Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Sep 18 2025 16:32:26 AWUS01 KWNH 181632 FFGMPD AZZ000-NVZ000-CAZ000-182230- Mesoscale Precipitation Discussion 1101 NWS Weather Prediction Center College Park MD 1231 PM EDT Thu Sep 18 2025 Areas affected...Southern and Eastern CA...West-Central to Southwest AZ...Far Southern NV Concerning...Heavy rainfall...Flash flooding likely Valid 181630Z - 182230Z SUMMARY...A gradual increase in the coverage and concentration of showers and thunderstorms will be expected over the next several hours. Increasingly heavy rainfall rates will pose a growing threat for isolated to scattered areas of flash flooding by mid-afternoon. DISCUSSION...The early-day GOES-W visible satellite imagery along with radar shows a broken axis of locally heavy showers and thunderstorms impacting portions of eastern CA as a deep layer fetch of tropical moisture/warm air advection and mid-level shortwave energy related to former Tropical Storm Mario advances up into the Southwest U.S. Despite the current regional coverage of cloud cover, there will be a gradual uptick in solar insolation across southeast CA and into southwest AZ over the next several hours which is where visible satellite imagery does show some thinning/clearing of the cloud deck. This will drive an uptick in surface-based instability over time, and with a well-defined instability gradient already in place (SBCAPE values of 1000 to 2000+ J/kg) from the Gulf of CA northward into the Lower CO River Valley, an increase in the coverage of heavy shower and thunderstorm activity will be expected. Differential heating boundaries near areas of higher terrain and also near the edge of the deeper layer cloud decks will be facilitators of additional convective initiation. Additionally, the flow aloft is rather divergent around the northeast flank of the mid-level shortwave energy offshore of southern CA, and this forcing interacting with the deeper layer southerly moisture/warm air advection regime will also promote an increase in convective coverage. PWs are well into the 95th+ percentile of climatology across southern and eastern CA in particular, with CIRA-ALPW showing substantial/highly anomalous concentrations of moisture in the 700/300 mb layer. This moisture coupled with the uptick in surface-based instability should easily support convective cells capable of producing 1 to 2 inch/hour rainfall rates by early to mid-afternoon. The latest 12Z HREF guidance suggests some localized backbuilding and cell-training concerns, and the concentration of convection will support rainfall totals by mid-afternoon of as much as 2 to 4 inches. Isolated to scattered areas of flash flooding will become increasingly likely with time given the high rainfall rates and storm totals over the next several hours. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!-dfCmWJ6CY2uuqTzuU-eee670QKSLFTWh2ApJ8Q9ixofl5V4_XFU_aHWvTI_InYcdTXh= XIBqvSTzKN_xCpJPU805pRE$ for graphic product... ATTN...WFO...FGZ...HNX...LOX...PSR...SGX...VEF... ATTN...RFC...RSA...STR...NWC... LAT...LON 36091632 36071540 35891429 35361298 34351203=20 33571209 33171309 32741382 32331443 32241510=20 32281583 32541653 33261700 34081727 35001829=20 35781806 36041724=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) .