520 AGUS76 KRSA 011615 HMDRSA Hydrometeorological Discussion National Weather Service / California Nevada RFC / Sacramento CA 815 AM PST Fri Dec 1 2023 ...A PAIR OF S/WV OVER THE PACNW TO BRING LIGHT SHOWERS TO THE CA/OR BORDER, NORTH COAST, NORTHERN NV THRU SAT AM... ...PLUME OF MOISTURE OVER THE WEEKEND THRU EARLY NEXT WEEK TO BRING LOCALLY HEAVY PRECIP AT TIMES TO NORTHERN AREAS (HEAVIEST NW CA COAST & SRN OR CASCADES)... .METEOROLOGICAL CONDITIONS (SHORT TERM: FRI AM - MON AM)... The upper trough that arrived yesterday now resides over soCal and through NV this morning. Observations over the past 12 hours report about 0.10-0.40" along the north coast and over the Sierra, 0.25- 0.50" for some of the far soCal mountains, and a few hundredths to a tenth of an inch surrounding those locations. A front and upper shortwave are also situated offshore of the PacNW while TPW satellite imagery shows some moisture building up offshore as the main plume for the weekend approaches. The upper shortwave will head inland into WA/OR later this morning while the southern edge of the surface front moves into the north coast. This will bring some light showers to the north coast and along the CA/OR border as well as into northern NV through tonight. Saturday morning, a similar set up is expected as another shortwave moves into WA/OR resulting an additional precip for the northern CA coast, across the CA/OR border, and over the Cascades. Models have the main moisture plume associated with the PacNW atmospheric river reaching the coast between late afternoon and evening on Saturday. The 00z ECMWF is a bit quicker than the GFS and is now the wetter of the two models with QPF. There also remain differences in how quickly the moisture plume will spread southward into CA on Sunday and into Monday. Both models are wetter than this time yesterday, but jumping around is not unexpected given that CA will be seeing only the southern edge of the moisture plume. Therefore, any shift to the north or south in each model run will result in either an increase or decrease in precip amounts for northern CA. The ensembles continue to show a decent spread in QPF reflecting the uncertainty in this forecast. Moisture will continue to stream into the west coast the rest of the weekend and into Monday as well keeping precipitation going across southern OR, northern CA, and northern NV. Highest precipitation accumulations are forecast along the north coast (particularly over Smith Basin) and over the crest of the soOR Cascades. For the QPF, went with a blend of WPC and the latest NBM. QPF for today to 12z Monday is as follows: 0.75-4.50" north coast (highest Smith Basin), 1.50-5.50" soOR Cascades, 0.50-1.50" around Shasta, 0.10-0.75" norCal coast between Sonoma and Mendocino counties, 0.10- 0.50" northern Sierra, and a few hundredths to a tenth of an inch for the central Sierra/Bay Area/northern Sac Valley. The ne NV mountains are looking at 0.75-3" with a few hundredths to a half an inch for the rest of northern NV. Freezing levels this morning 4-7 kft north of I-80 and 7-9 kft to the south. Levels will gradually rise into the weekend ahead of the moisture and as high pressure builds in west of CA. Saturday afternoon expecting 5.5-7.5 kft north of i-80 and 7.5-10.5 kft to the south (highest over coastal soCal). Higher freezing levels to spread northeastward the rest of the weekend with most areas above 8 kft by early Monday morning and up to 13 kft for soCal. .METEOROLOGICAL CONDITIONS (LONG TERM: MON AM - THU AM)... Ridge of high pressure moves over the west coast on Monday then shifts east on Tuesday as a broad trough over the NE Pacific approaches the west coast. A 1.5 inch moisture plume aims at the Pac NW and Nrn Ca coast on Monday and narrows on Tuesday (The 06Z det GFS drops south with the core aiming at Nrn CA and the 00Z EC shifts the core north of the region) Tuesday. A shortwave moves into the Pac NW/Nrn CA on Wednesday as the plume weakens and shifts south and retreats off the Central CA coast. Models and ensemble members variable with the track and timing of the moisture plume leading to low confidence in precip amounts and coverage and timing early next week for Southern Oregon and Northern CA. The forecast mainly used a blend of NBM with some WPC and previous forecast. Current forecast Precip amounts for Monday through Wednesday is 3-5 inches for the Smith Basin and 1-2 inches over Srn Or Cascades and Eel Basin and 0.5-1 inch for the Shasta Basin and up to around 0.5 inches for the Nrn Sierra and Russian Basin. The 24 hour QPF 50th percentile clusters ending 00Z Wednesday Dec 6 vary with only a few hundredths for the Smith Basin for Cluster 3 15% of members with majority EC to 1.5-3 inches over the Smith Basin in Cluster 2 28% of members with mainly Canadian and GFS. Freezing levels rise to 1000 ft and higher on Monday with ridge over the region then drop to around 6000 ft near the ORCA border and around 8000 ft over Nrn Sierra Wednesday with trough. QPF graphics are available at www.cnrfc.noaa.gov/qpf.php AS/Osborne $$