Analysis and Prediction of Summer Rainfall over Southwestern Utah

While many studies have examined intense rainfall and fl ash fl ooding during the North American monsoon (NAM) in Arizona, Nevada, and New Mexico, less attention has focused on the NAM ' s extension into southwestern Utah. This study relates fl ash fl ood reports and Multi-Radar Multi-Sensor (MRMS) precipitation across southwestern Utah to atmospheric moisture content and instability analyses and forecasts from the High-Resolution Rapid Refresh (HRRR) model during the 2021 - 23 monsoon seasons. MRMS quantitative precipitation estimates over southwestern Utah during the summer depend largely on the areal coverage from the KICX WSR-88D radar near Cedar City, Utah. Those estimates are generally consistent with the limited number of precipitation gauge reports in the region except at extended distances from the radar. A strong relationship is evident between days with widespread precipitation and afternoons with aboveaverage precipitable water (PWAT) and convective available potential energy (CAPE) estimated from HRRR analyses across the region. Time-lagged ensembles of HRRR forecasts (initialization times from 0300 to 0600 UTC) that are 13 - 18 h prior to the afternoon period when convection is initiating (1800 - 2100 UTC) are useful for situational awareness of widespread precipitation events after adjusting for underprediction of afternoon CAPE. Improved skill is possible using random forest classi fl cation relying only on PWAT and CAPE to predict days experiencing excessive (upper quartile) precipitation. Such HRRR predictions may be useful for forecasters at the Salt Lake City National Weather Service Forecast Of fl ce to assist in issuing fl ash fl ood potential statements for visitors to national parks and other recreational areas in the region.