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

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