Strange Floods: Downscaling Simulations of the Storms that Produce Them

We examine the upper tail of flood peaks in the US through downscaling simulations of a collection of storms that produce “strange floods”, which are flood events for which the peak discharge is much larger than observed floods in the region.  Downscaling simulations are carried out with the Weather Research and Forecasting (WRF) model; initial conditions and boundary conditions are principally taken from 20^th Century Reanalysis fields.  Flood events include the 14 June 1903 flood which devastated Heppner, Oregon (and the paired Eastern Oregon floods in Meyers Canyon on 13 July 1956 and Lane Canyon on 26 July 1965), the Republican River flood of 31 May 1935, Colorado Front Range floods of 16-17 June 1965 and the Costilla Creek, New Mexico flood of 22 July 1954.  Climatological analyses of 20CR fields are paired with downscaling simulations to characterize anomalies of the storm environments. Analyses point to fundamental difficulties in characterizing the upper tail properties of floods and the rainfall fields that are responsible for their occurrence. We also point to advances in climate modeling that are needed to assess hazards from extreme floods in both current and future climates.