To estimate property damage due to winds at a particular location, we must specify the wind conditions, the relationship between wind and damage, and the property value. We will write the damage at a grid point i as wipi. Here pi = f(Vi) is the fractional damage at the grid point and is a function of the wind speed Vi at the grid point. The weight given to the grid point, wi, is the total property value at that grid point and is a known constant.
 The relevant wind at a grid point is taken to be the near surface maximum sustained wind speed forecast over the course of the event. Winds used here are 10 m winds from the THORPEX Interactive Grand Global Ensemble (TIGGE) database [Bougeault et al., 2010] for each of the 50 ECMWF ensemble members. The forecasts begin every 12 hours from 00 UTC 23 August through 12 UTC 26 August 2011. The surface wind fields are archived by TIGGE every 6 hours on a reduced Gaussian grid with 320 latitudes in each hemisphere. This grid has a resolution of approximately 0.281° in latitude and 30 km in the zonal direction. We interpolate 10-mu- andv-wind components to a 1/4 × 1/4° grid.
 Empirical evidence shows that damage begins at some threshold value of wind speed, increases with wind speed, and asymptotes to 100% damage for high enough wind speed [Unanwa et al., 2000]. We use the parameterization of Emanuel , where damage increases with the cube of the excess of wind speed (V) over the threshold wind speed (Vt, t for “threshold”, taken here as 30 kt). The equation for fractional damage is f = α3/(1 + α3), where α = (V − Vt)/(Vh − Vt) for V > Vt and zero otherwise. The parameter Vh (taken here as 110 kt) is the speed at which 50% of property is damaged (h for “half”).
 We use U.S. census data on occupied household values from the 2010 5-year American Community Survey [U.S. Census Bureau, 2011]. This implies we consider only residential, not commercial, property. This database gives the number of occupied homes in each of twenty classes in each census block group. (Each home is assigned to the class with the closest property value. The class property values, in thousands of dollars, are 5, 12.5, 17.5, 22.5, 27.5, 32.5, 37.5, 45, 55, 65, 75, 85, 95, 112.5, 137.5, 162.5, 187.5, 225, 275, 350, 450, 625.5, 875 and 1000.) From this we estimate the total residential value in each census block group, which we then aggregate to the value of wi at the nearest grid point of the 1/4 × 1/4° grid. There are over 200,000 census block groups nationwide, but geographic size and population counts vary widely. Since the maximum recorded value is $1,000,000, property values in some wealthy neighborhoods will be underestimated. Figure 1 displays the resulting grid of residential property value. The dominant feature in the figure is the Washington to Boston corridor. (Note that values in billions of dollars per grid point are displayed with a base 10 logarithmic scale.)