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AGUS76 KRSA 302053

HMDRSA



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.





QPF graphics are available at www.cnrfc.noaa.gov/qpf.php







AS



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