Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 070454
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Tue Dec 6 2016

.SYNOPSIS...Cold dry air will be over Western Washington through
Wednesday. Easterly gap winds will increase Wednesday night. A
weather system will arrive from the south Thursday afternoon and
heavy lowland snow is possible from then through Friday morning.
Cool wet weather is likely Friday through the weekend.

&&

.SHORT TERM...Cold dry air lies over Western Washington. At 8 PM
temperatures were in the 20s and 30s, with dew points in the 20s. In
the far north there are dew points in the teens with KBLI showing 9.
Temperatures like this will continue through about Thursday morning.
This dry air will give mostly clear weather through Wednesday. Fraser
outflow will keep breezy conditions over Western Whatcom county and
into the San Juans. Easterly winds will increase Wednesday night
and Thursday, especially in the mountain foothills and passes.

Some truly interesting weather may be about to reach the area. A
weather system will move from south to north Thursday afternoon
through Friday morning. There will be precipitation out of this
system, into the cold dry air below. This is a good recipe for snow
in the lowlands, and a winter storm watch is now in effect. Four
inches of snow over Seattle and really all of the interior lowlands
is possible. The east winds could add an upslope component to the
snowfall around Hood Canal, where seven inches or more is possible.
Only the coast may be warm enough to have either rain or a rain/snow
mix. The onset and end times of snow varies from south to north.
Rain will replace snow by Friday morning in most places, but there
could be a real mess, for several hours at least, on Friday.
Temperatures should rise into the low 40s or so Friday afternoon.

There is also a winter storm watch in effect for the Cascades and
Olympics, somewhat later than the lowland watch. Precipitation keeps
falling even after the air mass warms a bit, so the mountains seem
likely to get a foot or two more of fresh snow. Burke

.LONG TERM...Previous discussion...There is a good chance that the
modified arctic front will be just north of the WA/BC border
Saturday and for most of Sunday. After that, the 12z GFS might have
a wintry solution around Sunday night but that`s super speculative.
The ECMWF pulls the gradient around to northerly next Monday night
and Tuesday--in fact dry cool weather is possible in strong offshore
flow by the middle of next week. But for now the forecast is closer
to climatology than to any of these interesting individual forecast
model solutions.

&&

.AVIATION...Dry northerly flow aloft ever Western Washington this
evening will continue tonight and Wednesday as an offshore upper
ridge moves slowly toward the region. Surface high pressure over
British Columbia will maintain cold dry northerly low level flow.
The air mass will be generally dry through Wednesday, although
locally BKN-OVC clouds around 080 will remain over the area this
evening. Patchy freezing fog is expected late tonight and
Wednesday morning only over the south Puget Sound region and
southwest interior.

KSEA.... Wind north-northeast 4-10 kt. Mainly clear skies tonight
and Wednesday. Snow is possible Thursday afternoon and night.
McDonnal

&&

.MARINE...High pressure over British Columbia will gradually
weaken through tonight, and northeast flow across Western
Washington and its waters to ease. Marginal small craft advisory
winds should continue over the northern inland waters through
Wednesday morning, mainly north of the San Juans.

The big issue is the evolution of the low currently offshore W of
the N CA coast near 40N/143W. Models agree that the low will drift
slowly NE toward the Pacific Northwest over the next few days.
What seems reasonably certain is that easterly offshore flow will
increase to gale levels over the coastal waters and through the
Strait of Juan de Fuca Wednesday evening and continue through
Thursday night as the associated front moves NE across the area. A
gale watch is in effect for Wednesday night through Thursday night
for the coast and parts of the Strait. What is much less certain
is the track and strength of the surface low, which will impact
winds Thursday night and Friday as the low nears the coastal
waters. Confidence in the wind forecast for that period is low.
Kam/McDonnal

&&

.HYDROLOGY...No river flooding is expected over the next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Watch from Thursday morning through Friday morning
     for Admiralty Inlet Area-Bellevue and Vicinity-Bremerton
     and Vicinity-East Puget Sound Lowlands-Eastern Strait of
     Juan de Fuca-Everett and Vicinity-Hood Canal Area-Lower
     Chehalis Valley Area-San Juan County-Seattle and Vicinity-
     Southwest Interior-Tacoma Area-Western Skagit County-
     Western Whatcom County.

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

PZ...Gale Watch from Wednesday evening through late Thursday night
     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.

     Small Craft Advisory until 10 AM PST Wednesday for Northern
     Inland Waters Including The San Juan Islands.

     Gale Watch from Wednesday evening through late Thursday night
     for Central U.S. Waters Strait Of Juan De Fuca-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html


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