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AGUS76 KRSA 311504
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
805 AM PDT Fri Oct 31 2025

...DRY CONDITIONS AND ABOVE NORMAL TEMPS THROUGH MONDAY FOR MOST
AREAS ASIDE FROM OCCASIONAL SHOWERS ALONG THE N COAST AS SYSTEMS
MOVE THROUGH THE PACNW...
...A LARGER LOW WILL APPROACH INTO MID-NEXT WEEK RESULTING IN MORE
WIDESPREAD PRECIP...

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

Dry conditions across most of the region along with above normal
temperatures into early next week as high pressure remains overhead.
Temperature anomalies expected at +5 to +10 deg F with some
locations reaching +15 deg F. A weak surface low will move into the
PacNW into tomorrow along with a cold front. The front will move
south into nrn CA later Saturday sending some showers into the north
coast and srn OR Cascades. Expecting about 0.10-0.30" or so over the
Smith basin quickly tapering down to a few hundredths over Cape
Mendocino for Saturday. By Sunday morning, the system will have
dissipated with dry weather then continuing through Monday morning.
At that point, another system will approach the PacNW sending
additional showers into the north coast later Monday through Tuesday
afternoon. More precip is expected along the north coast this time
with showers making it across I-5 and southward into the North Bay.
There is more uncertainty with this second round of precip as models
and ensembles disagree. QPF clusters ending 00z Weds differ with
just under half of the overall CMC/GFS/ECMWF membership favoring
1.50"+ along the north coast while the rest generally keep things
under 0.50". The official forecast is somewhere in between at about
0.50-1" across the north coast (up to 1.75" King Range) and 0.10-
0.50" or so inland into Shasta.

The main event will come into Wednesday as a large surface/upper low
traverses the Gulf of Alaska picking up tropical moisture before
heading for the west coast. Models start pushing higher precip
amounts into the north coast some time either Tuesday night or
Wednesday morning. There is quite a bit of uncertainty in the
forecast as models disagree and ensemble spread is high. The det
GFS/ECMWF are showing structural differences in the surface/upper
low that lead to different orientations of the precip band that will
arrive some time Tuesday night or Wednesday morning. The result of
this is a steeper north/south orientation out of the ECMWF that
keeps heavy rain offshore until about mid morning. The slope of the
precip band out of the GFS is less steep bringing heavier precip in
overnight and spreading it inland throughout the morning. The
flatter slope is also resulting in better interaction with the
coastal ranges and Shasta out of the GFS producing much higher
precip amounts over the mountains. From 00z-12z Weds the ECMWF has
generally less than a tenth of an inch from the north coast to
Shasta while the GFS has 0.50-1" along the north coast and up to
2.50" over the King Range. There is a huge spread of outcomes
looking at the individual ensemble members  with some holding off
any precip entirely until later Wednesday and others showing nearly
1.50" as far inland as the NV border before 12z. 24 hour QPF spread
at Arcata ending then ranges from 0-3" in the GFS and 0" to just
under 3.50" for both the legacy ECMWF and the AIFS. The official
forecast is about in the middle of this range closer to the det GFS
and in between the latest NBM and morning WPC guidance.

The rest of Wednesday, the front will travel across northern CA as
troughing from the upper low moves onshore. This will spread precip
inland across nrn CA and southward into the Bay Area. Precip will
diminish later in the day as the trough swings through leaving
lighter showers overnight. WPC has the cold front across srn CA by
the end of the forecast period. The same uncertainty remains a
concern for the rest of Wednesday with still relatively high
ensemble spread, though not as severely as Tuesday night. Looking at
the 48 hr QPF clusters ending 00z Fri there are 3 groups split to
47%, 28%, and 25% of the GFS/ECMWF/CMC ensemble members. The
differences mainly lie in how far inland across nrn CA to spread
higher totals (2.50-3.50"+) and how far south across the state to
carry showers. The first two clusters agree on the latter point
taking precip through Monterey County while the third cluster (0%
CMC, 50% GFS, 20% ECMWF) has showers as far south as Ventura county.

The official forecast is again in between carrying a few hundredths
into Santa Barbara but confining higher totals to the north
coast/Shasta. Mainly went with a combination of WPC and the NBM but
adjusting in the early period to confine the precip a bit more
westward. Want to emphasize again this is a low confidence forecast
and amounts will change over the coming days. For now QPF for this
system (00z Weds-12z Thurs): 2.50-4" north coast, 2-3" Shasta, 1-
2.25" northern Sierra, 1-2" rest of northern coastal CA, 0.75-1.50"
central Sierra/Sac Valley, 0.50-1.25" greater Bay Area, 0.10-0.75"
southern Sierra/SJ Valley, and a few hundredths to 0.25" or so from
Point Conception to SLO county.

Freezing levels Tuesday afternoon around 10-13.5 kft from the nw
border to the southern Sierra lowering overnight down to 7.5-10 kft
north of I-80 mid Weds am. By the evening expecting freezing levels
6.5-9kft north of I-80 and 8-12 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.

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



AS

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