Avalanche and Heavy Snow in Washington, December 10 to 13, 2021

Area Forecast Discussion
National Weather Service Seattle WA
820 PM PST Thu Dec 9 2021

.SYNOPSIS...A frontal system moving through Western Washington later
Friday and into Saturday will bring with it strong winds,
heavy mountain snow and lowland rain. By the start of the work week
things will still be unsettled but are expected to be generally
quieter. However, conditions will be colder and snow levels will be
dropping.

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.SHORT TERM /TONIGHT THROUGH SUNDAY/...Some showers present over the
western half of the CWA this evening while the convergence zone
driven activity over the Cascades finally seems to be tapering off.
That said, the inherited Winter Weather Advisory for the Central
Cascades set to expire at 10 PM tonight looks good to do so. The
aforementioned showers will move eastward during the overnight hours
but remain scattered, thus leaving some PoPs in the forecast
overnight tonight and into Friday. Inherited forecast is handling
the above well and foresee no evening update. For additional
forecast details, please refer to the Previous Discussion section
below.  18

From Previous Discussion...Winds on Friday will begin to pick up
during the afternoon hours at the coast and along the area waters.
These southerly winds are expected to peak during the overnight as
the cold front moves into the area. A high wind watch has been
issued for the typical windy locations of the coast and north
interior as gusts up to 60 mph are expected. The interior lowlands
will experience wind as well, but are only expected to reach
advisory strength, with gusts 40-50 mph. Winds will begin to ease
through Saturday but will remain breezy.

The big story when it comes to the precipitation will be the
mountain snow. Using the passes as a reference for elevation we
are expecting 2-3 feet to fall between Friday morning and Sunday
afternoon. Accumulations at the higher elevations will be
significantly hours.

There is one major hiccup when it comes to the snow forecast could
occur early Saturday morning as the warm front moves through. As
the warm front moves through their could be enough cold air
remaining right at the surface. What this means is there is the
potential for some freezing rain to occur during early Saturday
morning. These conditions are most probable in the Snoqualmie Pass
as well as the Amanda Park area along route 101. At this time
there is moderate confidence in freezing rain at the pass, with
slightly lower confidence along 101 there. Once the warm sector
moves through all precipitation is expected to either be rain in
the lowlands and snow in the mountains.

Rain and snow showers will continue into Sunday as an upper level
trough makes its way into the area. With this there is a pretty
decent chance for some thunderstorms to occur Sunday, primarily
along the coast. As the forecasted conditions stand right now,
there is enough instability and wind shear that any isolated
thunderstorms that do form could be strong enough to make their
way inland.

Butwin

.LONG TERM /MONDAY THROUGH THURSDAY/...From Previous
Discussion...From Monday onward we are looking to have a bit of a
break in the more active weather as a trough axis extends southward
to S. Oregon/N. California. While the main energy and moisture will
be headed their way, we will still have some moisture and showers in
the area. It is also going to be on the cold side (at least for our
standards) and snow levels will be dropping. So we wouldn't be
surprised if the lower elevations see some snow during this time
frame. That said, our day time temperatures are still on the warm
side for persistent accumulating snow. So we are not expecting an
major accumulations in the lowlands. We know the winter and snow
lovers are excited for this chance, but as it stands right now we
here are not too convinced there will be enough moisture (especially
for the Seattle area). But things can change between now and then.

Butwin

&&

.AVIATION...West to northwest flow aloft tonight and Friday. At
the surface, onshore flow tonight will turn more southerly on Friday
ahead of an approaching frontal system. The air mass will be
slightly unstable through early this evening then become stable
tonight. There will be a mix of low and mid clouds tonight and
Friday.

KSEA...Mostly low clouds tonight and Friday although the lower
ceilings might scatter at times, especially this evening. Southerly
winds 8-12 knots will ease a bit tonight. 33/16

&&

.MARINE...Onshore flow will ease tonight. West swell 10-13 feet
will continue over the Coastal Waters and West Entrance Strait of
Juan de Fuca.

A vigorous frontal system will move through the area Friday night
and Saturday. Winds will turn more southerly on Friday ahead of the
front then rise to gale force most waters Friday night and Saturday.
A brief period of storm force winds is not out of the question for
the Coastal Waters Friday night.

Winds will gradually ease on Sunday and then turn weakly offshore on
Monday. Light southerly winds are expected for Tuesday. 33/16

&&

.HYDROLOGY...From Previous Discussion...Heavy rain Friday night
through Saturday, 2 to 4 inches, combined with the brief rise in the
snow levels to around 4000 feet in the Olympics will likely drive
the Skokomish River above flood stage Saturday. Rivers over
Southwest Washington will also initially respond to the heavy rain
but lower snow levels over the Cascades will limit the amount of the
rises in these rivers. No additional river flooding expected in the
next 7 days.

Felton/Kovacik

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory from 7 PM Friday to 10 AM PST Saturday for
     Bellevue and Vicinity-Bremerton and Vicinity-East Puget
     Sound Lowlands-Everett and Vicinity-Hood Canal Area-Lower
     Chehalis Valley Area-Seattle and Vicinity-Southwest
     Interior-Tacoma Area.

     Winter Storm Watch from Friday evening through Saturday evening
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties-Olympics.

     Winter Weather Advisory until 10 PM PST this evening for
     Cascades of Snohomish and King Counties.

     High Wind Watch from Friday evening through Saturday morning for
     Admiralty Inlet Area-Central Coast-North Coast-San Juan
     County-Western Skagit County-Western Whatcom County.

PZ...Small Craft Advisory until 4 PM 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-Grays Harbor Bar-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning from 4 PM Friday to 4 PM 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.

     Gale Warning from 4 PM Friday to 7 AM PST Saturday for Admiralty
     Inlet-East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 PM PST Friday for Northern Inland
     Waters Including The San Juan Islands.

     Small Craft Advisory from 10 AM to 4 PM PST Friday for Puget
     Sound and Hood Canal.

     Gale Warning from 4 PM Friday to 10 AM PST Saturday for Puget
     Sound and Hood Canal.

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