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Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
800 AM PST Thu Jan 1 2026

...HEAVIER SHOWERS OVER SRN CA THIS MORNING DIMINISHING BECOMING 
MORE SCATTERED IN THE AFTN, T-STORMS POSSIBLE ESPECIALLY OVER 
COASTAL SRN CA...
...WET PATTERN PERSISTS THE REST OF THE PERIOD AS ADDITIONAL SYSTEMS 
MOVE IN FROM THE GULF OF ALASKA...

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

The upper low that has dragged tropical moisture into 
central/southern CA has begun to move inland this morning north of 
Point Conception as radar shows precipitation over srn CA and 
northward across much of the rest of the region. TPW imagery has 1+" 
PW values along the entire coastline and inland up to Shasta. 
Observations overnight report totals in the mountains at 1-4" with 
the highest amounts over Big Sur and the Transverse Range. Totals 
elsewhere generally 0.25-1". Precip during the current 6 hour window 
has come in much heavier over the San Diego than any of the models 
anticipated. There wasn't much data to put in to rectify this, so 
manual adjustments were made attempting to account for the first 2 
hours of precip observations from 12z-14z, though this may already 
be about 20-30% too low. That area over the past 3 hours has already 
seen 0.50" to over an inch with the heaviest precip moving through 
about now (7:15-7:30 am). 

The rest of today, the compact low will open into a trough as it 
moves inland through central CA allowing heavier precip to pull out 
of srn CA this afternoon and leaving behind more broadly scattered 
showers. Thunderstorms are still a concern, particularly along the 
soCal coast in the afternoon/evening. Showers will diminish into 
Friday though continue over nw CA ahead of the next system. A broad 
surface/upper low is set to arrive later in the day Friday rotating 
additional bands of precipitation into nrn CA some time in the late 
afternoon/early evening. Moderate precip will spread inland 
overnight and expand southward into central/southern CA Saturday 
morning. The rest of Saturday, the system will lift northward just 
offshore towards the PacNW and BC. Enough instability will be present 
to create a general risk of thunderstorms over nw CA for Saturday.

Another system will spin off the larger circulation over the Gulf of 
Alaska into Sunday sending additional precip into the region the 
rest of the weekend. Marginal risk of severe thunderstorms return 
for Sunday over the Sac Valley and to the north coast with a general 
risk of thunderstorms north of LA and to the foothills. Troughing 
will remain offshore Monday as well before closing off into a low in 
the evening and keeping precipitation in the forecast. Models 
continue to disagree early next week. The GFS has a larger low now 
traveling south offshore of central/srn CA later Tuesday and moving 
inland around the srn CA border mid to late Wednesday as another 
trough drops in from the nw. The det ECMWF as a smaller low moving a 
bit more quickly and slightly further to the south compared to the 
GFS. Ensembles are divided as well, just under half of the overall 
membership showing lighter showers over much of the coast while 
sparing inland areas. The next two clusters are progressively wetter 
with cluster 3 (15% CMC, 47% GEFS, 14% ECMWF) the wettest carrying 
heavy precipitation into coastal srn CA and the southern Sierra. 
Bottom line, increasing uncertainty in the forecast to end the 
weekend and through mid next week as models continue to disagree on 
the overall pattern and ensemble spread remains high. A wet pattern 
is expected to persist through most of the current forecast period.

As mentioned above, the morning forecast was focused on the current 
precip over the San Diego area though all periods and locations have 
been updated with the latest WPC guidance/NBM. Blended in the HRRR 
and made some manual adjustments over San Diego for the 12z-18z 
window to try and capture the current situation. The last run of the 
NBM only had 0.25" to less than 0.50" over that area and the 
GFS/ECMWF around 0.50" which quickly proved to be inadequate. Not 
counting the current period (ending at 18z), QPF totals through 12z 
Weds: 1.50-4.50" coastal srn CA (up to 6" mountains), 2-5.50" rest 
of the CA coast (8-13" King Range, 6-8" central coast mountains), 5-
13.5" northern Sierra, 2.50-7" central/southern Sierra, and 0.75-3" 
down the valleys.

Freezing levels today generally 8-10 kft dropping overnight over the 
Sierra but then rising again Friday morning up to 8-12 kft. Levels 
will re-lower into Saturday down to 6-7.5 kft north of I-80 and 7.5-
10 kft to the south mid morning. Lower levels spread Sunday with the 
Sierra at 6-8.5 kft mid Sunday morning and 5.5-7.5 kft early Monday. 
Some rise early next week, but generally remaining below 8.5 kft.