Subj : MESO: Heavy Rain/Flooding To : ALL From : Mike Powell Date : Tue Sep 12 2023 20:27:00 AWUS01 KWNH 122239 FFGMPD AZZ000-UTZ000-NVZ000-CAZ000-130345- Mesoscale Precipitation Discussion 1060 NWS Weather Prediction Center College Park MD 638 PM EDT Tue Sep 12 2023 Areas affected...southern NV into southwestern UT/northwestern AZ Concerning...Heavy rainfall...Flash flooding possible Valid 122236Z - 130345Z Summary...Thunderstorms may produce areas of localized flash flooding from southern NV into southwestern UT and northwestern AZ over the next 3-5 hours. Rainfall rates of 1-3 in/hr are expected from some of the stronger/slower moving cells. Discussion...22Z visible imagery from GOES West and area radars showed widely scattered thunderstorms ongoing across portions of southern NV into southwestern UT. A small convective cluster was observed in southwestern UT, dropping south and preceded by an outflow boundary. This region of the Southwest has seen favorable solar insolation for much of the day, resulting in a small pocket of MLCAPE with 500 to 1000 J/kg on the 22Z SPC mesoanalysis centered over Lincoln and Clark counties. 21Z GPS measurements of precipitable water were 1.0 to 1.1 inches at Las Vegas, slightly lower than the 12Z VEF RAOB of 1.25 inches, but still lying above the 90th percentile for early/mid-September. These anomalous moisture values are likely representative for the general tri-state region. Cell motions across southern NV into southwestern UT have averaged 5-15 kt, owing to a weakness in the low to mid-level flow over the region and further weakening of the mean flow is forecast by the RAP through 03Z. The region lies beneath a broad and low amplitude upper level trough, just ahead of a subtle mid-level vorticity max crossing the Sierra Nevada on water vapor imagery. While surface temperatures are likely at peak and no further increases in instability are expected over the next few hours, lingering instability will continue thunderstorm development over the next 3-5 hours before remaining instability is exhausted. Outflow boundary interactions and short term training may generate a few areas with 1-2 in/hr rainfall rates, but isolated rates near 3 cannot be ruled out. Across southwestern UT, should the ongoing convection survive as it heads south toward some of the sensitive slot canyons of the region, a localized flash flood threat could set up there as well with 1 inch of rain in 15-30 minutes, but weaker instability was estimated to be present in UT. Overall, the flash flood threat through 04Z should remain localized. Otto ATTN...WFO...FGZ...LKN...SLC...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 38531361 38491266 38131202 37581177 36911197 36641310 36061348 35481370 35241385 35011459 35321525 35961593 37021619 37711606 38111568 38391461 = = = --- * SLMR 2.1a * Arnold Layne, don't do it again! * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .