Objective Verification of the Weather Prediction Center’s Mesoscale Precipitation Discussions

Abstract

The Weather Prediction Center (WPC) issues Mesoscale Precipitation Discussions (MPDs) to highlight regions where heavy rainfall is expected to pose a threat for flash flooding. Issued as short-term guidance, the MPD consists of a graphical depiction of the threat area and a technical discussion of the forecasted meteorological and hydrological conditions conducive to heavy rainfall and the potential for a flash flood event. MPDs can be issued either during or in anticipation of an event and typically are valid for up to 6 hours. This study presents an objective verification of WPC’s MPDs issued between 2016 and 2022, complete with a climatology, false alarm analysis, and contingency table-based skill scores (e.g. critical success index, fractional coverage, etc). Regional and seasonal differences become evident when MPDs are assessed based on these groupings. MPDs improved in basic skill scores between 2016 and 2020, with slight decline in scores for 2021 and 2022. The false alarm ratio of MPDs has decreased between 2016 and 2021. The most dramatic improvement over the period occurs in the MPDs in the winter season (December, January, and February) and along theWest Coast (primarily atmospheric river events). The accuracy of MPDs in this group has quadrupled when measured by fractional coverage, and the false alarm rate is approximately one fifth of the 2016 value. Skill during active monsoon seasons tends to decrease, partially due to the large size of MPDs issued for monsoon-related flash flooding events.