Evaluation of Tools Used for Monitoring and Forecasting Flash Floods in the United States

This paper evaluates, for the first time, flash-flood guidance (FFG) values and recently developed gridded FFG (GFFG) used by the National Weather Service (NWS) to monitor and predict imminent flash flooding, which is the leading storm-related cause of death in the United States. It is envisioned that results from this study will be used 1) to establish benchmark performance of existing operational flash-flood prediction tools and2)to provideinformation toNWSforecastersthatrevealshow theexistingtools canbe readilyoptimized. Sources used to evaluate the products include official reports of flash floods from the NWS Storm Data database, discharge measurements on small basins available from the U.S. Geological Survey, and witness reports of flash flooding collected during the Severe Hazards Analysis and Verification Experiment. Results indicated that the operational guidance values, with no calibration, were marginally skillful, with the highest criticalsuccessindexof0.20 occurringwith3-h GFFG.Thefalse-alarm ratesfell andtheskill improvedto0.34 when the rainfall was firstspatially averaged within basins and then reached 50% of FFG for 1-h accumulation and exceeded 3-h FFG. Although the skill of the GFFG values was generally lower than that of their FFG counterparts,GFFGwascapableofdetectingthespatialvariabilityofreportedflashfloodingbetterthanFFG was for a case study in an urban setting.

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