Winter Storm Warning 1, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
847 AM PST Thu Jan 30 2025

.UPDATE...Stratus continues for the lowlands of Western Washington
this morning with increasing high clouds ahead of the next frontal
system approaching the area. Dry conditions for the majority of
today with rain increasing along the Olympic Peninsula beginning
later this afternoon and sliding east this evening. No major
forecast updates this morning.

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.SYNOPSIS...A frontal system will bring lowland rain and heavier
mountain snow tonight through Friday night. An upper low sliding
southwards across British Columbia will usher in a colder pattern
along with the potential for lowland snow Saturday into early
next week.

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.SHORT TERM /TODAY THROUGH SATURDAY/...Today begins what will be a
significant shift in the weather pattern from recent weeks. Deep
upper troughing digging southward from the Alaska panhandle will
merge with some energy offshore to carve out a broad, cold upper
trough over the region by the weekend as strong upper ridging
builds upstream over the Aleutians. This is shaping up to be an
impactful situation in terms of winter weather. So, let's break
down the general sequence of events that are expected as well as
some of the probabilities and uncertainties involved.

What's left of an upper level ridge today will give way to
an approaching frontal system. Rain associated with this front
will reach coastal areas late this afternoon or early evening and
spread inland overnight. Initially, snow levels will be in the
3000 to 4000 foot range as milder air associated with the system
infiltrates the region. The front is expected to slow and stall
somewhat offshore as some fairly healthy QPF (the most we've seen
in weeks) spreads over the area. This will translate to some
significant snowfall in the passes. The heavier snowfall will be
initially over the higher passes, but snowfall throughout the
event will warrant converting the existing Winter Storm Watch for
the Cascades above 3000 feet to a warning. Storm total snowfall
between Thursday night and Saturday will range from around a foot
at Snoqualmie Pass to 2 feet or more at Stevens Pass and Mount
Baker.

For the lowlands, we're talking rain Thursday night through much
of Friday evening. But then significant changes occur. Colder air
follows a front Friday night with 1000-850 millibar thicknesses
falling below the crucial 1300 meter threshold and 850 millibar
temperatures dropping to -7 or -8 C. Models are advertising a
fairly potent convergence zone Friday night into Saturday morning.
The numbers outlined above combined with the convergence zone's
ability to draw snow levels locally downward points toward
increasing probabilities for accumulating lowland
snowfall...especially Snohomish County northward. NBM
probabilities show a 50-70% chance of a fraction of an inch from
north King county northward by mid-morning Saturday. Those
percentages fall to 20-30% for an inch of accumulation. But keep
in mind that convergence banding can play a significant role in
localized higher amounts and this feeds uncertainty. A secondary
upper disturbance is expected to swing onshore Saturday night.
Portions of Puget Sound may see some shadowing by the Olympics,
but the air mass by this time is expected to be sufficiently cold
for accumulating snowfall down to sea level for most of the area.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...A vertically stacked
trough is expected to remain just offshore by the time we get to
Sunday. At the same time, a modified arctic boundary will be
sinking southward over southern British Columbia. Models have the
Williams Lake to Bellingham gradient approaching -20 millibars by
mid-morning Sunday which will initiate Fraser River outflow. The
modified arctic front is then expected to slowly sink southward
over Western Washington Sunday into Sunday night. While
uncertainty with regard to snowfall amounts is still high, the
probabilities of widespread snowfall accumulation are highest
during this period. Additionally, the Fraser outflow may act to
enhance upslope snowfall along the Strait of Juan de Fuca as
we've seen a number of times before.

Broad upper troughing remains just offshore Monday through
Wednesday with the the potential for additional precipitation to
move onshore for additional snowfall though uncertainty is quite
high in terms of amounts. 27

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.AVIATION...Flow pattern aloft is zonal with higher winds
towards the north up into Canada. Overcast ceilings remain over the
region this morning, with ceilings ranging from VFR/MVFR east of
Puget Sound, to IFR/LIFR in the Kitsap Peninsula and South Interior
areas (fog/mist has mixed out with increase of cloud coverage).
Cloud coverage will solidify late this morning/afternoon to VFR/MVFR
cloud decks (with a couple pockets of IFR/LIFR remaining n the
Olympics and portions of the North Cascades). Friday morning will
see lower end MVFR to IFR ceilings across the region. Precipitation
will expand inland Thursday evening as a cold front approaches the
region. Southerly to southwesterly winds will continue through the
period generally 3 to 6 kts.

KSEA...MVFR ceilings with calm/light SW winds early this morning.
There will be the potential for a brief improvement to VFR this
evening, before returning to MVFR with ceilings just above 1,000 ft
Friday morning. Rain will expand over the terminal late Thursday
evening through Friday. Southwesterly flow with winds 4 to 6 knots.

15/HPR

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.MARINE...Southerly winds will increase today as a frontal system
approaches, bringing in Small Craft Advisory winds across the
coastal waters and through the East Entrance of the Strait of Juan
de Fuca and Northern Salish Sea. Another front later on Friday
will maintain SCA winds for most areas through Saturday, with
gales possible through the strait. N/NE Fraser River outflow winds
are possible Sunday and Monday.

Seas will gradually build to 6 to 8 feet throughout the day today,
then continue to build on Friday to 10 to 13 ft. Seas will remain
elevated through Sunday before gradually easing to 4 to 7 feet by
the beginning of next week.

33/15

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.HYDROLOGY...No river flooding is expected over the next 7 days
as abnormally dry conditions continue. However, with the
precipitation coming towards the end of the week, the Skokomish
River is forecast to rise above Action Stage. There will be much
uncertainty with the river forecast all the way into the event
given the current below normal flows and the snow and ice build up
in the basin over this recent dry but cold period. As far as
other rivers, only low elevation basins like the Chehalis, Samish,
and Stillaguamish Rivers will see much response at all.

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from 7 PM PST this evening through Saturday
     afternoon for West Slopes North Cascades and Passes-West
     Slopes North Central Cascades and Passes-West Slopes South
     Central Cascades and Passes.

PZ...Small Craft Advisory from 10 PM this evening to 5 PM PST Friday
     for Admiralty Inlet-East Entrance U.S. Waters Strait Of
     Juan De Fuca-Northern Inland Waters Including The San Juan
     Islands.

     Small Craft Advisory from 1 PM this afternoon to 10 AM PST
     Friday for Coastal Waters From Cape Flattery To James
     Island 10 To 60 Nm-Coastal Waters From Cape Flattery To
     James Island Out 10 Nm-Coastal Waters From James Island To
     Point Grenville 10 To 60 Nm-Coastal Waters From James
     Island To Point Grenville Out 10 Nm-Coastal Waters From
     Point Grenville To Cape Shoalwater 10 To 60 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm.

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