Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Nov 21 2025 06:12:38 AWUS01 KWNH 210612 FFGMPD CAZ000-211210- Mesoscale Precipitation Discussion 1238 NWS Weather Prediction Center College Park MD 112 AM EST Fri Nov 21 2025 Areas affected...coastal southern CA Concerning...Heavy rainfall...Flash flooding possible Valid 210611Z - 211210Z Summary...Highly localized flash flooding will be possible from slow moving showers/thunderstorms capable of producing hourly rainfall up to ~1 inch. This threat will exist along portions of southern CA near the coast through at least 12Z. Discussion...GOES West water vapor imagery at 06Z showed a mid-level closed low centered over the central CA coast, slowly moving south. Several smaller scale vorticity maxima were observed within the cyclonic flow, with one of interest located over/near Santa Cruz Island. Cooling cloud tops were noted east of the vorticity center, northwest of a surface low within a mesoscale deformation zone. The 1009 mb low was located about 30 miles west of Santa Catalina at 06Z with an occluded/cold front extending southward. A weak plume of low level moisture transport was located ahead of the front, parallel to the southern CA coastline where a few showers have recently picked up in intensity with localized training along the Orange County coast. Weak instability up to a couple hundred J/kg was located along the southern CA coast from Santa Barbara to San Diego counties as noted in recent ACARS/model soundings. As the mid-level low continues to translate down the CA coast, the plume of low level moisture ahead of the front will push inland along with the initial round of stronger showers. Later in the night, some steepening of mid-level lapse rates should increase instability up to about 500 J/kg along the immediate coast and closer to 1000 J/kg offshore. The surface low is forecast to edge closer to the Los Angeles/Orange County coasts through 12Z. With this movement, another round of slow moving showers and thunderstorms is expected to track toward the coast from the offshore waters, with potential for highly localized hourly (or sub-hourly) rainfall between 0.5 and about 1 inch, due to slow movement. These spotty higher rainfall rates could result in flash flooding of urban areas or other sensitive terrain. Otto ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!5hy1OjoPIczj9JA_LYobi4X_8NO48jWfR8lBVWDPe-RK1XsCwgWROEbmiMan9UUefKhF= UopZquPsB1jFEV3OzPJESk4$ for graphic product... ATTN...WFO...LOX...SGX... ATTN...RFC...RSA...NWC... LAT...LON 34581936 34531776 33821649 32841623 32541631=20 32531634 32251753 32241756 32731790 33131881=20 33501962 34001978=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) .