Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 18 2023 18:58:36 AWUS01 KWNH 181858 FFGMPD COZ000-NMZ000-AZZ000-UTZ000-NVZ000-CAZ000-190057- Mesoscale Precipitation Discussion 0941 NWS Weather Prediction Center College Park MD 257 PM EDT Fri Aug 18 2023 Areas affected...Portions of the Colorado River Valley Concerning...Heavy rainfall...Flash flooding likely Valid 181857Z - 190057Z SUMMARY...Showers and thunderstorms with locally very heavy sub-hourly rainfall rates will be increasing in coverage going through the afternoon hours. Scattered instances of flash flooding are likely over and near the more sensitive basins including area dry washes, slot canyons, and burn scars. DISCUSSION...GOES-18 visible satellite imagery shows convective initiation taking place once again with the strong diurnal heating cycle and with a deep monsoonal moisture environment in place across large areas of the Colorado River Valley. Additionally, a feature that will be of strong interest over the next several hours is a strong MCV that is showing up over far northern AZ which is seen taking aim on southern UT. The latest RAP analysis shows SBCAPE values of as much as 1000 to 1500+ J/kg across areas of central and northern AZ into southeast NV and southwest UT. The MCV has helped to keep the boundary layer more stable over the last few hours across far northern AZ and southern UT given an abundance of cloud cover, but this cloud cover will be eroding with time and will set the stage for strong boundary layer heating and an increase in instability heading through the mid-afternoon time frame. This instability will be coupling with the persistence of a well-defined plume of deep monsoonal moisture, and orographics/upslope flow over the terrain for scattered to broken coverage of showers and thunderstorms going through the afternoon hours. The activity will be further aided by differential heating boundaries given the aforementioned MCV-related cloud cover and influence from rugged terrain. PWs across large areas of the Colorado River Valley are running upwards of 1 to 2 standard deviations above normal, with some of the greatest anomalies actually seen from southwest AZ through southeast CA, southeast NV and through southern and and eastern UT. The latest CIRA ALPW plots show notably concentrated moisture in the 700/300 mb layer across much of this region, and especially over southern UT near the MCV. Some weak mid-level shear is noted close to the MCV, and this coupled with the available thermodynamics and mesoscale-driven forcing should yield fairly slow-moving pulse to perhaps some loose pockets of multicell convection. The rainfall rates will be high though given the anomalously moist environment, and some of the stronger cells may produce sub-hourly rainfall rates of up to 1.5 inches in as little as 30 minutes. Some localized storm total amounts of 2 to 3+ inches will be possible, but these amounts and especially the high sub-hourly rainfall rate potential will likely result in scattered areas of flash flooding. The normally dry washes, local slot canyons, and burn scar locations will be most prone to seeing these impacts. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!-C-TkRIeezA0u7MmVXmksu84v__CLFfv72qbOTgj8lZ5WyQWmgRijC7CJcNk3_Em8VM9= En2lRv8NEwZWnK6W8EBypcU$ for graphic product... ATTN...WFO...ABQ...FGZ...GJT...PSR...SLC...TWC...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 39961144 39801032 39131012 38920930 38580885=20 37640855 36420865 35480900 34520868 33810909=20 33541037 33811285 33801449 34291549 35401610=20 36571587 37601511 38131431 38651314 39271244=20 =20=20=20=20=20=20=20=20=20=20=20=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .