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

Abstract

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 and 2) to provide information to NWS forecasters that reveals how the existing tools can be readily optimized. 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 critical success index of 0.20 occurring with 3-h GFFG. The false-alarm rates fell and the skill improved to 0.34 when the rainfall was first spatially 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, GFFG was capable of detecting the spatial variability of reported flash flooding better than FFG was for a case study in an urban setting.

Establishing a rainfall threshold for flash flood warnings in China’s mountainous areas based on a distributed hydrological model

“…Meanwhile, real-time forecast systems that are based on hydrological models might exhibit an increased probability of system failures because of model instability, incorrect updating procedures, and error propagation (Martina et al, 2006). Under these circumstances, an early warning method that is based on a rainfall threshold can serve as a satisfactory auxiliary reference, as proposed by several past studies (Mogil et al, 1978;Schmidt et al, 2007;Norbiato et al, 2008;Gourley et al, 2012).…”

“…Recently, Schmidt et al (2007) proposed a raster-based method to derive a gridded FFG (GFFG) in a distributed manner. Gourley et al (2012) reported that FFG performs better than the GFFG, but the GFFG can detect the spatial variability.…”

A review of advances in China’s flash flood early-warning system

“…This value is related to the characteristic of precipitation, soil moisture, and underlying surface (Diakakis 2011;Yucel 2015). Gourley et al (2012) evaluated Flash Flood Guidance (FFG) values and developed the gridded FFG used by the National Weather Service (NWS) to monitor and predict imminent flash flooding. Clark et al (2014) established the benchmark skill of the operational flash flood guidance (FFG) product used by NWS to forecast, monitor, and warn the public about dangerous flash flooding events.…”

Evaluating flash-flood warnings at ungauged locations using post-event surveys: a case study with the AIGA warning system

“…Even if both observed flood databases (NWS Storm Data and SHAVE) do not provide a comprehensive identification of all flash-flood events with accurate spatiotemporal description, they enabled Gourley et al (2012) to conduct a detailed comparison of the performances of FFG and recently developed gridded FFG (GFFG) used operationally in the NWS. Also Calianno et al (2013) used these databases to classify the impacts of flash floods in relation with socio-spatial attributes (such as land use and population density) and to evaluate whether flash-flood forecasting tools (namely FFG, GFFG and DHM-TF) could differentiate such categories of impacts.…”