126
AGUS76 KRSA 091502
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
800 AM PDT Sun Mar 9 2025

...DRY CONDITIONS THROUGH MON EVENING...
...A LOW WILL PASS BY SRN CA BRINGING SHOWERS TUES BEFORE A LARGER
COLD SYSTEM GENERATES MORE WIDESPREAD PRECIP MID TO LATE WEEK...

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

Dry conditions to continue for today as an upper ridge moves through
the west coast. A long upper trough is just behind the ridge
stretched from the Gulf of Alaska all the way southward past the srn
CA border. The trough will approach the rest of today splitting in
two by this afternoon with a shortwave offshore of the PacNW and a
closed low west of CA. The low will travel southeastward throughout
the rest of today and into tomorrow. Model trends remain in the
direction of slowing down the low and pulling it offshore delaying
the onset of precip for coastal srn CA until Monday night. The GFS
is further west with the core of the low than the ECMWF and
therefore does not show any showers reaching the coast until Tuesday
morning. It also generally does not have any precip over Santa
Barbara/Ventura counties while the ECMWF has light showers as far
north as Point Conception Monday evening. On Tuesday, the system
will cease its southerly movement and head eastward passing through
far southern CA/Baja in the afternoon/evening. The GFS is further to
the south with the now trough than the ECMWF meaning disagreement on
how far north across soCal showers will spread for Tuesday. The
majority of ensemble members now favor confining precip from this
low south of LA by varying extents with only a quarter of the
overall members (10% CMC, 20% GFS, 32% ECMWF) showing broader
coverage across the coast. The official forecast shows a few
hundredths to a tenth of an inch north of LA and 0.10-0.50" from
Orange County to San Diego with 0.10-0.25" inland.

As the low heads inland late Tuesday, another larger system will
approach from the northwest as a low moving offshore of BC dragging
a broad upper trough and front towards the west coast. Models do
have this system picking up a bit of tropical moisture (~0.75-1" PW)
as it approaches, but no persistent tap into CA. Showers forecast to
reach the north coast sometime Tuesday afternoon/evening. Timing
differences remain amongst the det models as well as ensembles with
some members delaying the onset of showers until as late as
Wednesday morning. The ECMWF is the early bird today and more
quickly carries the front/precip southeastward across the Bay Area.
Once the front makes landfall, expect more moderate precip along the
nrn CA coast spreading inland throughout the day. The trough will
then begin to move onshore behind the front continuing widespread
precip the rest of Wednesday and Thursday as the system heads inland
across CA/NV. Ensembles are in relatively decent agreement on higher
amounts across the Sierra/Shasta as well as along the entire CA
coast. The differences lay in the timing and exact amounts, but most
are generally above 1.50+".

Models then have an additional weaker front approaching for Friday,
but disagree on the details/timing. The det GFS keeps the system
further offshore with only light scattered showers over the region
while the ECMWF has a better organized front traveling inland across
CA Friday. The ensemble QPF for Friday is divided along the lines of
their parent models with 93% of the GFS and 70% of the CMC showing
roughly 0.10-0.75" across northern/central CA before Friday
afternoon and just over half of the ECMWF members predicting 1.50+"
for Shasta/nrn Sierra and across the nrn/cntrl coastal mountains.
The NBM/WPC lean towards the ECMWF but with slightly lower amounts.

Forecast was a blend of the morning WPC guidance and the latest NBM.
QPF for the second system (Tues pm - Sat am): 2-5.25" Sierra, 2.50-
5" Shasta, 2-4.50" nrn CA coast Sonoma County northward, 2-3.50"
central coast & Transverse mountains, 1-2.50" greater Bay Area, 0.75-
1.50" coastal soCal/NV mountains, 0.50-2" down the valleys, and 0.10-
0.50" for se CA. This will be a colder system as well with
relatively low freezing levels. Afternoon temperatures expected to
be 5 to 15 deg F below normal Wednesday and up to 20 deg F below
normal for some areas Thursday/Friday.

Tuesday morning, freezing levels 6-8.5 kft across the region
dropping from nw to se to 4-5.5 kft north of I-80 and 5-7.5 kft from
I-80 through the southern Sierra early Wednesday. Lower freezing
levels will spread across the region and further decrease overnight
reaching 2-4 kft north of I-80 and 3-5.5 kft from I-80 through the
southern Sierra early Thursday. Region wide expecting 2.5-5 kft by
the afternoon and 1.5-4.5 kft by early Friday. Levels will rebound
later Friday into Saturday up to 3.5-6 kft in the late afternoon and
increasing over srn CA to 6-8.5 kft early Saturday.


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



AS

$$