NGUS86 KRSA 202103 HCMRSA Hydrometeorological Coordination Message National Weather Service / California Nevada RFC / Sacramento CA 105 PM PST Wed Nov 20 2024 ...HEAVY PRECIP EVENT UNDERWAY FOR NORTHERN CA LASTING INTO FRI... ...COOLER WITH CHANCES FOR PRECIP SATURDAY INTO EARLY NEXT WEEK... .METEOROLOGICAL CONDITIONS (SHORT TERM: WED PM - SAT AM)... Precip continues in earnest across northern CA as the deep moisture plume intersects the coast with PW values at or just over 1.00-inch transporting efficiently inland under broad southwest onshore flow. Automated gauges over the past 12 hours show the greatest amounts from the Eel River basin down through the Russian River basin and then inland over the Shasta Lake drainage with anywhere from 1.50- to 3.00-inches and even a few localized amounts at 4.00-inches. The pattern will be slow to evolve over the next 48 hours leaving northern CA the focus for moderate to heavy precip. Deep troffing will continue over the northeast Pacific with the upr jet nosing toward the Pacific Northwest...rotating individual disturbances toward the coast This will keep the region in the warm sector of the system...allowing freezing levels to increase to between 8000- and 10000-feet across northern CA...peaking later Thursday...before dropping with the frontal passage on Friday. The PW plume will meander along the north coast over the next couple days ushering the high moisture content air inland through Friday morning under southwest onshore flow for ideal orographics...before the cold front starts to shift across the region from northwest to southeast...finally shunting the moisture plume across central and southern CA. This will bring a gradual decrease in precip and shift moderate amounts finally down across central CA. Afternoon update incorporates the latest WPC QPF and 20/19Z NBM with some manual edits to match gauge trends in the short term. 3 Day totals still show 5.00- to 10.00-inches across northern CA higher terrain with localized amounts approaching 15.00-inches. For the Sacramento Valley...a range of 3.00- to 4.00-inches across the southern areas near Sacramento to 7.00- to 8.00-inches near Redding are still looking solid. .METEOROLOGICAL CONDITIONS (LONG TERM: SAT AM - TUE AM)... Precip amounts generally decreased a little in the afternoon forecast except increased amounts (around a quarter of an inch or less) along the Central CA coast and into the Srn Sierra and transverse range Sunday afternoon into Monday afternoon. Although there is a lot of uncertainty in amounts and locations of highest precip amounts due to variability in details of models and in the ensemble members. Main low pressure system off the B.C. coast with associated cold front moving through the region and a trough extending into the Eastern Pacific with weaker lows to the southwest of main low on Saturday. Several weak disturbances moves through the base of the trough and into CA over the weekend. The main low heads south off the Pac NW/ Nrn CA coast on Monday. Moisture plume aimed around Point Conception and south along Srn CA coast (PW about 1.1 inches along Srn CA coast) for Saturday then stretches north up along the Central CA coast and into SJ Valley on Sunday then back down to Srn CA coast Sunday night into Monday and possibly a finger reaching out into the Bay Area Monday night into Tuesday. Showers are possible over most of the region (except far SE CA) Saturday into Monday. The highest precip amounts are expected over the Sierra with generally 0.75-1.5 inches on Saturday and again on Sunday and 0.5-1.25 inches along the Central CA coast and into the transverse Mtns on Sunday then shifting north to the Northern CA coast and Shasta Basin and the Nrn Sierra with 0.75-1.5 inches on Monday. About a third of the ensemble members (32% in Cluster 2) show amounts up to around 3 inches over the Sierra and up to 1.5 inches along the Central CA coast on the 24 hour QPF 50th Percentile ending 12Z Sunday. The transverse range (currently forecast 0.5-1 inch) may get higher amounts Sunday with southerly flow. Almost a quarter of the ensemble members show up to 2 inches over the transverse range and up 3 inches along the Sierra on Sunday on the 24 hour QPF 50th Percentile ending 12z Monday. Although 35% of the members (Cluster 1) show most of the precip staying around I-80 and north with 1-2 inches along Nrn CA coast and into Shasta Basin. Quite a bit of uncertainty on Monday with precipitation amounts and locations (especially with srn extent of precip). The 24 hr QPF 50th Percentile clusters ending 12Z Tuesday show up to 3-6 inches over Nrn CA coast (Cape Mendo to Russian Basin and into Shasta Basin and Nrn Sierra on 3% of the total Ensemble members (composed only of 6% of EC members then about a quarter of members (24% total- Canadian 25% GFS 17% and EC 28%) have amounts up to 2 inches along Central CA coast and up to 3 inches in Srn Sierra. Freezing levels generally drop to around 3500-4500 ft near the ORCA border and around 4000-6000 ft for the Nrn Sierra and and 5000-7000 ft for the Central Sierra and 7000-9000 ft for Southern Sierra and 10,000 ft and higher for Srn Ca on Saturday morning and drop down to around 5500-7500 ft in the Srn Sierra and 7000-10,000 ft over Srn CA Saturday afternoon/evening. Freezing levels generally fluctuate around 4000-5000 ft near the ORCA border and 5000-6500 ft in Nrn Sierra and 5500-8000 ft over Central and Srn Sierra and 9000-11,000 ft over Srn CA Sunday afternoon into Monday. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php *********************** Contact Information ************************ CNRFC Office Hours Phone: 1 (916) 979-3056 Extension 338 After-Hours Calling Service: 1 (800) 218-0858 Leave Name...Office...and Phone Number Kozlowski/Osborne $$