314
FXUS66 KSEW 310519
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
919 PM PST Thu Jan 30 2025

.SYNOPSIS...A frontal system will bring lowland rain, locally
breezy winds, and heavy mountain snowfall through Friday. A cooler
pattern will then develop over the weekend as an upper low slides
southward along British Columbia, with the potential for lowland
snowfall. The upper low will then linger offshore of Washington
and Oregon into midweek.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...It has been a few weeks,
but we`re seeing stratiform rain moving inland across western
Washington this evening, which has marked the beginning of a
pattern change as the upper ridge that has been in place flattens
over the region. The overall forecast tonight remains on track,
with no significant updates needed. The remainder of the previous
discussion follows below, with updates made to the aviation
section. 14

Precipitation will spread across Western Washington tonight with
steadier rain at times for the lowlands through Friday. Snow
levels during this period will initially range 2500 to 3000 feet
across the Cascades tonight before increasing to near 4000 feet
Friday afternoon ahead of a cold front. Snow levels will then fall
rapidly post-frontal with snow levels around 1000 feet by late
Friday night. Heavy mountain snowfall is forecast through
Saturday, including for the Cascade Passes. Current forecast
snowfall amounts for Stevens and White Pass range 18 to 26 inches,
with slightly lower amounts for Snoqualmie Pass (14 to 18 inches)
due to snow mixing with rain Friday afternoon. A Winter Storm
Warning continues for the Cascades during this period. If you`re
traveling over the Passes through the weekend, make sure to check
WSDOT for the road conditions and further information. In addition
to the lowland rain and mountain snow, breezy southerly winds are
anticipated, especially for Admiralty Inlet northward, and along
the coast, Thursday night into Friday morning. A strong westerly
push Friday evening will allow for elevated winds across Whidbey
Island, with HREF probabilities of gusts exceeding 45 MPH ranging
20 to 40%.

Precipitation will then transition to post-frontal showers later
Friday night and continue at times through Monday as an upper low
digs southward along the BC coast and towards the WA/OR coastline.
For reference, I have included the period of Saturday through
Monday in the section below to further discuss the snow & cold
potential for Western Washington:

Precipitation Type: The aforementioned upper low will dig
southwards towards Western Washington on Saturday, allowing for
temperatures aloft to drop rapidly, with 850mb temps by Saturday
AM generally -6 to -9C. Although sfc temps Saturday morning will
likely be a few degrees above freezing (with marginal dews), an
increase in precip rates within any heavier shower may result in a
rain/snow mix. We`re at the end range for HREF (through 400 AM
Saturday) which begins to highlight this. In addition, the UW-WRF
ensembles also increases the confidence in potential mixing
Saturday morning. Hi-res guidance focuses on the NW Olympic
Peninsula, as well as a potential convergence zone in the vicinity
of Skagit/Snohomish Counties for any mixing during this period,
although additional areas, especially south of Olympia cannot be
ruled out. Diurnally driven temperatures by Saturday afternoon
will likely limit snowfall accumulation for most areas, although
continued areas of mixing are possible. Of note, there is also a
slight chance for thunderstorms (probabilities ranging 15-20%) on
Saturday along the coast. Further cooling then occurs Saturday
night into Sunday morning, allowing for a better chance of snow
showers for most areas. This trend continues Sunday and
furthermore on Monday with a further cooling trend and lingering
rounds of moisture with the upper low offshore.

Snow Probabilities and Confidence: Taking a look at snowfall
probabilities from Friday night through Saturday, NBM highlights
the previous areas mentioned for the best potential of one inch
(probabilities range 40 to 70%). This includes the NW Olympic
Peninsula, as well as Snohomish County northwards. Of note, the
NBM may struggle with resolution for any convergence zone
developing near Snohomish/Skagit Counties, and we`ll have a
better idea of rates and any potential accumulation as additional
hi-res guidance becomes available.

Confidence then increases with a better chance for precipitation
type remaining snow Saturday night through Monday for most areas.
Although a rain/snow mix (along with lower rates and shadowing)
likely will reduce snow accumulation at times for Puget Sound.
Ensembles continue to highlight the following regions for elevated
snow potential during this period: NW Olympic Peninsula, Whatcom
County, as well as towards the Cascade Foothills. The latest 12z
EPS shows this well on the ECMWF EFI, where EPS members remain
anomalous to climo for snowfall Saturday through Monday. In
addition, there has been a slow uptrend in amounts over the last
few EPS cycles for these areas. NBM probabilities of 4" during the
48 hour period ending 12z Mon are approximately 75-95% chance for
the Pacific coast (highest along the NW Olympics), 50-90% for
Whatcom County and NE San Juan Islands, as well as near 70% for
the Cascade Foothills. Given all of this, there is enough
confidence to issue a Winter Storm Watch for these lowland areas,
and beginning 24 hours sooner for the NW Olympics (Friday
evening). With that said, uncertainty still remains despite these
higher probabilities with convective showers and localized banding
driving the snowfall amounts, and allowing for a likely wide
range of accumulation for these areas, rather than being uniform.

