Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Nov 17 2023 23:38:22 AWUS01 KWNH 172338 FFGMPD CAZ000-180536- Mesoscale Precipitation Discussion 1179 NWS Weather Prediction Center College Park MD 636 PM EST Fri Nov 17 2023 Areas affected...Southern CA Concerning...Heavy rainfall...Flash flooding possible Valid 172336Z - 180536Z SUMMARY...An increase in rainfall rates may lead to a localized flash flood risk across portions of southern CA over the next several hours. DISCUSSION...A well defined deep layered low pressure is easily seen on satellite imagery off the CA coast and is moving slowly off to the northeast. Satellite imagery shows a narrow southwest to northeast moving moisture plume off the coast that will push into southern CA over the next few hours. Given the orientation of the closed low and its northeastward movement, portions of southern CA will see a longer residence time within the moisture plume...unlike areas further north that should see a briefer window before dry air moves in from the west. Coastal Santa Barbara and Ventura Counties are also in a position to best take advantage of the deep layered southerly flow from an orographics perspective. While not necessarily evident at the surface, recent HRRR runs show some uptick in 925-850mb convergence as the moisture plume moves across...both from speed convergence as stronger offshore flow hits land, and directional convergence as winds offshore take on more of a westerly component. HRRR runs also indicate as much as 500-1000 J/KG of CAPE getting into coastal areas. This combination of instability and convergence may be enough to result in a few heaver convective cells. Recent HRRR runs do indicate some heavier convection could develop over these counties by ~02z, and suggests cells that develop could briefly train given the deep southerly flow and terrain interactions. The 21z HRRR has hourly rainfall locally exceeding 1", with 15 min rainfall locally exceeding 0.5". Meanwhile 18z HREF data shows ~50% chance of 0.5"/hr rainfall and ~20% of exceeding 1"/hr. While some brief cell training is possible, the overall moisture plume is progressive enough to limit rainfall durations. For that reason total rainfall will probably peak in the 1-2" range. Thus it is really the potential for high rainfall rates, not total magnitudes, that is driving the possibility of localized flash flooding. Not yet seeing an uptick in convective intensity offshore and model guidance remains inconsistent, so not a guarantee we end up realizing the higher rate potential. However there is enough of a risk to suggest localized flash flooding is at least a possibility, and radar trends will need to be closely monitored. Chenard ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!4Lx6CoXzS3YFnhUdkz-VG4XDnKdpuiL_OAlVKnA7ZUAnH9Evet5DXXPGxLLlsngItnk2= oC2Fbp4WMGAjaHATbl22AWk$ for graphic product... ATTN...WFO...LOX... ATTN...RFC...CNRFC...NWC... LAT...LON 35732120 35692094 35672074 35342056 35082016=20 34891984 34761944 34681912 34501900 34251903=20 34161931 34182008 34192038 34482074 34942101=20 35132110 35452132 35652134=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) .