4th September 2023
Callum Higgins, senior Product manager, Moody’s RMS, comments:
"Major Hurricane Idalia made landfall as a Category 3 hurricane near Keaton Beach, Florida at around 11:45 UTC (07:45 local time) on Wednesday, 30th August. It is the first time since records began in 1851 that Taylor County, Florida, experienced a Category 3 hurricane or higher and Idalia matches or surpasses the strongest hurricanes on record to strike Florida’s Big Bend region.
The day prior to landfall the storm rapidly intensified with wind speeds increasing by 45 mph, fueled by the anomalously warm waters in the Gulf of Mexico. As noted by Dr. Jeff Masters, this makes Idalia one of only ten historical storms since 1950 to intensify at least 40 mph in the 24 hours before US landfall. Five of these ten storms have now occurred in the past seven years, all impacting the Gulf Coast, with Idalia becoming the eighth major hurricane to make landfall in the Gulf region since 2017.
This rapid intensification of Idalia led to the storm briefly attaining Category 4 status on its approach to Florida, but a combination of dry-air entrainment and the beginning of an eyewall replacement cycle weakened the system, with maximum sustained wind speeds falling to 125 mph prior to its centre officially making landfall.
Nevertheless, this makes Idalia one of only 21 hurricanes known to strike Florida with winds of at least 125 mph. It's the fourth hurricane since 2017 to make landfall in Florida with winds equal to or exceeding 125 mph, following Irma, Michael, and Ian.
Despite its intensity a couple of crucial factors may mitigate the impact of Idalia to an extent. First, its landfall location. The Big Bend region of Florida has significantly lower population and exposure density relative to much of the state. Based on the Moody’s RMS 2023 United States Hurricane Industry Exposure Database, the average total insured value for the counties of the Big Bend region is over 85% less than the average for all counties in Florida.
However, this lack of exposure is likely to be somewhat counteracted by the greater vulnerability of properties in the region. The Big Bend area has the lowest design wind speed of any Florida coastal area in the Florida building code, with peak 3-second gusts of 130 mph. Additionally, according to Ian Giammanco at IBHS over 50% of homes in the Big Bend counties were built in the 80s and 90s, before the implementation of modern building codes, and there are almost as many mobile homes as single-family dwellings. Properties with asphalt shingle roofs that are older than ten years are likely to be particularly susceptible to damage, even from wind speeds in the 90-100 mph gust range, with any roof damage exacerbated by subsequent water intrusion.
The other mitigating factor is the relatively small wind field of Idalia, with a low radius to maximum winds(9 miles at landfall according to Moody’s RMS HWind) and the intensity of wind hazard rapidly declining outwards from this, reducing the spatial extent of the most damaging winds."
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