Along with the areas already under a Winter Storm Watch, snowfall
is expected elsewhere in the lowlands, although forecast amounts
remain lower through Monday morning. Current snow probabilities
of 2" are approximately 40 to 75% for much of the Puget Sound
(specifically I-5 corridor south of Snohomish CO) from Saturday
night through Monday morning. We`ll continue to get a better idea
of the location of any heavier showers over the next few days
which will likely adjust these probabilities, and additional
headlines may be necessary. Finally, also want to point out the
north Olympics from Port Angeles to Sequim for upslope snow as
Fraser Outflow winds begin in the Sunday night and Monday
timeframe. At the moment, there remains uncertainty in timing of
the development of Fraser Outflow winds. Looking at individual
ensemble members (GEFS, EPS, GEPS), the majority of GEFS members
begin NE winds as early as Sunday morning. Meanwhile, EPS/GEPS
members wait longer till Sunday night and Monday morning with more
of a S/SE component continuing on Sunday. However, despite the
timing differences, moisture is expected to remain on Monday,
allowing for increasing upslope snow potential for Port Angeles to
Sequim, and have increased QPF (towards 90th PCT NBM) during this
period to address this. NBM probabilities will continue to
underdo this region as model guidance struggles with this
mesoscale detail, and this will be something to monitor the next
few days.

Cold Temperatures: As mentioned above, a slow cooling trend in
surface temperatures is expected over the weekend. There remains a
rather widespread range of potential temps by Monday morning.
This is shown well on a plume of all ensemble members, with the
10 to 90th PCT spread in Monday AM temps for SEA being 22 to 35
degrees. Interestingly, GEFS mean remains the coldest solution for
most of W WA (actually located outside of the 25 to 75th PCT at
SEA), likely due to Fraser Outflow beginning sooner on the GEFS
suite. This is also a theme seen Tuesday into Wednesday, although
to less of an extent. For areas that do drop below freezing Monday
morning, refreezing at the surface may also be a concern.

.LONG TERM /MONDAY THROUGH THURSDAY/...The forecast for Monday was
combined into the short term section above. As for Tuesday and
Wednesday, the upper low continues to slowly meander southwards
along the WA/OR coastline. With that said, there remains
uncertainty in the position and depth of the trough offshore, in
this case, the EPS has a deeper trough off the PacNW coast, and
less so on the GEPS. In general, precipitation potential wanes
Tuesday night through Thursday as the upper low sinks southward
offshore. Current POPs are around 15% up north midweek, and higher
near 50% along the southern tier of the CWA. Temperature profiles
would support snow at this time, if precipitation were to occur.
The primary concern will be the cold overnight temperatures,
although uncertainty remains in temperatures looking at the
ensemble spread. Apparent temperatures will also be a concern
across Whatcom County (at the least) due to Fraser Outflow early
next week. It will be important to continue to monitor the
forecast over the next week due to the active, cold pattern
expected through this period. JD

&&

.AVIATION...Zonal flow aloft is becoming more southwesterly
tonight and into Friday as an upper level low swings a warm cold
front through on Friday. Ceilings this evening remain VFR/MVFR
this afternoon, and will likely fall down to IFR/MVFR overnight
into Friday morning (with lower ceilings towards the South
Interior). Rain continues to spread inland tonight into Friday,
before tapering back in the afternoon. Snow is likely at higher
elevations (greater than 3,000 feet). Dense fog is not expected,
but there could be visibility reductions due to mist and heavy
rain at times. Winds remain breezy in spots ahead of the front
tonight at 10 to 15 kt (with gusts to 20-25 kt possible), and
decrease Friday morning to around 8 to 12 kt, before picking up
again Friday afternoon/evening.

KSEA...MVFR conditions at the terminal this evening as rain moves
in. Rain will continue to move into the terminal through Friday.
Ceilings currently around 3,500 ft and will continue to lower to
1,500 feet by Friday early morning. Mist and visibility reductions
due to rain will remain possible. Expect winds to be breezy at
times at 8 to 12 kt (gusting to 18 kt) by Friday morning,
decreasing to 5 to 10 kt during the day and picking up again
Friday evening.

HPR/14

&&

.MARINE...South winds increase tonight into Friday as a
frontal system moves inland. A switch to strong post-frontal
onshore flow will develop behind the front Friday night. Seas
building to 10 to 12 feet over the Coastal Waters this weekend.
There`s potential for gusty N/NE Fraser River outflow winds Sunday
through Tuesday, impacting the Northern Inland Waters and Strait
of Juan de Fuca. 33

&&

.HYDROLOGY...No river flooding is expected over the next 7 days.
Precipitation with the upcoming systems moving through this weekend
into next week will bring the Skokomish River up to action stage.
The combination of mixed precipitation (snow in the higher
elevations and likely the lowlands later in the weekend) will also
likely slow rivers down (in combination with cooler temperatures).

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Saturday evening through Monday morning
     for Central Coast-East Puget Sound Lowlands-Lower Chehalis
     Valley Area-San Juan County-Western Whatcom County.

     Winter Storm Watch from Friday evening through Monday morning
     for North Coast-Western Strait of Juan De Fuca.

     Winter Storm Warning until 4 PM PST Saturday for West Slopes
     North Cascades and Passes-West Slopes North Central
     Cascades and Passes-West Slopes South Central Cascades and
     Passes.

PZ...Gale Warning from 5 PM Friday to 4 AM PST Saturday for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 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 until 4 AM PST Saturday 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.

&&

$$