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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
735 AM PST Sun Jan 4 2026

...STEADY BAND OF PRECIP MOVING THROUGH CNTRL/SRN CA TODAY ALONG 
WITH T-STORM CHANCES AND SCATTERED SHOWERS ELSEWHERE...
...OFFSHORE LOW EARLY TO MID WEEK TO BRING ADDITIONAL SHOWERS BEFORE 
DRY CONDITIONS RETURN INTO FRIDAY...

.METEOROLOGICAL CONDITIONS (SUN AM - SAT AM)...

An upper trough approaches from offshore as a band of precipitation 
is currently moving through central CA while scattered showers 
appear elsewhere. TPW imagery shows a narrow band of 1" PW moisture 
associated with the heavier precip around the Monterey Bay and 
northeastward through I-80 into the Sierra. Observations over the 
past 12 hours report totals along the coast of 0.50-2" and up to 4-
5.50" over Shasta. This more moderate band of precip will continue 
to move through central CA through this afternoon, along with the 
trough, before shifting into srn CA this evening while becoming more 
terrain focused. HREF exceedance probabilities show a 10-50% chance 
of 0.50+"/hr rain rates. This band should then dissipate overnight. 
Thunderstorms remain a possibility today for much of CA, but not to 
the extent seen yesterday.

An upper low is then expected to form offshore tomorrow traveling 
south off the CA coast before moving inland somewhere over Baja. 
Details remain a source of uncertainty as models continue to be 
inconsistent with the low. The 06z GFS has slowed back down compared 
to yesterday's 12z run and now takes the low inland Wednesday 
night/Thursday morning further to the south than previously. The 
ECMWF has been a bit more consistent with the timing than the GFS, 
but how close the low tracks to the coastline will be significant. 
Right now it appears that most of the precip will fall along the 
coast between Cape Mendocino and Point Conception as well as inland 
over Shasta and the nrn Sierra for Monday into Tuesday. There are 
differences in the ensembles for Monday on exactly how widespread 
higher values are. This depends on how close the low tracks to the 
coast and how quickly it moves through. Models often struggle with 
these offshore lows. 

QPF clusters for Monday afternoon to Tuesday afternoon depict the 
possibilities. There are 3 clusters each focusing precip on a 
different part of the coast. Cluster 1 (5% CMC, 93% GEFS, 50% ECMWF) 
has 0.25-0.50" or so from Point Arena to Big Sur and into the 
northern Sierra. Cluster 2 (55% CMC, 7% GEFS, 22% ECMWF) has higher 
totals from the Monterey Bay to the western Transverse (1-3") and 
cluster 3 (40% CMC, 0% GEFS, 28% ECMWF) has similar totals shifted 
further south along the coast from Point Conception to the San 
Bernadino Mountains. To summarize, an offshore low will bring 
additional precip to CA early to mid week but confidence is low due 
to uncertainty in timing and path of the system. 

Wednesday, a trough is progged to dig into nrn CA from the PacNW 
potentially bringing additional showers. The GFS now has the trough 
further northward over the PacNW while the ECMWF digs the trough 
into nrn CA. The result is that the GFS is mostly dry for Wednesday 
while the ECMWF has about 0.25-0.75" over nrn CA. Most of the 
ensembles have at least some precip over the area for Wednesday, but 
it doesn't look like it will be anything significant. Potential for 
showers to linger into Thursday morning as the trough swings inland 
before high pressure builds in over the eastern Pacific returning 
dry conditions to the region to end the period.

The morning forecast was again a combination of the latest NBM and 
morning WPC guidance. Blended in the 12z HRRR over some areas within 
the next 24 hours. Also made manual adjustments to the current 
period (ending 18z) to account for recent observations. The main 
band of precipitation currently traversing CA has moved through a 
little earlier than models had suggested yesterday. QPF for the 
period: 1-2.50" coast north of the Golden Gate, 0.50-1.50" coast 
south of SF (up to 2.50" or so over the mountains, 1.50-4" Sierra, 2-
4" Shasta, and 0.25-1.50" down the valleys.

Freezing levels will lower gradually throughout the week starting at 
4.5-6.5 kft north of I_80 and 6-8 kft to the south today and 
hovering around there through Wednesday. Things will lower further 
across nrn CA Thursday down to 3.5-6 kft north of I-80 and 5.5-7.5 
kft to the south. Freezing levels will come back up into Friday from 
west to east as the ridge builds in offshore.


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.