Jeff Masters: Hurricane Irma an Extreme Storm Surge Threat to the U.S. and Bahamas

[highplainsdem]

https://www.wunderground.com/cat6/hurricane-irma-extreme-storm-surge-threat-us-and-bahamas

Irma’s large wind field is putting in motion a vast amount of water, which is spiraling into the center of Irma and creating a large mound. In the open ocean, that water is forced downward, pushing deeper water outward, and the sea surface is not elevated more than a few feet. However, once the hurricane drives that mound of water into a shallow area near land, the water cannot flow downwards, and instead piles up and is forced on land, creating a storm surge. In the Turks and Caicos Island and in the southeastern and central Bahamas, a highly destructive storm surge of 15 – 20 feet above ground is expected near the coast to the right of where the eyewall hits.

A potentially catastrophic storm surge for Florida, Georgia, and South Carolina

If Irma makes a trek up the East Coast from Miami to southern South Carolina as a Category 3 or 4 hurricane, as the models currently suggest, the portions of the coast that the eyewall touches will potentially see a massive and catastrophic storm surge, breaking all-time storm surge records and causing many billions of dollars in damage. Even areas up to a hundred miles to the north of where the center makes landfall could potentially see record storm surges. The area of most concern is the northern coast of Florida, the coast of Georgia, and the southern coast of South Carolina, due to the concave shape of the coast, which will act to funnel and concentrate the storm surge to ridiculous heights. If we look at wunderground’s storm surge maps for the U.S. East Coast, we see that in a worst-case Category 3 hurricane hitting at high tide, the storm tide (the combined effect of the storm surge and the tide) ranges from 17 – 20’ above ground along the northern coast of Florida, and 18 – 23 feet above ground along the Georgia coast. If Irma is a Cat 4, these numbers increase to 22 – 28 feet for the coast of Georgia. This is a Katrina-level storm surge, the kind that causes incredible destruction and mass casualties among those foolish enough to refuse to evacuate.

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Figure 7. South Florida is not at as great of a risk of a high storm surge, since there is deep water offshore, and the mound of water the hurricane piles up can flow downward into the deep ocean instead of getting piled up on land. The worst-case storm tide from a Category 4 hurricane for the coast from Miami Beach to West Palm Beach is 7 – 9 feet. However, that deep water allows much larger waves to build up, and Irma will create big waves that will pound the coast and cause heavy damage. There is a region of the coast from downtown Miami southwards, including Biscayne Bay, where the water is shallow, and the storm tide can be up to 15 feet in a Category 4 hurricane. The Great Miami Hurricane of 1926, a Category 4 storm, brought a 10 – 15’ storm surge to the coast of Miami along Biscayne Bay.

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Figure 8. The Atlantic (Florida Straits) side of the Florida Keys also has deep water offshore, limiting the maximum storm surge in a Cat 4 to 8 – 10 feet. The risk is higher on the west (Florida Bay) side of the Keys, where the water is shallower; a worst-case storm tide of 12 – 15 feet can occur there. Any storm tide over six feet is extremely dangerous in the Florida Keys, due to the low elevation of the land. The greatest risk in the Keys, if the current NHC forecast verifies, would be on the Florida Bay (west) side of the Upper Keys, after the center of Irma moves just to the north. The counter-clockwise flow of air around the hurricane will then bring winds out of the southwest that will drive a large storm surge into the west side of the Upper Keys.

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Check the link, both for more text and for the images showing the risk.

[Lochloosa]

1. Water is what kills you in a hurricane

[Dawson Leery]

2. kick

[longship]

3. Stay safe FL DUers!

Do what you can.