Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Feb 24 2023 20:13:00 AWUS01 KWNH 242012 FFGMPD CAZ000-250300- Mesoscale Precipitation Discussion 0075 NWS Weather Prediction Center College Park MD 312 PM EST Fri Feb 24 2023 Areas affected...southern CA Concerning...Heavy rainfall...Flash flooding likely Valid 242008Z - 250300Z SUMMARY...Periods of very heavy rain will approach southern CA ahead of a cold front through the afternoon/evening. Peak rainfall rates will likely peak above 1 in/hr and possibly approach 1.5 in/hr through 03Z. Areas of flash flooding appear likely with an additional 3-5" (local/peak values) through 03Z. DISCUSSION...Areas of heavy rain have been noted over portions of southern CA with reports over the past 24 hours showing a broad 1-3 inches stretching across San Luis Obispo, Santa Barbara, Ventura and Los Angeles counties, with 3 to 5+ inch totals embedded within. Several observed rain rates have already peaked above 0.8 in/hr and the heaviest rates are yet to come. 1930Z GOES West visible satellite imagery and surface observations helped place an occluded/cold front extending from the northern San Joaquin Valley to just west of Point Conception. 850 mb winds ahead of the cold front have been steady in the 35-50 kt range, oriented from SSW into the Transverse Ranges. Orographic lift has been aided by lift within the right entrance region of a 110-130 kt jet aloft, located southeast of the closed upper level low advancing southward off of the CA coast, and increasing upper level diffluence in the 500-300 mb layer. 12Z CAM guidance and recent runs of the HRRR have supported the development of a frontal band of convection, aided by weak instability of up to a couple hundred J/kg CAPE, containing rainfall rates of 0.5 to 1 in/hr. However, the 3 km nature of the models may be underplaying the potential for localized rainfall rates of up to 1.5 in/hr where orographics and dynamic lift ideally combine, coupled with precipitable water values nearing 0.9 inches along the coast and local enhancement to low level flow. Snow levels are forecast to rise into the 5000-6000 ft range for the central and western Transverse Ranges through 00Z, prior to lowering from west to east with the cold frontal passage. Additional rainfall, beneath snow levels, is expected to peak in the 3-5 inch range, with rates peaking in magnitude just prior to the cold frontal approach. Given increasing moisture into area soils and rising creeks/streams, areas of flash flooding will become likely through the remainder of the afternoon into the early evening hours. Flooding will especially be true if higher rates can overlap with any recent burn scar locations which will carry an increased risk for runoff. Farther north, into the southern San Joaquin Valley, total rainfall so far has averaged 0.5 to 1.5 inches. While rain rates will likely remain below 0.5 in/hr for the Valley, a continued duration of moderate to heavy rain will likely lead to an additional inch or so of rain in the Valley through early evening, increasing the threat for localized areas of flooding. Otto ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!4tCgG4C3g-7NWtkt_1p-GAyefmmhMFGq4pibzsQvvVj1m93vrze318ac7VOGmBEJapL4= zitNfZaXxMi_p2h1EhMbCrE$ for graphic product... ATTN...WFO...HNX...LOX...MTR...SGX... ATTN...RFC...CNRFC...NWC... LAT...LON 36291953 36251911 36191889 35981871 35811867=20 35511867 35011869 34801853 34641831 34451816=20 34391804 34361799 34341795 34321792 34301785=20 34291779 34271773 34221766 34211761 34241745=20 34191722 34021724 33851729 33641745 33391779=20 33091877 33351988 34492088 35162076 35652056=20 36131993=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) .