Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 20 2024 09:11:57 AWUS01 KWNH 200911 FFGMPD CAZ000-202110- Mesoscale Precipitation Discussion 0070 NWS Weather Prediction Center College Park MD 410 AM EST Tue Feb 20 2024 Areas affected...Portions of the Transverse and Peninsular Ranges including the Los Angeles Basin Concerning...Heavy rainfall...Flash flooding likely Valid 200910Z - 202110Z SUMMARY...The tail-end of an atmospheric river continues to impact areas of southern CA, and some uptick in rainfall concentration and intensity is expected going through the morning and midday hours. Given the extremely sensitive conditions on the ground locally, any additional heavy rain is likely to foster more runoff problems and flash flooding. DISCUSSION...The latest GOES-W WV suite shows a well-defined subtropical moisture fetch associated with the tail-end of an atmospheric river impacting southern CA along with proximity of a stalled out frontal zone. Shortwave energy offshore of southern CA situated near and adjacent to 30N 130W will be advancing off to the east over the next 12 hours and this will be interacting with the deeper layer moisture axis and nearby front for what should be a gradual increase in shower activity offshore of the Transverse and Peninsular Ranges. This activity heading through the morning hours will overspread the coastal terrain, including the Los Angeles Basin, and with an uptick in forcing and stronger IVT magnitudes, the rainfall rates should also increase. IVT magnitudes by 12Z are forecast to rise to as much as 500 kg/m/s across portions of Los Angeles and Orange County, and adjacent areas to the south into San Diego County will also see increasing moisture transport that will facilitate a threat of heavier shower activity. A combination of a generally orthogonal orientation of the IVT axis relative to the terrain along with the subtropical origins of the moisture transport should favor rather efficiently high rainfall rates that should increase to occasionally over a 0.50"/hour. The 00Z/06Z HREF guidance suggests additional rainfall totals of as much as 2 to 4 inches with isolated heavier amounts possible for the southwest-facing slopes of the San Gabriel and San Bernadino mountains by midday on Tuesday. Areas farther back up the coast involving eastern portions of the Santa Ynez mountains which have already been hard hit by this most recent atmospheric river event will also see locally heavy rains of 1 to 3 inches. Given the extreme sensitivities locally on the ground, these additional rains will favor more runoff problems and a likelihood of additional flash flooding which will include more debris flow and landslide activity. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!42mXAgfYoBN1eBrLHsc-B8dSrx4lsQ0HtQO6r6RM-ida4vhZzjsqN2UPU2WWYikiTJNt= KJrIIc1jGBAw82obENRrvQQ$ for graphic product... ATTN...WFO...HNX...LOX...SGX... ATTN...RFC...CNRFC...NWC... LAT...LON 34811972 34791864 34541791 34431717 34111669=20 33681664 32941668 32761704 32971732 33371768=20 33591797 33871842 33931879 34111936 34341998=20 34652003=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) .