Hurricane Ian brought a historic and deadly storm surge to southwest Florida when it came ashore two weeks ago yesterday.
As of Monday evening, the Florida Medical Examiners Commission reported 105 deaths from Ian in Florida, with over half in Lee County alone which saw the brunt of Ian’s storm surge. Nearly 60 percent of deaths were caused by drowning. Water, both at the coast and inland, was the big story with Ian.
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But just how high the catastrophic storm surge reached takes a little time to unpack. We knew almost instantly from the terrifying videos shared on social media from people trapped in their homes that the coastal surge made it to the second floor of some places.
Given a story is about 10 feet high, in the absence of data, it’s safe to assume the storm surge reached at least 10 feet above ground in spots.
Simply put, storm surge is water above the normal tide caused by the strong winds of a coastal storm pushing seawater ashore. Contrary to popular belief, only a tiny fraction (less than 10 percent) of a hurricane’s storm surge is due to its low pressure.
Storm surge is produced on the shallow part of the ocean floor (called the continental shelf) that borders the coastal U.S. Along western Florida, the ocean floor is especially shallow for a long way out, which makes it more susceptible to big storm surges.
The variation in tide in the Gulf is generally modest, unlike on the U.S. East Coast. In southwest Florida, for example, tides vary by a foot or two each day, but up in the northeast tides can range by 10 feet or more. So, whether the surge comes in at high tide or low tide can play a significant role in how bad the flooding gets.
Scientists account for this by measuring storm tide, which is the combination of storm surge and astronomical tide. When the National Hurricane Center issues its storm surge forecasts, they’re actually issuing a forecast for storm tide, assuming the surge occurs at the time of high tide.
The NHC storm surge forecast for the hardest hit areas was raised from 8-12 feet to 12-18 feet on the morning of Wednesday, September 28th, the day Ian came ashore.
Both NOAA, the parent organization of the National Weather Service, and the U.S. Geological Survey (USGS) send out teams in the immediate weeks following landfall to collect data about the extent and magnitude of the storm surge to assess exactly how high the water got.
Most of the storm surge observations we receive come in the form of high-water marks – post-flood debris lines.
They’re not always the most reliable or accurate and can be tainted by waves, especially in enclosed spaces (storm surge forecasts don’t explicitly account for waves, which can add temporarily to the “stillwater” rise). Nevertheless, assessment teams grade the high-water marks they measure, and higher-quality debris lines provide a good idea of how high the surge reached.
The USGS has collected hundreds of high-water marks over the past few weeks. Filtering for only high-water marks with a +/-0.2 ft. accuracy reveals the worst storm surge flooding the Gulf side from Sanibel Island to Bonita Beach, focused on Fort Myers Beach in Lee County.
The highest storm surge was confined primarily to Lee and northern parts of Collier Counties about 25 to 30 miles south of where Ian came ashore, near its radius of maximum winds of onshore flow. High water marks on Fort Myers Beach reached near 16 feet in spots.
In addition to high-water marks, the USGS also collected high-fidelity storm tide data from pressure sensors scientists mounted ahead of the storm. These pressure sensors are among the best storm tide data we have available.
A sensor mounted on Fort Myers Beach Pier indicated a storm tide reaching about 13 feet above the pre-Ian beach, with waves helping water levels reach 15 feet. The beach itself was scoured away by the powerful waves, losing about 5 feet of sand from pre-Ian levels.
Overall, the provisional data suggest a storm surge of 10-15 feet focused on Bonita Beach and Fort Myers Beach. This is consistent with the preliminary findings of NWS assessment teams shared this week on social media.
It’s worth noting that only six deaths so far have been reported in Charlotte County where storm surge levels were much lower, despite Ian’s center tracking directly over.
As history has shown, water is the deadliest hazard of a hurricane and storm surge has the biggest potential for large loss of life. In the case of Ian – alongside Katrina (2005), Ike (2008), Michael (2018), and Laura (2020) – it will go down as one of the biggest surge producers we’ve witnessed this century, making it unfortunately also one of the deadliest hurricanes in recent years.