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AGUS76 KRSA 121559
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
800 AM PST Wed Nov 12 2025

...WIDESPREAD MODERATE TO HEAVY PRECIP LATER TODAY THRU SAT DUE TO A
LARGE LOW CARRYING TROPICAL MOISTURE INTO THE REGION...
...T-STORMS FOR PARTS OF NRN/CNTRL CA TOMORROW AND SRN CA FRI...

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

A broad area of tropical moisture is approaching CA funneled in
between a ridge inland and an elongated trough offshore. TPW
satellite has the core of this moisture at nearly 1.50". 12z
soundings showed about 0.7" or so of PW along the coast, so the main
moisture has not yet arrived. Light precip is expected to begin
along the coast of nrn/cntrl CA later this morning out ahead of a
developing surface low. The southern end of the elongated trough
offshore is forecast to break off into a closed low by tonight with
a surface low beneath it. This low and the tropical moisture
together are expected to bring widespread precip to the region into
the weekend.

Moisture will begin to move inland along the coast from the OR
border into the central coast tonight as the low forms. The main
front will sweep across CA to the southeast throughout Thursday
along with the main moisture plume. Expecting the focus of precip to
shift into srn CA into Friday. HREF exceedance probabilities show a
15-50% chance of rain rates exceeding 0.50"/hr with the core of the
precip and a 10% or less chance of 1"/hr embedded for some
locations. Additionally, as the front moves through expect potential
for thunderstorms first for nrn/cntrl CA tomorrow and then srn CA
Friday.

There is some disagreement between the models on the initial
position of the low as more moderate precip begins. These
differences propagate throughout the rest of the lifetime of this
system making for a difficult forecast. The det GFS and ECMWF
disagree on how soon more moderate/heavy precip will spread to the
southeast along the low`s cold front tomorrow as well as the
position/timing/movement of the low as this occurs. This matters in
how efficient the orographics will be into the coastal mountains,
Shasta, and the Sierra. Favorable wind directions could make a huge
difference which all depends on the position of the low. Another
point of disagreement is in how long heavier precip will fall along
the nrn CA coast. For example, 12z-18z Thurs the GFS has 1-1.50"
along the Russian/Eel basins into Shasta while the ECMWF has 0.50-
1". Later Thursday, the ECMWF breaks down the upper low splitting it
with a shortwave over the north coast Friday morning and a closed
low west of Point Conception while the GFS still has a tighter and
stronger vertically stacked system west of Mendocino. The ECMWF is
also about 6 hours ahead on the location of more moderate/heavy
precip along the srn CA coast.

The two catch up a little later Friday as the closed low sits west
of Point Conception, but still differences remain as the GFS hangs
on to the corresponding surface low and the ECMWF shows a much more
disorganized and less wet feature. The difference in terms of QPF
06z-12z Saturday is 0.10-0.50" along coastal srn CA if the ECMWF has
its way and 1-1.50+" over Santa Barbara and Ventura counties
according to the GFS. One thing both models agree on is that
tropical moisture getting pulled back a bit northward along the
soCal coast early Saturday and allowing for a resurgence of heavier
precip into the evening, but again to varying degrees and in
different locations. The differences in the det models carry over
into the ensembles as well. The GEFS (77% of members) generally
favors wetter conditions over the northern Sierra/Shasta and along
the coast over the Eel/Russian from 00z Thurs to 00z Sat while being
slower to take heavier precip into srn CA. The CMC (80% of members)
spreads higher totals (~3-4") further into the central Sierra and is
quicker to take heavier precip into the Transverse. The majority of
the ECMWF membership (88%) looks more similar to the CMC in terms of
coverage but is less enthusiastic down the valleys and with the
overall totals.

The offshore upper low looks to finally begin to head inland
somewhere around central or southern CA overnight Saturday into
Sunday. Troughing and lingering showers for Sunday as the main
system weakens throughout its eastward journey. A wet pattern may
continue into early next week as well, though there is disagreement
on the details. Ensemble QPF clusters range from mostly dry to 0.25-
1"+ across central and southern CA with some showers across all of
NV for Monday.

To summarize, tropical moisture and a large low are expected to
impact CA/NV later today through Saturday resulting in widespread
moderate to locally heavy precip with lingering showers on Sunday.
Model differences and ensemble spread make for high uncertainty in
the timing and amounts of precip. The forecast was generally a blend
of WPC guidance and the latest NBM. QPF 00z Thurs-00z Mon: 2.50-
4.50" across the Sierra/central coast mountains (locally higher Big
Sur), 2.50-5.50" Shasta, 4-6.50+" Transverse, 2-4" coastal srn CA, 1-
2.50" Bay Area/central coast/valleys, and 0.75-1.50" north coast.
For early next week (00z Mon-12z Tues), right now expecting another
0.50-1" across the Sierra, 0.25-0.75+" for coastal srn CA, and 0.10-
0.50"+ across most of the rest of the state.

The main system looks to be relatively warm as well with freezing
levels only bottoming out across the Sierra down to 8-10.5 kft n to
s on Friday and otherwise generally staying above 10 kft before the
upper low moves inland over the weekend.

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.

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



AS

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