Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 13 2024 20:01:31 AWUS01 KWNH 132001 FFGMPD CAZ000-ORZ000-140600- Mesoscale Precipitation Discussion 1174 NWS Weather Prediction Center College Park MD 300 PM EST Fri Dec 13 2024 Areas affected...Northern California...Southwest Oregon Concerning...Heavy rainfall Valid 132000Z - 140600Z SUMMARY...An approaching storm system accompanying a strengthening atmospheric river will result in an areal increase of excessive rainfall rates this afternoon and into tonight. Areas of flash flooding and landslides are possible. DISCUSSION...GOES-West satellite imagery shows a steady fire-hose of Pacific moisture being directed at the U.S. West Coast today with periods of rain well underway across northern CA and southern OR. As the afternoon unfolds, the approaching warm front and storm system will force southerly 925-850mb winds to accelerate into the far northern CA coast. The warm front's approach will also force freezing levels to rise as high as 5,000ft in some cases. This provides both a deeper warm cloud layer and would allow for higher elevations (up to around 5,000ft) to be at-risk for excessive rainfall rates. By 00Z, the triple-point of the frontal system will be tracking through the CA/OR border and the warm front will reside along the CA Coastal Range. A surge in 850mb moisture flux along the northern CA coast will accompany an IVT >750 kg/m/s that surpasses the 99th climatological percentile according to NAEFS. This robust IVT is the catalyst for a >12hr period for excessive rainfall from as far north as southwest OR to as far south as some of the northern Bay Area suburbs overnight. 12Z HREF probabilistic guidance shows high chance probabilities (>70%) for >0.5"/hr rainfall rates around the Eureka area as early as 21Z. These probabilities then spread as far north as the OR/CA coastal border between 00-03Z. By 03Z, the cold front will be on approach and the northern CA coast will become placed within the storm's warm sector. Low-end MUCAPE values (generally <200 J/kg) could be available for potential convective enhancement in northern CA tonight, while the strong SW flow aloft supports strong upslope enhancement into the Trinity/Shasta Mountains between 03-06Z tonight. These two areas could see rainfall rates approach 1"/hr in some cases. Overall, through ~06Z Saturday, additional rainfall totals of 2-4" are expected within most of the highlighted region with localized totals surpassing 5" possible. The 12Z HREF depicts low-to-moderate chance probabilities (30-50%) for 12-hr QPF >5" along the coast near and south of Eureka, as well as in the southwestern facing slopes of the Trinity/Shasta Mountains. Previously saturated soils have recovered to some extent given the drier than normal stretch of weather over the past couple weeks, which should help limit the areal extent for potential flash flooding. That said, the atmospheric parameters mentioned above are more than enough to support the potential flash flooding and landslides in parts of northern CA this afternoon and into tonight. Mullinax ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!4qLvtWeaBcIOYHZkoGSpo2fqD1WRjZoZ7yQJcmzH6-Uedhy7O7s0XLSR8fnL2zcKeXvT= NoWtsLIC1SZ_CJNrFGFsldM$ for graphic product... ATTN...WFO...EKA...MFR...STO... ATTN...RFC...CNRFC...NWRFC...NWC... LAT...LON 42442432 42422394 42172361 41792363 41342343=20 40972324 40832293 41082230 40862213 40252285=20 39652302 39112306 38872348 39072392 39622409=20 40142453 40582460 41482437 42082453=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .