Tuesday, September 5, 2017

Catastrophic Hurricane Irma — now a Cat 5 — is on a collision course with Florida


Hurricane Irma is an “extremely dangerous” Category 5, barreling toward the Greater Antilles and Southern Florida. It’s already the strongest hurricane ever recorded outside the Caribbean and the Gulf of Mexico, and it’s likely to make landfall somewhere in Florida over the weekend.

If it does, the impact could be catastrophic.

The storm is life-threatening for the United States and British Virgin Islands, Puerto Rico, the Dominican Republic, Haiti, Cuba and the Bahamas, as well. Hurricane warnings have been issued for portions of the Leeward Islands and the Greater Antilles, including Puerto Rico. A hurricane watch was issued for Hispaniola.

With maximum winds of 180 mph, Irma is the strongest hurricane in the Atlantic Ocean outside the Caribbean or the Gulf of Mexico. By wind-speed, it’s the third-strongest storm on record anywhere in the Atlantic, behind Hurricane Wilma in 2005 and Hurricane Allen in 1980. And in its Tuesday morning discussion, the National Hurricane Center said the storm is in an environment “ideal for some additional intensification.”

The hurricane is expected to remain at least a Category 4 for the next few days with minor fluctuations in intensity. It could even become slightly stronger, but it is already nearing historical precedent and a theoretical limit for how strong it can get.

It cannot be overstated that Hurricane Irma is extremely dangerous and will produce the full gamut of hurricane hazards across the Caribbean and potentially in South Florida, including a devastating storm surge, destructive winds and dangerous flash flooding.

All of Florida — especially South Florida and the Keys — should be preparing for a major hurricane landfall on Sunday. Tropical-storm-force winds are expected to arrive as soon as Friday.

U.S. landfall threat

Computer models are in strong agreement that by Saturday, Irma will be approaching the Florida Keys — where dangerous storm conditions are likely. Then, they show a sharp northward turn by Sunday morning. The precise timing and location of the turn has huge implications for Florida.

Model ensemble guidance out to ten days from the European model (top) and the U.S. model (bottom). (B. Tang, UAlbany)
It is impossible to say with certainty if Irma will track up along the eastern side of the Florida peninsula, the western side, or straight up the peninsula. Since the weekend, models have generally shifted westward with the storm’s forecast track, which means interests along the coast of the Gulf of Mexico should also closely monitor this storm.

For a major hurricane, the exact track of the storm’s eyewall — the zone surrounding its calm center — is critical as it will determine where the most severe effects tend to concentrate. The most violent winds coincide with the eyewall, and the biggest storm surge occur just to its right (or north).

But as Irma is such a large and powerful hurricane, very dangerous weather will also occur up to 200 miles away from the eyewall — including coastal surge, flooding rains and potentially damaging winds.
“The hurricane force winds in Irma are wider than Florida,” tweeted Bryan Norcross, hurricane specialist at the Weather Channel. “You won’t need a direct hit to get Wilma-type winds and storm surge on both coasts.”

Beyond the weekend, the scenarios really depend on which side of Florida it tracks. But for now, it’s safe to say that the southeast United States, including the Florida panhandle, Georgia and the Carolinas should also brace for potential impacts, such as flash flooding, storm surge and strong winds.

Seven-day cumulative rainfall forecast. (NOAA/WPC)
Impact on the Leeward Islands, Virgin Islands and Puerto Rico

At 8 a.m. Tuesday, the powerhouse storm was positioned 270 miles east of the island of Antigua in the northern Leewards, where it is forecast to make a direct impact early Wednesday. The storm was moving eastward at 14 miles per hour.

Hurricane warnings are in effect for the northern Leeward Islands, the Virgin Islands and Puerto Rico, and new warnings will be added to the west as the storm tracks just north and parallel to the Greater Antilles. Storm surge heights of 7-11 feet are possible in the warning areas, as well as heavy rain that can produce flash flooding and mudslides.

Irma is likely to become the strongest hurricane on record to hit the Leeward Islands, even more intense than David, which raked across the islands in 1979. “David was a horrible hurricane for Leeward Islands: 56 fatalities in Dominica,” tweeted Phil Klotzbach, hurricane expert at Colorado State University.

Antigua, Barbuda, Saint Kitts and Anguilla — in particular — are right in the path of the storm.

“Really feel for the northern Leeward Islands,” tweeted National Hurricane Center forecaster Eric Blake. “A hurricane this strong there only comes around once a generation or two.”

After passing the northern Leeward Islands, the hurricane is directly hitting the British Virgin Islands.
The U.S. Virgin Islands and Puerto Rico may remain south of the storm’s center, so less prone to Irma’s most hostile conditions. But even so, damaging winds and torrential rains are likely.

Irma’s place in history

Irma’s peak intensity so far ranks among the strongest in recorded history, matching the likes of Katrina, Andrew and Camille.
If Irma makes landfall as a Category 4 or higher in the United States, joining Hurricane Harvey, it will become the first time two storms so strong struck the United States in the same season.

Correction: The storm is third-strongest anywhere in the Atlantic, by wind speed. A previous version of this story said it was the second-strongest.

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