Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 17 2023 18:10:43 AWUS01 KWNH 171810 FFGMPD AZZ000-UTZ000-NVZ000-180009- Mesoscale Precipitation Discussion 0937 NWS Weather Prediction Center College Park MD 209 PM EDT Thu Aug 17 2023 Areas affected...Central/Southern UT...Central/Northern AZ...Southeast NV Concerning...Heavy rainfall...Flash flooding possible Valid 171809Z - 180009Z SUMMARY...Scattered showers and thunderstorms with locally heavy sub-hourly rainfall rates may result in at least isolated instances of flash flooding through the afternoon hours. DISCUSSION...GOES-18 visible satellite imagery shows convective initiation underway across the higher terrain of central and southern UT, including portions of the Wasatch and extending down south down through much of central and northern AZ including the Mogollon Rim. Morning cloud cover has been eroding with time and is giving way to very strong boundary layer heating and an increase in instability which is currently being characterized by SBCAPE values of 1000 to 1500+ J/kg. This instability will be coupling with the persistence of a well-defined plume of monsoonal moisture, and orographics/upslope flow over the terrain for scattered areas of showers and thunderstorms going through the afternoon hours. The activity will be also aided by some smaller scale differential heating boundaries for convection to initiate and locally expand in coverage. PWs across central and northern AZ are running upwards of 1 to 1.5 standard deviations above normal, however, these anomalies are notably stronger farther north through southern and central UT, and reach as high as 2 to 2.5 standard deviations above normal with PW values of 1 to 1.25 inches locally. The anomalies are also quite strong farther west back into areas of eastern NV given moisture concentrations seen around the northeast side of the mid-level trough/closed low near the West Coast. The rather well-established monsoonal moisture profile through the vertical column which is well depicted by the latest CIRA ALPW data should work in tandem with the increasing instability to produce notably heavy rainfall rates. There will not be a lot of shear to favor well-organized convection, but certainly there will be enough orographic focus and smaller-scale forcing for slow-moving pulse cell convection. Also, there is evidence in visible satellite imagery of a small-scale MCV over northwest AZ which will be gradually drifting north over the next several hours and may provide some weak dynamical support for convection. Some sub-hourly rainfall rates may reach 1 to 1.5 inches in as little as 30 minutes, and there are some low-end HREF probabilities supporting this. Some spotty storm totals where some of these cells persist may reach as high as 2 to 3 inches. There will therefore be at least isolated concerns for some flash flooding which will include the normally dry washes, local slot canyons, and any burn scar locations. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!_wMo0Va5LPc43pBn3jNr8Kbnb4UgueAvlrrDEQ0pVjS-Q1eVPX01CDIfJhLB4FQE8To3= Nyg7xsP5GLcSQaw5T-bbwXU$ for graphic product... ATTN...WFO...FGZ...SLC...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 40581073 39201068 36961160 35311115 34551013=20 33881037 34201238 34951380 36031489 37051524=20 37911469 38431332 39371226 40481142=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) .