As Hurricane Irma looks to be hurdling straight for a direct hit on Southern Florida, meteorologists from Weather Underground are warning that the most devastating impacts of the storm could be felt much further north in towns along the coast of Georgia and South Carolina where the storm surge could be a catastrophic 20-28 feet high in certain areas. To put that in perspective, Hurricane Katrina in 2005 set a record for the largest storm surge ever recorded along the U.S. coast at 27.8 feet.
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.
So, which coastal towns are most at risk? As Weather Underground notes, Savannah in Southern Georgia could see a surge of up to 23 feet if Irma strikes as a Category 3 storm. Obviously, the surge would be even larger if Irma manages to maintain Cat-4 winds.
Maximum of the "Maximum Envelope of Waters" (MOM) storm tide image for a composite maximum surge for a large suite of possible mid-strength Category 3 hurricanes (sustained winds of 120 mph) hitting at high tide (a tide level of 3.5’) along the coast of Georgia. What’s plotted here is the storm tide--the height above ground of the storm surge, plus an additional rise in case the storm hits at high tide. Empty brownish grid cells with no coloration show where no inundation is computed to occur. Inundation of 19 – 23’ will occur in a worst-case scenario along most of the coast.
Meanwhile, further north in Charleston, SC the surge could also exceed 20 feet and flood areas many miles inland from the shore.
Maximum of the "Maximum Envelope of Waters" (MOM) water depth image for a composite maximum surge for a large suite of possible mid-strength Category 3 hurricanes (sustained winds of 120 mph) hitting at high tide (a tide level of 2.5’) along the coast of South Carolina near Charleston. If Irma is a Cat 3 in South Carolina, a worst-case 17 – 21’ storm tide can occur.
Of course, as we noted earlier, this data has already prompted the governors of Georgia and South Carolina to declare a state of emergency and to call for citizens of coastal areas to begin evacuations immediately.
Ironically, even though Irma will be her strongest when washing ashore in Southern Florida, Weather Underground notes that deep water just offshore helps to subdue the storm surge from Miami to Fort Lauderdale...
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.
...which also should help to somewhat protect the coast on the western shores of Florida.
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.
Be that as it may, with winds in excess of 120 mph expected pretty much across the entire state of Florida, one might be best suited to prepare for the worst no matter how close you are to the shore.