Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Aug 23 2025 18:39:01 AWUS01 KWNH 231838 FFGMPD NVZ000-CAZ000-240037- Mesoscale Precipitation Discussion 0983 NWS Weather Prediction Center College Park MD 237 PM EDT Sat Aug 23 2025 Areas affected...Southern and Eastern CA...Sierra Nevada Concerning...Heavy rainfall...Flash flooding likely Valid 231837Z - 240037Z SUMMARY...Deep layer monsoonal moisture advancing up across southern and eastern CA and into the higher terrain of the Sierra Nevada will foster scattered to broken areas of heavy showers and thunderstorms this afternoon. Scattered areas of flash flooding are likely, and especially over the Sierra Nevada. DISCUSSION...The deeper layer subtropical ridge over the Southwest is in a very favorable position to facilitate the northwestward transport of monsoonal moisture up across southern and eastern CA and well into the Sierra Nevada. The latest CIRA-ALPW data shows moisture near or above the 90th percentile of climatology in the 700/500 mb and 500/300 mb layers respectively across the region, with PW anomalies overall running a solid 2 to 3 standard deviations above normal. This moisture coupled with placement of elongated mid-level vort energy and deeper layer upslope flow over the higher terrain will combine with increasing boundary layer instability for scattered to eventually broken areas of heavy shower and thunderstorm activity. Cloud cover over portions of the Transverse and Peninsular Ranges of southern CA have mitigated the build up of instability to a degree, but the proximity of vort energy and steeper mid-level lapse rates have already been focusing an axis of showers and thunderstorms. This activity should tend to expand a bit in coverage as differential heating boundaries and greater instability adjacent to the terrain help to drive additional mesoscale-enhanced circulations for new convective development. The Sierra Nevada overall is the area that will likely see the greatest concentration of heavier rainfall though given the level of anomalous moisture in place along with upslope flow. Even the hires CAM model guidance which can suffer with convective details over the Sierra Nevada has a footprint for impactful rainfall, including a signal for locally repeating cell-activity. The most recent visible satellite imagery along with LightningCast data shows convective initiation underway and suggestive of multiple rounds of convective potential in conjunction with the full diurnal heating cycle. Given the high moisture environment, rainfall rates up to 1.5 inches/hour will be possible, with some localized storm total amounts by late this afternoon of 2 to 4 inches possible where any cells tend to become more terrain-anchored. Scattered areas of flash flooding are likely with today's convection, with the greatest threat over the Sierra Nevada where there will also be some burn scar sensitivities. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!6jxbe72gLfvIl9oz_JQC2znK8LlCdDdfWl3Xj-JLfB9zPHkxSOiX7WrfOspbc1NhzrPw= ZSbUFG5s2dL5xWppXQEBwZU$ for graphic product... ATTN...WFO...HNX...LOX...PSR...REV...SGX...STO...VEF... ATTN...RFC...RSA...STR...NWC... LAT...LON 39211974 39091885 38021760 36821708 35361691=20 34501635 33481530 32731533 32551597 32621671=20 33251717 33711785 34061855 34571913 35431918=20 36151891 37041920 38001996 38692010=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) .