Winter Storm Warning 7, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
301 AM PST Sat Nov 26 2022

.SYNOPSIS...Drier weather today with scattered showers mainly for
the Cascades and northern areas. A stronger front will bring
lowland rain, heavy mountain snow, and breezy winds tonight into
Sunday. Convective showers Sunday afternoon into Monday morning
for additional mountain snow and lowering snow levels. A much
cooler weather pattern through much of next week for potential
lowland snow mid-week.

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.SHORT TERM /TODAY THROUGH MONDAY/...A mix of clouds and fog
across the area interior early this morning as shown on
satellite, with the majority of fog mainly south of Seattle, with
low clouds across Snohomish/Skagit counties with weak convergence
and light showers. Mainly drier weather today, however, light
showers will continue for Snohomish County northward, especially
in the Cascades. The next system will move into Western Washington
Saturday night and continue into Monday for additional impacts as
highlighted:

Mountain Snow: Snow levels will initially be 3000-3500 feet this
evening through tonight for the Cascades then drop to 1000-2000
feet Sunday through Sunday night. The heaviest stratiform
precipitation will move southward tonight across the area for
heavy snowfall in the mountains. Winter Storm Warnings have been
issued for the Cascades for this front. Snoqualmie Pass will
likely initially have a rain/snow mix tonight with the front
before transitioning to all snow with falling snow levels. Stevens
Pass is expected to remain all snow for this event. Snowfall
accumulations will range 14-21" for the Passes tonight through
Monday morning. Travel impacts are expected due to the heavy snow,
make sure to be familiar with WSDOT pass updates through Monday.

Wind: Winds will increase late this afternoon and peak tonight
with the front. HREF highlights the strongest winds being near the
water of the northern interior and Puget Sound, particularly
Western Whatcom County, and towards western Snohomish/King
Counties. Wind gusts of 30 to 40 MPH are expected to be widespread
in these areas, as well as along the north Olympic coast. In
addition, increasing onshore flow and tight pressure gradients
late tonight will promote west winds through the Strait of Juan de
Fuca towards Admiralty Inlet and Whidbey Island. West wind gusts
of 35 to 45 MPH may be possible during this period. Will have to
continue to monitor if headlines are necessary, but localized
power outages may be possible. Winds will taper off into Sunday,
albeit continuing to be breezy in some areas.

Lowland Precipitation: Rain will be heaviest across the lowlands
tonight as the front quickly moves southward. Sunday may feature
drier weather in some places, with likely convergence banding and
heavier convective showers beginning during the day and continuing
through Sunday night. In addition, snow levels will drop Sunday
night into Monday morning to near 500-1000 feet. Due to the
more convective nature of the showers during this period, snow
levels may briefly be lowered to near sea level in the heaviest
precipitation for possible rain/snow mix, especially late Sunday
night into Monday morning. We're certainly just on the cusp of
hi-res guidance for this period, but brief mixing is expected in
the heaviest bands, which includes any convergence activity.
Little to no snow accumulation is expected for most areas.
However, the Cascade Foothills/Valleys will need to be monitored
for potential light accumulations due to their higher elevation.
Precipitation will taper off on Monday.

.LONG TERM /TUESDAY THROUGH FRIDAY/...A period of brief, dry
weather is expected Monday night into Tuesday morning with lows
dropping below freezing for most areas. Re-freeze possible for any
untreated road surfaces. The primary focus on the longer-term
period will be the next system to impact the Pacific Northwest
between Tuesday-Thursday. We briefly discuss a few aspects with
this system below.

