Heavy Snow in Washington, December 18 to 19, 2021

Area Forecast Discussion
National Weather Service Seattle WA
722 PM PST Fri Dec 17 2021

.UPDATE...The current forecast remains on track as precipitation
spreads across the region at this hour. Changes are not planned
for this evening.

Pullin

&&

.PREV DISCUSSION... /issued 244 PM PST Fri Dec 17 2021/

SYNOPSIS...A strong weather system will move across the area
Saturday into Saturday night for lowland rain, gusty winds, and
mountain snow. The system will slide south of the area on Sunday
for a few showers. A front will move north Monday for additional
precipitation. An upper low offshore will bring potential for
precipitation into next week.

SHORT TERM /TONIGHT THROUGH MONDAY/...A system is moving towards
the area with rain offshore spreading inland this evening and
across the area through tonight, with increasing snow for the
mountains. Rain will continue for the lowlands on Saturday morning,
especially for northern areas. The heaviest rain will move through
late Saturday afternoon and into the evening with the front. The
winds will increase this evening, and peak Saturday morning for
most areas, with gusts of 60 MPH for the Admiralty Inlet and the
San Juan Islands, and gusts to 50 MPH for the coast, Eastern
Strait of Juan de Fuca and Western Whatcom/Skagit Counties. Gusts
of 25-40 MPH for the Puget Sound on Saturday. Mountain snow will
be increasing tonight, with snow levels increasing into Saturday.
There is potential for freezing rain at Snoqualmie Pass, with the
best chance Saturday morning, as temperatures warm into the low to
mid 30s, a transition to rain is expected during the afternoon.
Late Saturday afternoon into Saturday night will feature the
heaviest mountain snow for the Passes. Winter Storm Warnings are
in effect for the Cascades with a Winter Weather Advisory for the
Olympics. Amounts of 8 to 12" for Snoqualmie Pass with 18-24"
possible for Stevens Pass. Temperatures will be a bit warmer on
Saturday with the system, high temperatures will be in the upper
40s to low 50s.

The front will slide south of the area late Saturday night into
Sunday with some lingering rain showers possible for the Lowlands
into Sunday morning. Mainly dry weather for Sunday afternoon with
the front across southern WA and into northern Oregon, though some
chance of showers will be possible with the trough. In addition,
winds will increase from the north on Sunday. Fraser Outflow winds
will increase across Whatcom County and the San Juan Islands for
potentially stronger winds. Cold air will filter into Western
Washington into Sunday night. The forecast is a bit more
interesting on Monday as the front located to the south slides
northward across the area for precipitation during the day. There
is uncertainty as to the exact amount of cold air in place on
Monday, but the potential exists for at least a rain/snow mix
across areas of King County and northward during the day.
Precipitation may taper off as it slides northward with the
weakening front and air that is drier filtering into Whatcom
County. Little to no snow accumulation for the area, but at least
the chance for rain/snow mix is possible. Temperatures will also
be chillier Sunday and Monday with highs in the low to mid 40s.

LONG TERM /TUESDAY THROUGH FRIDAY/...A trough will be located
along the Western coast through most of next week with periods of
precipitation possible as bands slide across the Pacific
Northwest. A round of lowland rain and mountain snow on Tuesday
night into Wednesday for generally light accumulations in the
mountain areas. Additional precipitation later next week with the
trough offshore and another system sliding south towards the area.
The cooler pattern will continue into the week, though some
uncertainty as colder air filters into the area later in the week,
mainly Friday. Any precipitation through the week is currently
forecast to be rain in the lowlands, though lower snow levels
possible along the Foothills and Whatcom County area. JD

AVIATION...Rain associated with the next vigorous frontal system
is spreading inland this evening with westerly flow aloft then shift
inland on Saturday. The air mass will be moist and stable. Ceilings
are currently MVFR area wide this evening, with localized drops down
to IFR expected as the front moves through. Low clouds should linger
all day Saturday as the front drags through. Wind speeds will remain
elevated over the coast and northern inland areas, with gusts
between 45-50 kts in those areas possibly. Direction will largely be
southerly. Additionally, there are area wide concerns for potential
LLWS, generally between 030-050 with southerly wind speeds up to 50-
60 kts.

KSEA...Rain has returned to the terminal this evening, as well as
MVFR ceilings. Wind speeds are between 5-7kts out of the southeast.
Low clouds will return later tonight as a front arrives. Southeast
wind 4-8 knots tonight will become south 10-20 knots with higher
gusts Saturday morning, generally between 15Z-17Z. High res model
guidance is suggestive of some LLWS between FL 030-050 as the front
passes through, as wind speeds in this layer could be southerly at
50-60 kts. Above that layer, winds will take on more of a westerly
component and still be between 45-55 kt. Freezing levels will be
between 2,500-3,500 feet through 09Z-10Z, before rising to 6,000-
7,000 feet through the rest of Saturday.

Kristell

MARINE...A vigorous frontal system will move through the area
tonight and Saturday. Gale force southerly winds are likely for
the Coastal Waters, East Entrance Strait of Juan de Fuca,
Admiralty Inlet and Northern Inland Waters tonight and Saturday
morning with small craft advisory strength winds elsewhere. There
will also be a surge of west winds down the Strait on Saturday
behind the cold front.

Offshore flow will develop on Sunday the increase Sunday night,
peaking early Monday morning. Fraser outflow will develop Sunday
night with gale force northeast winds possible for the Northern
Inland Waters. Offshore flow will gradually ease later Monday into
Tuesday. Winds will turn more southerly Tuesday night and
Wednesday as a frontal system offshore reaches the area.

16/Kristell

HYDROLOGY...A weather system will bring increasing snow levels
Saturday. Most rivers are not expected to flood, the Skokomish
River near Potlach will need to be monitored with this weather
system.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 4 AM PST Sunday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

     Wind Advisory until 4 PM PST Saturday for Central Coast-Eastern
     Strait of Juan de Fuca-North Coast-Western Skagit County-
     Western Whatcom County.

     High Wind Warning until 4 PM PST Saturday for Admiralty Inlet
     Area-San Juan County.

     Winter Weather Advisory until 4 PM PST Saturday for Olympics.

PZ...Gale Warning until 4 PM PST Saturday for Admiralty Inlet-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-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands.

     Small Craft Advisory until 10 PM PST Saturday for Central U.S.
     Waters Strait Of Juan De Fuca-Puget Sound and Hood Canal-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

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