Subj : Heavy Rain/Flooding UT/AZ To : All From : Mike Powell Date : Thu Aug 22 2024 08:40:00 AWUS01 KWNH 221322 FFGMPD AZZ000-UTZ000-221920- Mesoscale Precipitation Discussion 0919 NWS Weather Prediction Center College Park MD 921 AM EDT Thu Aug 22 2024 Areas affected...southern UT...northern AZ Concerning...Heavy rainfall...Flash flooding likely Valid 221320Z - 221920Z Summary...Localized short-term totals in excess of 2" are likely to lead to isolated to widely scattered instances of flash flooding. An instance or two of flash flooding may be significant, with particular concern for slot canyons, normally dry washes, and other sensitive low-lying areas). Discussion...The Southwestern U.S. remains wedged between an upper low/trough to the northwest (off the Northwest U.S. coast) and a upper high/ridge to the southeast (over TX), enhancing the seasonal monsoonal pattern with moisture streaming northward in the lower to mid levels (925-700 mb). Widely scattered convection is evident via GOES and MRMS imagery over portions of southern UT and northern AZ at this early morning hour, unusual for even this more rainy time of year. The mesoscale environment is characterized by pockets of 500-1000 J/kg of SB CAPE, a broad plume of precipitable water values ranging from 0.9 to 1.3 inches (between the 90th percentile and max moving average, per VEF and FGZ sounding climatology), and effective bulk shear of 20-30 kts. This parameter space should continue to support at least isolated convection through midday, despite somewhat limited CAPE and unfavorable diurnal timing, particularly because convection is already ongoing and shear is anomalously high (near the 90th percentile, owing to the unseasonable strength of the Northwest U.S. trough). While hi-res CAMs notoriously struggle with monsoonal convection, there is some HREF signal (from the 06z run) to support a flash flood risk in the near term. While the HRRR is consistently one of the weakest members of the ensemble, there is still a 10-20% chance for localized 2" exceedance, per 40-km neighborhood method via the 06z HREF. These values are quite significant across this hydrologically sensitive portion of the country, corresponding to 20-30% chances of exceeding the 10-year average recurrence interval (ARI), and even a 10% chance of exeecind the 100-year ARI (maximized over portions of southwestern UT, where much of the rainfall so far this morning has already occured). Given this favorable enviornment and the local sensitivity, isolated to scattered instances of flash flooding are likely (and may be locally significant, particularly in the vicinity of slot canyons, normally dry washes, and other sensitive low-lying areas). Churchill ATTN...WFO...FGZ...GJT...SLC...VEF... ATTN...RFC...CBRFC...NWC... LAT...LON 38531152 37410974 35441022 34611246 35131352 35791393 37291388 38061317 --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .