Because damaging winds are rarer far inland, the hurricane risk to the interior of the U.S. can be easy to overlook.

But storms can travel hundreds of miles after landfall and, on rare occasions, hurricane remnants sometimes reintensify after transitioning into extratropical cyclones or combining with pre-existing mid-latitude systems. Moreover, exposed inland properties tend to be more vulnerable compared to coastal construction.

For these reasons, a robust hurricane model needs to extend far beyond coastal counties—and even coastal states—to reflect the full spatial extent of potential losses.

A Century of Evidence

The potential for hurricanes to penetrate far inland has been understood for over a century. In the year 1900, a Category-4 hurricane made landfall in Galveston, Texas, before traveling deep into the U.S. interior. The storm moved into the Upper Midwest, bringing damaging winds to more than half a dozen states, including Illinois, Indiana and even Vermont. Were this event to recur with present-day exposures, AIR Worldwide estimates insured losses to onshore properties of $38.5 billion, including $1.3 billion to inland states.

In 1954, Hurricane Hazel made landfall at Category-4 strength near the North Carolina/South Carolina border, transitioning into an extratropical system a few hours later. The storm retained damaging wind speeds over the Mid-Atlantic states and into central Pennsylvania and western New York, where peak gusts of 90 mph were recorded. A recurrence today of Hurricane Hazel would cause an estimated $1.7 billion of insured loss in Pennsylvania alone.

Most recently, in 2008 Hurricane Ike dissipated into a tropical depression some 170 miles from its landfall location, at a rate fairly typical for a Category-2 hurricane. The sheer size of the storm, however, led to greater inland penetration than would have occurred with a more compact system. Furthermore, Ike's remnant energy continued moving north, where it combined with a pre-existing extratropical system moving through the Midwest. This new, complex storm system produced heavy rainfall and high winds in 10 states in the Midwest and the Northeast. Final damage estimates issued by ISO's Property Claims Services (PCS) for seven inland states exceeded $2.5 billion.

In total, of the 88 storms for which PCS issued losses from 1950 to the present, 16 percent have caused insured losses in inland states.

Well-Studied Phenomenon

Based on the National Hurricane Center's HURDAT database, the average time it takes for a landfalling hurricane to dissipate is about 34 hours. Considering a range in forward speeds of 10-20 mph, this translates to an overland distance of 340-480 miles—a considerable length from the coast. The mean central pressure for storms after 34 hours from landfall is approximately 989 mb, enough to produce damaging winds far inland.

A storm's rate of decay, or filling, as it moves over land has been a continuous subject of research, motivated by the need to forecast damaging winds—not just at the coast, but also far inland. Over the past 25 years, scientists have developed empirical filling models that relate decay rates to the storm evolution after landfall. AIR's filling-rate formulations are based on an examination of these models, combined with a comprehensive analysis of the latest observational data. Consistent with historical experience, AIR's model also allows a small percentage of storms to reintensify after landfall, as in the case of Hurricane Ike.

Fully understanding inland risk also requires consideration of regional variations in the vulnerability of structures to the observed winds. To that end, the damage functions in the latest release (2010) of our model takes into account findings from AIR's multi-year, peer-reviewed study of the adoption and enforcement of building codes throughout the United States; changes in building materials and construction practices; structural aging and mitigation features; as well as other factors that affect vulnerability. 

AIR's view of the relative hurricane risk across the United States is consistent with the historical record, which has repeatedly demonstrated that damaging winds from hurricanes can penetrate hundreds, and sometimes thousands, of miles inland.