Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 25 2025 20:09:37 AWUS01 KWNH 252009 FFGMPD NVZ000-CAZ000-260208- Mesoscale Precipitation Discussion 1001 NWS Weather Prediction Center College Park MD 408 PM EDT Mon Aug 25 2025 Areas affected...Sierra Nevada...Southern CA Concerning...Heavy rainfall...Flash flooding likely Valid 252008Z - 260208Z SUMMARY...Scattered thunderstorms with heavy rainfall rates are likely through this afternoon across portions of the Peninsular Range of southern CA, and also into the central and southern Sierra Nevada. Scattered areas of flash flooding are likely, and this may include a threat for burn scar flash flooding and debris flow activity. DISCUSSION...The early afternoon GOES-W Day Cloud Phase RGB satellite imagery shows convective initiation underway across the higher terrain of the Peninsular Range of southern CA and also up across the central and southern Sierra Nevada. Daytime heating and the build of surface-based instability along and adjacent the terrain is fostering favorable differential heating boundary and terrain-enhanced circulations/ascent for convection which should generally expand in coverage over the next several hours. Monsoonal moisture remains quite prevalent and especially in the 850/500 mb layer based on the latest CIRA-ALPW data. This is helping to drive PW anomalies again that are 1.5 to 2.5 standard deviations above normal. This moisture along with the increasing instability and orographic footprint of the convection should yield rainfall rates this afternoon reaching as high as 1.5 to 2 inches/hour, with some of the heaviest rates probably focused over the Peninsular Range of southern CA where moisture here is a tad deeper (noting the 500/300 mb layer) relative to areas farther north. Also, there is some very moist low-level southeast flow off of the northern Gulf of California playing a role here in driving greater nearby instability. The convection today should again follow the diurnal heating cycle, and by late today, some spotty rainfall amounts of 2 to 4 inches will be possible. This is supported locally by the latest experimental WoFS guidance, and especially over the Peninsular Range. Given the sensitivities with localized burn scar areas and the rugged terrain in general, scattered areas of flash flooding are likely. Some debris flow activity will be possible where any burn scar flash flooding occurs. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!4ndsX6KA9fEXjpy0wdelhQQz5tqrzimNfEaoVjf97bEyQm3PQrit_tXPEeD8qz5kZXqZ= mrIcsIrA4pRFv1MUuipHEN0$ for graphic product... ATTN...WFO...HNX...LOX...PSR...REV...SGX...STO...VEF... ATTN...RFC...RSA...NWC... LAT...LON 39441960 38271829 36641772 35181763 34091633=20 32511581 32361649 33041699 33451730 34271799=20 34491839 34811881 34981884 35391887 36041883=20 36871906 37631967 38742031 39392040=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .