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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
1255 PM PST Tue Dec 30 2025

...NORTHWARD MOVING CUTOFF TO BEGIN SPREADING PRECIPITATION ACROSS 
SOUTHERN CALIFORNIA TOMORROW...
...UPPER TROUGH TO SWING IN FROM GULF OF ALASKA ON THURSDAY INTO 
FRIDAY...

.METEOROLOGICAL CONDITIONS (TUE PM - MON AM)...

An upper low continues to approach southern CA sending tropical 
moisture into the coast while a ridge sits over nrn CA and the 
PacNW. Expect increasing high clouds as the low continues to 
approach the rest of today with some light scattered showers 
potentially across srn CA later today as the layer begins to 
saturate. Radar imagery is still showing echos streaming into srn 
CA, with observations reporting some areas getting just under 0.10" 
over the past 6 hours. Additional more widespread shower activity is 
expected along the coast, mainly south of the Monterey Bay, by 
tomorrow morning before spreading inland and northward in the 
afternoon. Models have the low becoming more compact as it arrives 
offshore of Point Conception early Thursday with continued showers 
across much of the region. The system will weaken throughout 
Thursday as it heads inland across srn CA.

The next system, an upper trough, will approach from the Gulf of 
Alaska later Thursday sending additional showers into the nrn CA 
coast ahead of the main surface low and cold front. There are still 
timing issues on the arrival of the front and more steady precip 
over nrn CA with the GFS being on the earlier side. There are also 
structural differences between the GFS/ECMWF regarding this system. 
Either way, precip is expected into Saturday as the low moves 
northward along the coast towards the PacNW while another upper 
level system develops off the back side of the broader troughing 
pattern over the eastern Pacific sending additional precipitation 
into the area through the end of the forecast period. Disagreements 
persist for this latter time frame making for uncertainty in the 
QPF. Overall ensemble spread remains high as well, 24 hr QPF out of 
Blue Canyon shows daily ranges of roughly 2 to 3".

Afternoon forecast was a blend of the latest NBM and recent WPC 
guidance. This has trended wetter than the morning forecast 
resulting in increases in QPF over the period by 0.25-0.75" for most 
areas and up to 1-1.50" over the northern Sierra, Shasta, and the 
Transverse Range. QPF through 12z Mon: 2.50-12" s to n across the 
Sierra, 5-8.50" Shasta, 4-7.50" Russian/Eel basins (7-10" King 
Range), 3.50-7" Big Sur/Transverse Range, 1.50-3" Sac Valley/most of 
the Bay Area, and 1-3" coastal srn CA.