How Often Should We Expect Losses on the Scale of Harvey or Irma?

In 2017 North Atlantic hurricane season, as many as four hurricanes made landfalls in the US including its Caribbean territories, causing catastrophic property loss from extreme wind and flooding. Major hurricane Harvey made landfall at Category 4 intensity with a maximum sustained wind speed of 130 mph on Aug. 25, 2017 in Rockport, Texas. Another Category 4 hurricane named Irma made landfall on Sunday Sept. 10 at Cudjoe Key in the Florida Keys with a wind speed of 130 mph. On the same day in the afternoon Irma made a second landfall in Marco Island, Florida as a Category 3 storm with a wind speed of 115 mph. Hurricane Irma's landfall in the Florida Keys was just 16 days after Hurricane Harvey’s landfall in Texas, and together the storms caused devastating property losses across the Gulf states and Florida.

Harvey produced devastating winds, storm surge, and extremely heavy and excessive rainfall, producing historic flooding over areas east of the landfall. Strong and extreme wind gusts damaged or destroyed many structures, residences, and business in Rockport and the Fulton area in Texas. The highest maximum storm tides at the Aransas Wildlife Refuge reached more than 12 feet above ground level. Harvey stalled over South and Southeast Texas for days, dumping more than 4 feet of rain and producing deadly flash and river flooding. Wind gusts of 130 to 160 mph extended throughout much of the area around Rockport and west of Aransas. As a powerful and large cyclone Irma’s hurricane force winds were experienced along much of the east coast between Miami and Jacksonville. Storm surge flooding occurred well away from the storm center, including the Jacksonville area on the northeast coast. Peak measured wind gusts of 120 to 145 mph occurred from the lower Florida Keys north to the southwestern coastline of Florida. Life threatening and highly damaging storm surge impacted the Keys and both coasts of Florida. Texas and Florida residents will remember these hurricanes for years to come.

To assess the impact of major hurricanes Harvey and Irma in the mainland US, CoreLogic’s high-resolution, directionally varying time-stepping wind field model and high resolution storm surge and inland flood models are used to simulate the evolution of the hazard from these two storms. The modeled hazards are validated using observed data. The maps and scatter plots show that model estimated hurricane winds and flooding are reasonably comparable to the observations available. Property losses and their return periods to residential and commercial buildings are estimated using the hurricane wind, storm surge and inland flood risk models.

It was not the extreme winds but the deluge from Harvey that caused catastrophic damage in Texas and the Gulf. Residential building and contents insured loss in the 70-county area in Texas and Louisiana affected by inland, flash and storm surge flooding from Harvey is estimated to be between $6.5 billion and $9.5 billion, which includes homes covered by the National Flood Insurance Program (NFIP) and by private insurers which is estimated around $0.5 billion. Insured loss attributed to Harvey’s hurricane winds is estimated in the range of $1 to $2 billion. Residential uninsured flood loss for the same area from flooding is estimated to be between $18 billion and $27 billion, which is about 70 percent of the total loss. The return period of inland flood loss of hurricane Harvey’s severity or greater in Texas alone is estimated to be on the order of 250 to 350 years. Contrary to Hurricane Harvey, Hurricane Irma was a large, fast moving storm with much higher extreme wind damage. Irma’s total insured and uninsured loss from wind and flood for residential and commercial properties is estimated to be between $42.5 billion and $65 billion. Irma’s extreme winds caused an estimated $13.5 billion to $19 billion in insured loss for both residential and commercial properties. Our analysis shows that an Irma like event landfall in southwest Florida with a 130 mph or more sustained wind is likely to be a 43 year event, whereas a 115 mph or more sustained wind is likely to be a 20 year event. The return period of a loss of Irma’s severity or greater in Florida is likely to be once in 15 to 20 years, whereas along the Gulf and Atlantic coast the estimated return period is 8 to 11 years. All the return periods are consistent with historical activity in the basin.