Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 25 2023 19:30:49 AWUS01 KWNH 251930 FFGMPD AZZ000-UTZ000-260130- Mesoscale Precipitation Discussion 0793 NWS Weather Prediction Center College Park MD 330 PM EDT Tue Jul 25 2023 Areas affected...Northern Arizona, Southern Utah Concerning...Heavy rainfall...Flash flooding possible Valid 251930Z - 260130Z Summary...Scattered showers and thunderstorms will increase in coverage this afternoon and advect M/NE through the evening. Rainfall rates may reach 0.5-1"/hr at times, producing locally more than 1 inch of rain. Flash flooding is possible. Discussion...The GOES-E visible imagery this afternoon is showing rapid development of diurnal Cu across the Four Corners states, with the most agitated activity currently setting up over terrain features. The deepest Cu, and associated convective elements noted by the onset of lightning, is occurring across northwest AZ west of the Mogollon Rim, which is downstream of a shortwave lifting northward out of the low deserts of AZ. Enhanced ascent through subtle PVA/height falls will continue as this feature shifts northward, rotating along the periphery of the monsoon ridge to the east. PWs across the area are measured via GPS to be 0.7 to 1.1 inches, above the 75th percentile according to the SPC sounding climatology, and are collocated with SBCAPE that has eclipsed 1000 J/kg. The result of this overlap of ascent and thermodynamics will be the continued expansion of thunderstorms through the aftn. The current convective coverage is outpacing all available high-res models including the HRRR, UA WRF, and experimental RRFS, suggesting the ascent is more intense than the models are predicting. With this already the case, and forcing/thermodynamics expected to remain favorable, anticipate coverage of storms will continue to increase. The HRRR sub-hourly fields suggest rainfall may reach as high as 0.25"/15 minutes, but this may also be underdone in the most intense cells. While 0-6km mean winds are progged at 10-15 kts indicating generally progressive storms, enhanced ascent downstream of the northward moving shortwave aligned with the greatest instability could result in some slowing noted by Corfidi vectors falling to around 5 kts. This is also observed in current radar with some near-zero storm motions immediately downstream of the shortwave. While storms will generally be progressive to limit rainfall accumulations, where brief slowing can occur, rainfall totals may eclipse 1 inch. This region has experienced pockets of heavy rain the last 7 days noted by AHPS departures that are in places more than 150% of normal, and FFG is as low as 0.5-0.75"/1hr. Although HREF exceedance probabilities for this FFG are minimal, it is possible that any slow moving cell could quickly lead to runoff and isolated instances of flash flooding. This will be most likely should a slow moving storm move across normally dry washes or sensitive terrain like slot canyons or burn scars. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!5alQMxt5XpGSHHGeogfgYVGwhI-befUGk6Qn_cdmA84DTpCA-FrOAKj_uGQOJh6OgPrF= nsncCu9820sqFlFBc7xd5TU$ for graphic product... ATTN...WFO...FGZ...GJT...SLC...VEF... ATTN...RFC...CBRFC...NWC... LAT...LON 38991222 38911111 38371061 37571041 36451068=20 35341100 34771146 34481210 34521254 34681334=20 35061386 37001403 38771345=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) .