Describing floods in terms of an Average Recurrence Internal (ARI) or “return period” (e.g. 100-year) has been used for decades to convey the rareness of flooding at stream gauges. Describing the intensity of heavy precipitation events in a similar manner has not been as routine, but provides an equally objective perspective of extreme events. Official, gridded NOAA/NWS precipitation frequency estimates (PFEs) from documents such as NOAA Atlas 14 provide the statistical basis for translating observed or predicted precipitation at any location in the United States into an equivalent ARI between 1 and 1,000 years. Coupling PFEs with gridded high-resolution quantitative precipitation estimates (QPEs) or observed measurements allow for ARI maps of the continental United States to be created. Maps of this nature provide a unique and useful perspective of precipitation that most people understand, even for those not familiar with the precipitation of a particular area. Although the ARI of rainfall does not necessarily equate to a flood of the same ARI, real-time ARI maps of precipitation provide excellent flash flooding guidance, particularly forecast ARI precipitation maps.

This paper will focus on the concept of generating forecast ARI precipitation maps using Quantitative Precipitation Forecasts (QPFs) from the Weather Research Forecast (WRF) mesoscale numerical weather prediction model. Advances in the science of numerical weather prediction have significantly increased skill and resolution of QPF over the last decade, and mesoscale models such as WRF provide excellent automated forecast guidance for the one to three day forecast period, especially for strongly forced events typical of those that lead to widespread heavy rainfall. Communicating the potential for high impact precipitation events through the use of the ARI technique can be a powerful way to heighten the public's awareness of impending flood potential. This paper will share the results of a 6- and 24-hour case study in the United States.