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AGUS76 KRSA 211532
HMDRSA

Hydrometeorological Discussion
National Weather Service / California Nevada RFC / Sacramento CA
735 AM PST Fri Feb 21 2025

...WEEKEND INTO MONDAY SEES INCREASING PRECIP FOR NW CA AND SW OR...

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

A dry s/wv trof is dropping southward pretty much along the UT/NV
border this morning...while a positively tilted upr ridge is
situated from just off the southern CA coast north-northeast across
northern CA and the interior of the Pacific Northwest. This will
bring dry conditions for Friday under weak northerly flow aloft.

Offshore...a fetch of lower latitude moisture stretches from the
northeast Pacific back toward the west-central Pacific west of
Hawaii. Into Saturday...this surge of moisture will make its way
toward the Pacific Northwest and northern CA coast...while a series
of disturbances move through the west-southwest flow toward BC and
the Pacific Northwest. For northern CA and southwest OR...as the
higher moisture content air reaches the coast with PW values close
to 1.00-inch and condensation pressure deficits decrease...a ramping
up of WAA/isentropic lift will bring an increase to precip along
coastal areas mainly from the Eel River basin northward...and then
inland from the Shasta Lake drainage up to the crest of the southern
OR Cascades. At this time...best opportunity for precip on Saturday
looks to fall over the Smith River basin during the evening and
overnight periods...totaling from 2.00- to 3.50-inches by Sunday at
12Z.

Into Sunday...the moisture plume will narrow while the
WAA/isentropic lift decrease across northwest CA and southwest OR.
Precip will be on the decrease as the next s/wv trof approaches the
Pacific Northwest coast. Best precip totals continue to look to fall
over the Smith River basin with 1.00- to 2.00-inches and the bulk
falling within the first 6 to 12 hours between 23/12Z and 24/00Z.

Freezing levels during the periods of precip on Saturday and Sunday
near the CA/OR border will generally range from 7500- to 9000-feet.
For the 3 day period ending at 12Z on Monday...precip totals will be
highest over the Smith River basin with 4.00- to 5.50-inches.
Amounts will rapidly drop off south and inland with amounts of 1.00-
to 2.50-inches across the lower Klamath River basin and 0.25- to
1.00-inch for the Eel River basin...while the crest of the southern
OR Cascades will see 1.00- to 2.00-inches and 0.25- to 0.75-inch for
the Shasta Lake drainage. A few hundredths to 0.10-inch might make
its way down to the upper Russian River basin and portions of the
far northern Sierra.


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

Shortwave moves into the Pac NW/B.C. and brushes the north Monday
into early Tuesday bringing a chance of showers to Srn OR and
portions of Nrn CA. PW plume aimed at Nrn CA coast with around 0.8
inch PW along the coast on Monday morning then drops south and
erodes and retreats off of Central CA coast Monday evening. Showers
will be along the NW CA coast and Srn OR Monday morning and will
spread inland into the Shasta Basin and south down through the Eel
Basin Monday afternoon/evening as the trough moves onshore.
Lingering showers possible overnight into Tuesday morning. The
current forecast is a blend of latest NBM and WPC and some of
previous forecast. Precip amounts for 12Z Monday into 12Z Tuesday
are forecast to be around 0.5-1 inch for the Smith Basin and around
a half an inch for the Srn OR Cascades and King Range and around a
quarter of an inch or less for the Shasta Basin and Eel Basins and
less than a tent of an inch for the Feather and Russian Basins.
Precip amounts for early Tuesday are expected to be around a tenth
of an inch or less along the NW CA coast and Srn OR Cascades.
Although on the 24 hr QPF clusters ending 12Z Tuesday show a little
over a quarter of the ensemble members (28%- Canadian 30%, GFS 23%,
and EC 30%) show amounts up to 2 inches for the Smith Basin and King
Range and up to an 1 inch for the Shasta Basin and about another
quarter (26%- Canadian 45%, GFS 43% EC 8%) show precip mainly
confined to the Nrn CA coast (just N of pt Arena) and the Srn OR
Cascades.  There is lower confidence in amounts and the southern and
eastern extent of precip for Monday into early Tuesday due to
differences in models and ensemble members.

The trough moves east of the region and ridge of high pressure
builds into the region Tuesday afternoon into Wednesday for dry
conditions. Another shortwave trough behind the ridge splits with a
closed low developing over the Eastern Pacific offshore of the CA
coast on Wednesday night. The models vary on timing and strength and
track of this low but will bring a chance of showers to portions of
CA, mainly over central and Srn CA, later next week. The 06z GFS is
quicker bringing in low to coast of CA near north Bay Thursday
afternoon/evening and inland into Central CA on Friday. 00Z EC keeps
closed low spinning offshore then into Srn CA Saturday.

Freezing levels around 7000-8000 ft along Srn OR Cascades and NW CA
coast and 8000-13,000 ft (N-S) elsewhere Monday morning then drop to
around 4000-5000 ft near the ORCA border and around 6000-12,000 ft N-
S elsewhere Monday night. Freezing levels rise to around 8000-13,000
ft over CA behind the trough and lower to around 6000-11,000 ft over
NV Tuesday night.  Freezing levels around 10,000 ft and higher for
the region under high pressure by Wednesday night.


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



Kozlowski/Osborne

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