Hurricane Irma is poised to pull off the remarkable feat of traveling the entire length of Florida, from south to north, and spreading catastrophic storm surge flooding and damaging winds along with it.
The storm’s track would mean that Miami and West Palm Beach, two of the most populated areas with vulnerable shoreline property, will escape their worst-case scenarios, but still experience Category 1 or 2 hurricane force wind gusts.
Unfortunately, the projected path, which takes the center of Category 3 or 4 Hurricane Irma from the Florida Keys northward to near Ft. Myers, and along or just off the west coast from there, puts in play highly vulnerable, low-lying areas that are especially prone to storm surge flooding. Typically, storms in this region travel either from the east to the west, or southwest to northeast.
But Irma’s south to north motion will pile up water on its eastern flank, leading some areas to see between 10-15 feet of water above ground if the peak surge hits at the time of high tide. The National Hurricane Center (NHC) is forecasting this amount of surge to affect areas from Cape Sable to Captiva, including the city of Naples.
To the north of there, from Captiva to Ana Maria Island, the NHC is forecasting 6 to 10 feet of storm surge flooding, depending on the timing of the peak surge. This region includes Sarasota and Bradenton, two populous beachfront communities.
Then, from Ana Maria Island to Clearwater Beach, which includes Tampa Bay, the NHC is predicting 5-8 feet of storm surge flooding, depending on the timing of the peak surge. This forecast prompted additional evacuations in this region, which had only been expecting about half this amount of storm surge based on previous forecasts that had the center of the storm moving well east of the area.
The east coast of Florida won’t escape storm surge flooding, though, since the right side of the storm will push water onshore across the entire east coast of the state. The NHC is forecasting 4-6 feet of surge from North Miami Beach to Card Sound Bridge, including Biscayne Bay. This is down from previous forecasts, but not insignificant.
Storm surge expert Hal Needham wrote that areas of Florida’s west coast that lie to the east, or left side, of Irma’s path could first see a negative storm surge, with water being pushed out of bays and estuaries by an offshore wind. Then, after the center of the storm moves past, a rush of water would follow it, leading to flooding.
This is the scenario predicted for Tampa Bay, though variability in where the center of the storm comes onshore — if it does so at all in that part of the state — will be crucial to the surge forecast there.
Needham wrote that an offshore track just west of Florida’s west coast could cause a “shelf wave” to form from Clearwater on northward, which he described as a “bulge of water” that gets caught between the storm and the coast. If this happens, it could exacerbate the surge in parts of western coastal Florida.
Water, rather than wind, kills the most people in U.S. landfalling hurricanes, and the southwest Florida coast is known to be particularly prone to high levels of storm surge. This is because of the shape of the coastline, including the bathymetry just offshore, with an expanse of shallow waters that can easily generate large amounts of surge if the wind is blowing in the right direction.
The forecast track of the storm means that virtually entire state of Florida will be on the right hand side of Hurricane Irma as it slowly makes its way to the north on Saturday night through Monday.
This tends to be the side that contains the strongest surface winds, in part because the counterclockwise circulation around the storm gives it the added component of the hurricane’s forward motion. Hurricane force winds are expected to be felt everywhere from Key West to Jacksonville, and possibly northward into Georgia and South Carolina.
While these winds won’t cause widespread destruction to the level seen in the Caribbean, it could knock out power to 1.8 million customers across Florida and Georgia, according to a forecast from a University of Michigan research group.
Restoring power after the storm is going to be especially difficult in southern Florida, since crews will need to come down the length of the state in order to get there, and that will take time.