Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 31 2022 16:46:07 AWUS01 KWNH 311646 FFGMPD CAZ000-010430- Mesoscale Precipitation Discussion 1182 NWS Weather Prediction Center College Park MD 1145 AM EST Sat Dec 31 2022 Areas affected...central to southern CA Concerning...Heavy rainfall...Flash flooding possible Valid 311636Z - 010430Z Summary...Heavy rain from an impressive atmospheric river event will impact portions of central CA through late morning before translating southward for the afternoon/evening. Hourly totals in excess of 1 inch and localized flash flooding will be possible. Discussion...GOES West water vapor imagery from 16Z showed a well-defined vorticity max near 35N 133W tracking ESE, with a strong area of lift inferred downstream across the north-central coast of CA. At the surface, a low was estimated to be near 36N 126W along a frontal boundary that extended through the San Francisco Bay area. Heavy rain has been noted in and around the San Francisco Bay region over the past 2-3 hours with hourly rainfall totals over 1 inch on a localized basis in the northern Santa Cruz Mountains, not captured by 00Z or 12Z components of the HREF. Surrounding rates over 0.5 in/hr have also been observed. Areas of heavy rain also extended across the Valley between Sacramento and Modesto into the Sierra Nevada where 24 hour rainfall reports of 5 to 10 inches have been observed. These rains have contributed to numerous reports of flooded roadways and debris flows through this morning. Precipitable water values were highest along the coast from Monterey Bay to the North Bay Coastal Mountains at 1.1 to 1.3 inches (GPS and 12Z OAK sounding data). 850-700 mb winds were 40-50 kt (KMUX VAD wind) which matched recent short term RAP forecasts. As the shortwave and embedded upper trough axis continue to approach the coast over the next 5-12 hours, the surface low is forecast to reach San Francisco near 21Z, followed by a continued eastward movement and subsequent southward moving cold front in its wake, according to RAP forecasts. In the short term, areas of heavy rain will continue to impact north-central CA with a surge in rainfall intensity expected for The Santa Cruz into the northern Santa Lucia Mountains through 21Z as the surface low nears along with a renewed rise in 850 mb wind speeds in excess of 50 kt. Hourly rainfall totals of 1 to 1.5 inches will be possible in the Santa Lucia Range and up to an 1 inch in an hour for the upslope regions of the Sierra Nevada due to areas of weak instability up to a few hundred J/kg. Given initially high snow levels near 8000 ft above MSL across the Sierra Nevada, snow melt may contribute to excess runoff from heavy rain, posing a threat for flooding. High rainfall rates over recent burn scars will also pose a threat for flash flooding, mud slides and debris flows. As the cold front begins to slide southward down the coast between 21-04Z, heavy rain will reach the Transverse Ranges of southern CA where peak hourly rainfall near 1 inch will be possible. While impacts from heavy rain are expected to maximize across the terrain, localized flooding in the San Joaquin Valley will also be possible, especially across any urban areas. Otto ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!-YKcClZqsOO3HnTvx_km1tNbbqupDEYDTvuhvrkaOgmDiZhOtbfsHFuac2cVhZvOH8ob= 4GnGeYqAg589IHjPCqqMwRU$ for graphic product... ATTN...WFO...HNX...LOX...MTR...SGX...STO... ATTN...RFC...CNRFC...NWC... LAT...LON 39242110 39172061 38362006 37241926 36571868=20 35591838 35101796 34761780 34671771 34571755=20 34421730 34271720 34091740 34091883 34392052=20 35652147 36862247 37512282 37852312 38272313=20 38512300 38882199=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) .