Guidance is in general agreement an upper trough will slide
southeast along the British Columbia coast Tuesday-Thursday,
and ensembles have come into a bit better agreement in the depth
and intensity of the trough, but minor fluctuations of the systems
storm track will greatly influence sfc precipitation type during
this period. As such, have focused on looking at the WPC cluster
analysis (all 90 ensemble members from GFS/CMC/ECMWF) for the
24-hour period ending 00z Thurs. The Cluster Analysis tool is
useful in identifying ensemble uncertainty, as well as potential
extremes and the forecast. Per the Cluster 500mb heights,
approximately 55% of the distribution indicates a deepening
trough offshore of Vancouver Island, with a more elongated trough
noted on only 21% of the cluster (primarily GFS). This would
suggest a deeper, stronger system is likely. One of the main
impacts from a stronger system would be increased southwesterly
flow, and thus warmer temperatures both at the surface and aloft
on Wednesday. This can be seen on the cluster analysis max/min
temperatures, with temperatures a tad warmer than the mean with a
deeper trough. With all that said, temperatures will still be
below average during this period due to the previous trough moving
overhead Sunday-Monday, and thus making for a rather tricky setup
for potential wintry precipitation in the lowlands due to the
lower snow levels. Reviewing NBM guidance for the 72-hour period
ending 12z Friday shows widespread 60-90% chance of >1" of snow
for the lowlands. Potential for 4 inches or more favors the
Cascade Foothills/Valleys (50-80% chance). In addition to the
western Kitsap County area (60-90% chance) due to colder air
sticking around that area longer. As for the I-5 corridor a
general theme of higher probabilities exist north of Seattle
(40-70%), and less southward (25-50%). Minor fluctuations in the
storm intensity/track will lead to any further adjustments in
thinking for lowland snow. For that reason, it will continue to be
important to stay updated on the weather forecast during this
period. Finally (if you're still reading), the trough will
reinforce the cold airmass for the Pacific Northwest late next
week. Temperatures will be well below normal for this time period,
with widespread lows in the teens/20s and highs in the 30s on
Friday. JD

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.AVIATION...Western Washington currently remains in a brief lull
between weather systems early this morning. Latest satellite
imagery does show some clearing, which has led to the development
of fog at some area TAF sites overnight. As a result, conditions
remain a bit of a mixed bag across the region, with most sites
outside of fog experiencing MVFR conditions and sites within fog
experiencing IFR to LIFR conditions. Expect low clouds and fog to
stick around through morning hours, with conditions expected to
improve somewhat towards more widespread MVFR/VFR between 18-21Z.
A strong frontal boundary will then move across the region tonight
into Sunday, bringing with it another round of rain and lower
cigs/vsbys. Likely onset of rain along the coast and north
interior is 22-01Z. Rain then looks to move inland between 03-06Z
Sunday. Expect southerly winds to increase across the area
terminals this afternoon, peaking overnight into Sunday. S winds
generally persisting at 15-20 kt, with gusts to 25-35 kt possible
at times.

KSEA...LIFR conditions in fog at the terminal. Expect conditions
to improve towards MVFR/low end VFR between 18-21Z.S winds will
increase this afternoon, persisting at 15-20 kts. Gusts 25-35 kt
will be possible at times this evening into Sunday. Rain ahead of
the next system looks to make it into the terminal between 03-06
Sunday. 14

&&

.MARINE...A strong frontal system will move across the region
tonight into Sunday, bringing several rounds of headlines to the
area waters. Southerly winds will start to increase across the
waters this afternoon, mainly persisting at 15-25 kt. Thus, have
issued small craft advisories for all area waters ahead of this
next system. Latest hi-res guidance still showing the potential
for frequent gusts to gale as this frontal system traverses the
waters. Thus, have issued gale watches for increasing southerly
winds tonight into Sunday for the coastal waters and for the
interior waters of Puget Sound, Admiralty Inlet, the eastern
Strait, and the Northern Inland waters. Winds will then shift
westerly in the wake of the system, with a strong surge expected
down the Strait of Juan de Fuca. Thus, a gale watch has also been
issued for the west and central portions of the Strait. Winds
along the eastern Strait will shift from strong southerly to strong
westerly by early Sunday. Overall, expect winds to peak overnight
and will then gradually ease again through the day Sunday. An
active pattern will persist heading into next week as additional
systems traverse the region, potentially bringing several rounds
of additional headlines to the area waters.

Seas will continue to subside to 7 to 9 ft this morning, before
increasing again to 10-14 ft by Sunday. Seas look to remain
elevated, hovering around 10-12 ft, into the start of next week.
14

&&

.HYDROLOGY...No river flooding expected over the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning from 1 PM this afternoon to 7 AM PST Sunday
     for West Slopes North Cascades and Passes.

     Winter Storm Warning from 7 PM this evening to 10 AM PST Monday
     for West Slopes North Central Cascades and Passes-West
     Slopes South Central Cascades and Passes.

PZ...Small Craft Advisory from 1 PM this afternoon to 7 PM PST this
     evening for Central U.S. Waters Strait Of Juan De Fuca-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from this evening through late tonight for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Watch from this evening through Sunday morning for
     Admiralty Inlet-Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands-Puget Sound
     and Hood Canal.

     Small Craft Advisory from 10 AM this morning to 7 PM PST this
     evening for Admiralty Inlet-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands-Puget Sound and Hood Canal.

     Small Craft Advisory from 10 AM this morning to 4 PM PST this
     afternoon 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.

     Gale Watch from this afternoon through late tonight 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.

&&