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

But storms can travel hundreds ofmiles after landfall and, on rare occasions, hurricane remnantssometimes reintensify after transitioning into extratropicalcyclones or combining with pre-existing mid-latitude systems.Moreover, exposed inland properties tend to be more vulnerablecompared to coastal construction.

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

A Century of Evidence

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

In 1954, Hurricane Hazel made landfall at Category-4 strengthnear the North Carolina/South Carolina border, transitioning intoan extratropical system a few hours later. The storm retaineddamaging wind speeds over the Mid-Atlantic states and into centralPennsylvania and western New York, where peak gusts of 90 mph wererecorded. A recurrence today of Hurricane Hazel would cause anestimated $1.7 billion of insured loss in Pennsylvania alone.

Most recently, in 2008 Hurricane Ike dissipated into a tropicaldepression some 170 miles from its landfall location, at a ratefairly typical for a Category-2 hurricane. The sheer size of thestorm, however, led to greater inland penetration than would haveoccurred with a more compact system. Furthermore, Ike's remnantenergy continued moving north, where it combined with apre-existing extratropical system moving through the Midwest. Thisnew, complex storm system produced heavy rainfall and high winds in10 states in the Midwest and the Northeast. Final damage estimatesissued by ISO's Property Claims Services (PCS) for seven inlandstates exceeded $2.5 billion.

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

Well-Studied Phenomenon

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

A storm's rate of decay, or filling, as it moves overland has been a continuous subject of research, motivated by theneed to forecast damaging winds—not just at the coast, but also farinland. Over the past 25 years, scientists have developed empiricalfilling models that relate decay rates to the storm evolution afterlandfall. AIR's filling-rate formulations are based on anexamination of these models, combined with a comprehensive analysisof the latest observational data. Consistent with historicalexperience, AIR's model also allows a small percentage of storms toreintensify after landfall, as in the case of Hurricane Ike.

Fully understanding inland risk also requires consideration ofregional variations in the vulnerability of structures to theobserved winds. To that end, the damage functions in the latestrelease (2010) of our model takes into account findings from AIR'smulti-year, peer-reviewed study of the adoption and enforcement ofbuilding codes throughout the United States; changes in buildingmaterials and construction practices; structural aging andmitigation features; as well as other factors that affectvulnerability. 

AIR's view of the relative hurricane risk across the UnitedStates is consistent with the historical record, which hasrepeatedly demonstrated that damaging winds from hurricanes canpenetrate hundreds, and sometimes thousands, of milesinland.