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AGUS76 KRSA 211530
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
830 AM PDT Fri Mar 21 2025

...SHOWERS OVER THE NORTHERN PORTION OF THE REGION ESPECIALLY ALONG
THE NORTH COAST AND ALONG THE SRN OR CASCADES INTO SATURDAY AS A
MOISTURE PLUME AND SHORTWAVE MOVES THROUGH THE PACNW...
...DRY CONDITIONS AND ABOVE NORMAL TEMPERATURES SUNDAY THRU EARLY
NEXT WEEK AS HIGH PRESSURE BUILDS IN FROM THE ERN PACIFIC...
...CHANCE OF PRECIP RETURNS TO THE NORTH ON WEDNESDAY...

.METEOROLOGICAL CONDITIONS (SHORT TERM: FRI AM - MON AM)...

Short wave trough off the B.C./Pac NW coast this morning and moves
onshore this afternoon and inland into ID early Saturday. A weak
disturbance moves through Nrn/Central CA and NV on Saturday.
Moisture plume about 0.9 inches along the OR and NW CA coast this
morning and spreads south to around the Bay Area tonight into
Saturday morning. Scattered light showers moving through Srn OR and
Nrn CA and NW NV this morning. Precipitation will increase,
especially along the NW CA coast and Srn OR Cascades this afternoon
into tonight then showers taper off Saturday afternoon/evening. The
weak disturbance may bring a few light showers along the Nrn and
Central Sierra Saturday afternoon.  Precipitation amounts for today
into tonight are expected to be around 1-3 inches for the Smith
Basin/ NW CA coast and west slope of the Srn OR Cascades and 0.5-1.5
inches over the King Range and 0.25-0.75 inches over higher terrain
of Nrn CA and NE NV and generally less than half an inch to nothing
elsewhere over the north.  Precip amounts for Saturday into Saturday
night are forecast to currently be around 0.25-0.75 inches along the
NW CA coast and the west slope of the Srn OR Cascades and generally
a tenth of an inch or less to nothing elsewhere over the north and
the Sierra.  High temperatures this afternoon are below normal up to
15 degrees across the north and near normal to 10 degrees above
normal across the south then generally warm in the north on Saturday
to near normal to 10 degrees below normal and remain near normal to
10 degrees above normal in the south. Low temperatures tonight and
Saturday night are generally near normal to around 10 degrees above
normal.

High pressure builds into the region from the Eastern Pacific on
Sunday bringing warmer temperatures and dry conditions.  High
temperatures warm up to near normal to around 12 degrees above
normal for the region on Sunday. Low temperatures generally above
normal up to 20 degrees Sunday night.

Freezing levels around 4000-5000ft near the ORCA border and 6500-
8000 ft for the Nrn Sierra and 5000-7000 ft for Nrn NV this morning.
Snow level radar at Happy Camp shows snow levels almost 4000 ft
(3900 ft) this morning.  Freezing levels rise to around 5000-6000 ft
near the ORCA border and remain near 6500-8000 ft for the Nrn Sierra
and rise to 6000-8000 ft over Nrn NV by this evening. Freezing
levels rise to around 6000-7500 ft near the ORCA border and around
8000-9000 ft for the Nrn Sierra and 7000-8000 ft for Nrn NV by
Saturday evening.  Freezing levels continue to rise on Sunday (to
around 9000-13,000 ft over the region (N-S) by Sunday night) as high
pressure builds into the region from the Eastern Pacific.


.METEOROLOGICAL CONDITIONS (LONG TERM: MON AM - THU AM)...

Warm and dry conditions to start the extended as an upper ridge
builds into the west coast. Afternoon high temperatures are expected
to be 10 to 20 deg F above seasonal normals Monday and Tuesday. A
large low pressure system will approach for mid-week from the
northwest beginning to nudge the ridge east of the area. Much of the
region will remain well above normal for Wednesday as temperatures
begin to ease over nw CA thanks to the approaching low. Lots of
uncertainty for Wednesday as models disagree on the arrival timing
of this next system. The det ECMWF brings initial showers into the
northern CA coast early to mid Wednesday morning while the CMC
delays until closer to the early afternoon and the GFS holds out
until early in the evening. Spread on the timing amongst the
ensembles is similar between early Wednesday morning and the
evening. QPF clusters ending 00z Thursday split this timing into
three groups. Cluster 1 (65% CMC, 30% GFS, 56% ECMWF) is the slowest
showing about 0.10-0.50" along the north coast drying out quickly
inland. Cluster 2 (10% CMC, 53% GFS, 16% ECMWF) is very similar but
also carries some showers further inland to Shasta. Cluster 3 (25%
CMC, 13% GFS, 28% ECMWF) is the fastest with 0.75"+ spread across
the north coast into the Mad/Eel basins and Shasta with up to 0.75"
across the northern Sierra. This cluster also has showers as far
south as San Francisco while the others barely carry anything south
of Cape Mendocino.

WPC and the NBM are a bit quicker overall somewhere between clusters
2 and 3. The official forecast was a blend of these bringing a few
hundredths of an inch into the north coast early to mid Wednesday
morning spreading inland across nrn CA and south to Marin County in
the afternoon. Expect this to change over the coming days as
hopefully models converge towards a solution. As of now, there is
high uncertainty in the QPF. No precip for Monday and Tuesday due to
the ridge, all QPF in the extended is for Wednesday. Expecting 0.50-
1" along the north coast, 0.25-0.75" Shasta, 0.10" to nearly 0.50"
northern Sierra/srn OR Cascades, and a few hundredths to a tenth of
an inch or so across the North Bay/central Sierra.

Freezing levels 10-13.5 kft across the region most of Monday and
Tuesday. Levels will lower from nw to se on Wednesday, when and by
how much will depend on the timing of the approaching low pressure
system. For now, expecting 6.5-9.5 kft north of I-80 by late
Wednesday afternoon and 3.5-7 kft early Thursday.


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



Osborne/AS

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