Verifying Flash Floods: Pondering Precipitation Proxies

The prediction of flash floods is a notoriously challenging forecast problem, requiring not only accurate prediction of heavy rainfall magnitudes, but also on the spatiotemporal distribution of that rainfall; the hydrologic interactions between precipitation, terrain, and the land surface; and also on antecedent precipitation and its effects on soil conditions. Further exacerbating the flash flood forecast problem is the considerable difficulty in verifying flash flood events, an essential component to forecasting any phenomenon. Flash flood reports and warnings are subject to population bias, with report databases often missing transient floods in very rural areas, and also to varying reporting and report encoding practices in different regions of the United States. It is often attractive to consider flash flood potential in a simplified framework considering only the quantitative precipitation estimates (QPEs) or forecasts (QPFs), as the case may be. There are numerous ways to estimate risk of flash flooding directly from QPEs, such as exceedances of some precipitation threshold within a prescribed time interval (e.g. 3”/3 hr.); by exceedances of average recurrence intervals (ARIs), which determine amounts required for a fixed frequency of exceedance, varying depending on the local precipitation climatology; or by dynamically varying thresholds that account for the hydrologic characteristics of the area and antecedent conditions, such as exceedances of Flash Flood Guidance (FFG), a product issued routinely by NWS River Forecast Centers (RFCs). Through these come many different ways to assess whether a flash flood is likely to occur (in the forecasting context) or has occurred (in the analysis context), including the precipitation accumulation interval length, which can vary from subhourly to over an entire day, QPE threshold selection, and source for determining QPEs. Three popular QPE sources—the Multi-Radar/Multi-Sensor System (MRMS), NCEP Stage IV Precipitation Analysis (ST4), and the Climatology Calibrated Precipitation Analysis (CCPA)—are compared over a 2.5 year record from January 2015 through June 2017 for exceedances of different thresholds—including fixed amounts, ARIs, and FFGs—used as candidate proxies for flash flooding, and compared against flash flood warnings and reports.