hammered and that really is one of the big catastrophic events we have been worried about for some time,” said Kyle Mandli, assistant professor of mathematics at Columbia University.

But Mr. Mandli warns the entire state could remain at risk if the hurricane tracks up the middle of the state and causes storm surges on both coasts, though those would probably not be as high.

With Irma now projected to make landfall in the Florida Keys about daybreak Sunday, weather experts say the flooding could begin hours earlier because surges from a hurricane start to hit land in advance of the storm’s center. The surge peaks as the hurricane eyewall crosses onto land, said Robert Bea, professor emeritus at the University of California’s Center for Catastrophic Risk Management. “We’re talking several hours of surge,” Mr. Bea said.

Storm surges, created when the high wind of a hurricane forces ocean waters onshore, account for half of the deaths and most of the destruction caused by the majority of hurricanes, weather experts say.

Miami-Dade County Mayor Carlos Gimenez cited a possible life-threatening storm surge when he expanded the county’s evacuation zone on Thursday, now affecting more 650,000 residents.

Much of the estimated $62 billion in U.S. damage from superstorm Sandy in 2012 was caused by the storm surge that slammed the Eastern seaboard, according to an analysis by the U.S. Geological Survey.  Storm surge was cited by the National Oceanic and Atmospheric Administration as the major cause of the $75 billion in destruction along the Gulf Coast from 2005’s Katrina, which leveled beachfront communities in Mississippi and inundated the city of New Orleans.

On Florida’s coasts, which will face the brunt of the Category 4 hurricane’s destructive force, about 3.5 million residential and commercial properties are at risk of storm-surge damage and almost 8.5 million properties are at risk of wind damage, according to data provider CoreLogic .

The last Florida storm that was the size of Hurricane Irma, which was downgraded to Category 4 from Category 5 on Friday, was Hurricane Andrew in 1992. That storm was originally classified as Category 4 but was reclassified in 2002 to a Category 5.

Catastrophe-modeling firm Karen Clark & Co. said a repeat of Hurricane Andrew on the same path as in 1992 would cause $50 billion in insured losses. The same storm directly hitting Miami today would cause more than $200 billion in losses, the firm said.


Miami, however, is protected by a rapid drop offshore thanks to the continental shelf, which is unlike Florida’s mostly shallow Gulf of Mexico coast. As a result, the surge hitting Miami from a Category 4 storm like Irma is expected to total up to 9 feet, compared with as high as 20 feet if it were to hit more along the Gulf Coast, according to NOAA.

The highest waves are typically centered on the leading right side of the storm, where counterclockwise winds in the Northern Hemisphere push the bulk of a hurricane’s destructive force. The surge waves are made even higher when they travel across shallow coastal waters, said Robert Bohlin, a meteorologist with the Central Pacific Hurricane Center in Honolulu.

Historically, the biggest storm surges in U.S. history have taken place in shallow Gulf waters. Hurricane Katrina in 2005 produced the nation’s highest recorded surge of 27.8 feet at Pass Christian, Miss. At least 1,500 people died in Katrina—many from the surge—and entire beachfront neighborhoods were washed away by the waves, NOAA officials said.

But Irma is forecast to take such an unusual track—essentially up the length of the Sunshine State—that hurricane experts aren’t exactly sure how the surge pattern will play out. If it shifts slightly to the west, much higher surge could inundate parts of Florida’s Gulf Coast, said Columbia’s Mr. Mandli.

“Even a shift of a few kilometers could be the difference between a huge disaster and something more manageable,” Mr. Mandli said.

Damage from a storm surge is considered flooding, which isn’t covered by standard homeowners insurance policies. Flood damage is largely covered by the federal government’s National Flood Insurance Program, which provides homeowners up to $250,000 to repair a home and $100,000 for personal possessions.

Homeowners in high-risk flood zones are required by their mortgage providers to buy flood insurance, but consumers outside those areas often forgo the coverage.

Businesses can buy federal flood insurance, which covers up to $500,000 for damage to a building and $500,000 for its contents. Commercial-property insurance for large businesses often includes flood coverage...

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