Winter Storm Warning 12, NWS Seattle, WA

Area Forecast Discussion
National Weather Service Seattle WA
935 PM PST Tue Dec 11 2018

.SYNOPSIS...The first is a series of strong frontal systems will
move onshore tonight with locally windy conditions and heavy
mountain snow. There will be a lull on Wednesday followed by the
next system Wednesday night and Thursday. This system will hang up
over the area before pushing inland on Friday. Additional fronts
will affect the area Saturday night through Tuesday.

&&

.UPDATE...Cold front continues to push out of the area this
evening with precipitation turning more showery behind the front.
Winds have subsided a bit from today's peak though are still
breezy in some locations. A rather healthy convergence zone formed
in Snohomish county and is currently pushing across northern King
County, though in a weakened state. Snow levels have come back
down with a return to snow at pass levels. Snow will taper off
Wednesday morning. No major changes to tonight's forecast.
Previous discussion follows. CEO

.SHORT TERM from PREVIOUS DISCUSSION...A strong frontal system is
currently moving inland. Breezy to locally windy conditions will
continue tonight with strong onshore flow behind the front. Snow
levels spiked up today which has turned the snow to rain in spots
and limited accumulations a bit. Totals so far include 14" at
Mount Baker and 10" at Stevens Pass. Snoqualmie Pass turned to
rain today and only picked up about 5". Snow levels will come down
again this evening and good orographics should produce several
more inches of snow tonight so the winter storm warning will
continue. Snow in the Cascades will taper Wednesday morning.

Wednesday will be a relative lull in between fronts. The next in
a series of frontal systems will bring more rain, wind, and
mountain snow Wednesday night and Thursday. Advisory strength
winds are likely again coast and Northwest Interior. Headlines
might also be needed again for the Cascades where good shot of
snow is possible. It looks like the north Cascades will get the
most snow this time - maybe a foot a Mount Baker - with lesser
amounts further south.

This Thursday system will hang up across the area, weaken, and
then lift north as a warm front Thursday night. This is all in
response to another system strengthening offshore. This frontal
wave will move through the area as a cold front on Friday. An
associated deep surface low will track offshore and move into
Vancouver Island. Models are in better agreement with this idea
but there are still differences with timing, location, and
strength of the low. Regardless, it will likely be another breezy
to windy day in spots Thursday night and Friday with additional
wind related headlines possible. Schneider

.LONG TERM...Saturday will be a relatively quiet weather day in
between fronts. The next in the parade of storms will bring
another round of rain and wind Saturday night and Sunday. There
will be a brief break Sunday night/Monday then a very wet frontal
system is expected to move into the region Monday night and
Tuesday. This system has the potential to cause some flooding.
Please refer to the hydrology section below for more details.
Schneider

&&

.AVIATION...Cold front is passing through the area this evening
with precipitation turning more showery in its wake. Winds have
shifted to the west/southwest at the surface and have subsided a
bit from earlier peak. Many locations though will continue to see
breezy winds through the night. Ceilings have lifted to higher-end
MVFR and VFR. Will see a brief lull in the precipitation for TAF
sites during the day Wednesday before the next system, similar to
today's moves onto the coast in the afternoon hours and inland
through the evening/overnight hours. Will see another round of
breezy southerly winds and MVFR conditions toward the end of the
TAF period.

KSEA...In the wake of the cold front, will see scattered showers
tonight into early Wednesday. Cigs remain in the MVFR category
tonight but should see some improvement to VFR Wednesday
morning/afternoon before the next system arrives Wednesday night.
This will bring another period of MVFR conditions and breezy
southerly winds.

CEO

&&

.MARINE...System will continue to move east out of the area
tonight into early Wednesday. Small craft to gale force winds
continue across most of the waters this evening into early
Wednesday morning. Thereafter winds should drop off rather
quickly and we see a brief respite in marine headlines. This will
be short-lived as the next system approaches Wednesday PM.
Guidance suggest winds should pick up by as early as 21-00z
tomorrow with a return to gale force winds tomorrow night into
Thursday. Additional systems past Thursday will impact the area
with small craft or gale conditions from time-to-time into next
week. Kovacik/CEO

&&

.HYDROLOGY...Precipitation amounts over western Washington today
have been 2.5 to 4.5 inches of precipitation over the Olympics. The
Cascades have received 1 to 2 inches of precip so far, mostly snow
in the higher elevations. Rivers are rising around the region with
the Skokomish being under a flood warning as it has already
exceeded flood stage.

After a break on Wednesday the next storm in a series impacts the
area starting Wednesday night and may hang on into Friday. This will
renew flooding on the Skokomish and continue pushing other area
rivers higher. Over the weekend into next week another potent storm
will impact the area and early indications are it could produce some
widespread flooding.
jbb

&&

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

     Winter Storm Watch from Wednesday afternoon through Thursday
     morning for Cascades of Whatcom and Skagit Counties.

PZ...Gale Watch from Wednesday afternoon through Thursday morning 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-Puget Sound
     and Hood Canal-West Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Gale Warning until 4 AM PST Wednesday for Admiralty Inlet-
     Central U.S. Waters Strait Of Juan De Fuca-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.

     Small Craft Advisory for rough bar until 4 AM PST Wednesday for
     Grays Harbor Bar.

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

     Small Craft Advisory until 10 PM PST this evening for Puget
     Sound and Hood Canal.

&&

www.weather.gov/seattle