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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
715 AM PST Wed Dec 31 2025

...INCREASING SHOWERS TODAY BECOMING HEAVY OVER SRN CA OVERNIGHT TO 
MID THURS AM AS A LOW APPROACHES FROM THE SW, T-STORMS POSSIBLE...
...WET PATTERN PERSISTS INTO EARLY NEXT WEEK AS ADDITIONAL SYSTEMS 
DROP IN FROM THE GULF OF ALASKA... 

.METEOROLOGICAL CONDITIONS (WED AM - TUE AM)...

An upper low continues to approach CA from the southwest this 
morning pulling 1-1.50" PW of tropical moisture into the 
central/soCal coasts. Radar shows widespread reflectivity echos 
across the Bay Area/central coast and southern CA. Observations 
however are not reporting much accumulation on the ground thus far, 
maybe a couple hundredths of an inch here and there. A look at the 
12z morning soundings shows why. Though PW values at KOAK, KVBG, and 
KNKX are all above an inch, the actual soundings show saturation 
only down to about 700 mb so far along the coast with a dry layer 
beneath down to the surface. A good bit of that precip the radar is 
picking up is likely evaporating before it has a chance to reach the 
surface. Expecting that to change at some point this morning with 
showers forecast along the coast, mainly south of the Golden Gate 
and inland to the southern Sierra spreading northward this afternoon 
and becoming more moderate along the coast. These coastal areas may 
experience a few thunderstorms as well. 

The low will continue to approach during this time becoming more 
compact in size as it arrives off the coast of Point Conception late 
tonight/early Thursday morning. This is the time period of greatest 
concern as this will set up favorable wsw moist flow into the 
Transverse Range through about 18z. HREF exceedance probabilities 
show a 30-80% chance of 0.50+"/hr rain rates and a 10-30% chance of 
embedded 1+"/hr rates. Once the low opens into a trough and starts 
to move inland into the central coast after about 18z the winds 
should shift and cut off that heavier precip. However, thunderstorms 
remain a possibility particularly for coastal srn CA which SPC has 
under a marginal risk of severe thunderstorms. These may produce 
heavier rainfall rates into the afternoon.

The trough will gradually dissipate the rest of Thursday as the next 
surface/upper low approach from the Gulf of Alaska. Scattered 
showers will persist ahead of this next system into Friday with the 
main front and precip band arriving some time in the 
afternoon/evening. The latest det models have slowed down this 
arrival by about 6 hours or so compared to yesterday. More moderate 
precip is expected overnight across nrn/cntrl CA as the front and 
main precip band arrive. The surface/upper low will lift northward 
along the coast the rest of Saturday allowing precip to become more 
scattered. Shortly thereafter, another system will rotate down from 
the Gulf of Alaska sending additional troughing towards CA. At this 
point, models disagree on what this looks like with the GFS 
generating a broader surface/upper low that keeps precip in the 
picture through at least the end of the current period while the 
ECMWF has a more compact low further offshore resulting in much 
lighter accumulations for Monday. Ensembles are pretty divided for 
those last couple of days spread across four different solutions 
that focus heavier precip over different areas or lighter precip 
generally across nrn/central CA. Uncertainty in the forecast for 
Sunday onwards.

To summarize, a wet pattern looks to continue through the forecast 
period thanks to a series of systems. Highest totals look to be over 
the coastal mountains, Shasta, and the Sierra. QPF through 12z Tues: 
2.50-6" along the coast LA County northward (6-12" for the coastal 
mountains, 1.50-2.50" south of LA along the coast (up to 5.50" 
mountains), 6-16.50" northern Sierra, 4-9.50" central/southern 
Sierra, and 1-4" down the valleys.

Freezing levels start out today with the region 10.5-12.5 kft 
lowering tomorrow to 8-10.5 kft. For Friday, expecting 7.5-10 kft 
north of I-80 and 10-12.5 kft to the south. Freezing levels will 
lower over the weekend down to 6.5-8 kft north of I-80 Saturday then 
down another 500 ft for Sunday reaching a minimum mid Monday of 4.5-
6.5 kft and then 6-8 kft from I-80 to the southern Sierra.


Please Note: This product may NOT be routinely updated.
Please refer to the following product issued by the CNRFC
www.cnrfc.noaa.gov/Daily-Briefing for a graphical summary of
weather and hydrologic conditions.