Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 032356
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 PM PST Sat Dec 3 2016

.SYNOPSIS...A vigorous cold front will push across the area later
tonight. Rain and mountain snow will continue into Sunday morning,
then taper to showers. Upper level low pressure will bring even
colder air aloft into Western Washington by Monday with snow levels
falling to around 500 feet. Snow showers or a mix of rain and snow
will be possible across the lowlands, especially away from the water
and on hilltops. Temperatures falling to below freezing Monday night
could allow for light accumulations before showers end. Cold and
mainly dry weather is anticipated Tuesday into Wednesday. A system
arriving Wednesday night and Thursday could bring some low elevation
snow before warm air brings rising snow levels.

&&

.SHORT TERM...Showers are picking up in coverage and intensity this
afternoon. The coastal radar is showing more solid stratiform
precipitation approaching the coast ahead of a strong cold front
expected to arrive later tonight. Moist westerly flow over the
eastern Pacific evident on satellite imagery will bring steady
precipitation into the area tonight with westerly flow favorable for
enhancing orographic snowfall along the Cascades. Impressive snow
amounts are expected in the mountains and Cascade passes through
Sunday morning, with generally 1 to 2 feet likely. Passes will be
impacted with 10 to 20 inches expected, including Stevens pass. Some
rain and snow mixed is expected at 3000 feet this evening ahead of
colder air behind the front. Snoqualmie pass is the one pass
location that will probably get a bit less snow, up to around 10
inches, mostly falling later tonight and early Monday. Rain and
mountain snow will taper to showers later Sunday morning and
afternoon.

An upper low along the B.C. coast will slowly dive south toward the
area Sunday and Sunday night bringing even colder air, -38C at 500
mb, into the Pacific Northwest. A surface trough will bring mainly
light showers into the area by Monday morning with snow levels
falling to 500 feet. Meso models and ensemble forecasts show a
possibility for an inch or two of snow across the lowlands, mainly
during the morning hours. However, ground temperatures will be
relatively warm to start and southerly gradient should cause some
mixing of rain and snow below 500 feet. Accumulations should
initially be confined to above 500 feet on hilltops and away from
water. Grassy surfaces and rooftops may get a bit white but roadways
will probably just get a bit slushy at few hundred feet or higher.

Colder air may filter in Monday night as Fraser outflow increases.
Models are not in good agreement on the extent of outflow and low
dew point air invading the Puget Sound region which could bring a
period snow and accumulation down to sea-level. Monday evening
through Tuesday morning could see near or sub-freezing temperatures
with a chance of residual snow showers. Although amounts would be
light if it occurs, roads in the metro area could become icy during
this period. Will need to continue monitoring the system as it
evolves. In the very least, sub freezing lows will bring a threat of
black ice Tuesday and Wednesday mornings.

.LONG TERM...The next system to watch will bring precipitation back
into the area Wednesday night and Thursday. With cold air in place
to start, precipitation could fall as snow initially in the
lowlands. Models suggest a few inches of snow could fall before
changing over to rain Thursday morning or midday. The flow is strong
enough to scour out cold air by afternoon but there could be several
inches of snow before this occurs.

Snow will fall again in the mountains, adding to the snowpack toward
the end of the week. Precipitation will be rain in the lowlands
later Thursday and Friday. Some extended model runs show a stronger
low tracking north of our area Friday or Saturday, possibly bringing
wind and higher snow levels. There is not much agreement toward the
weekend except that the active weather pattern will continue. Mercer

&&

.AVIATION...Westerly flow aloft through Sunday. A frontal system
well offshore will move rapidly eastward and push through the area
between 06Z and 12Z late tonight. Onshore flow will diminish behind
the front Sunday. The air mass will be somewhat stable and moist
tonight and will become moist and unstable behind the front late
tonight through Sunday.

The lowest conditions (MVFR) continue to be from about Seattle
northward in the interior today. Conditions elsewhere are mainly
VFR. Conditions will deteriorate to MVFR across the area early this
evening ahead of the incoming frontal system

Behind the front very late tonight and Sunday, expect showers and
MVFR conditions to become increasingly tied to the terrain and a
central Puget Sound Convergence zone with mainly VFR conditions
elsewhere. Albrecht


KSEA...South winds 10 to 15 kt with gusts to 22 kt tonight. VFR
conditions at the terminal will lower to MVFR by 03Z and will
continue until frontal passage at 09Z. A Puget Sound Convergence
Zone will likely develop around the terminal area behind the front
by 12Z late tonight and persist into the afternoon hours on Sunday
giving MVFR ceilings and light showers. Albrecht

&&

.MARINE...A Pacific frontal system well offshore will move rapidly
eastward through the waters tonight and well east of the area early
Sunday morning. Small craft advisories are in effect for most of the
waters with the exception of the Northern Inland Waters and
Admiralty Inlet through tonight. A 14 to 17 foot swell train
continues to move onshore late this afternoon. This swell train will
slowly subside late tonight and Sunday. Small craft advisories are
in effect for the coastal waters and the western portions of the
Strait of Juan de Fuca due to a combination of wind and hazardous
seas.

A broad and weak surface low will move southward through the waters
on Monday with south to southeast flow gradually shifting to
northerly. As the low tracks south on Monday, a cold dome of high
pressure will move south through British Columbia. This will cause
the flow across Western Washington to become northerly on Monday.
Cold Fraser River outflow winds of 20 to 30 kt will likely develop
across the northern inland waters and Strait of Juan de Fuca late
Monday afternoon or evening, and gales are possible.

A strong Pacific front will approach the waters Wednesday night and
will move through the waters on Thursday. Gales are likely for most
waters except for Puget Sound and Hood Canal as this front
approaches. Albrecht

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory Olympics and Cascades of Whatcom and
     Skagit county.

     Winter storm warning Cascades from Snohomish county south to
     Lewis county.

PZ...Small Craft Advisory until 6 PM PST Sunday 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-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Small Craft Advisory for hazardous seas until 3 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-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 6 PM PST Sunday for
     Grays Harbor Bar.

     Small Craft Advisory until 9 AM PST Sunday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

     Small Craft Advisory until 2 AM PST Sunday for Puget Sound and
     Hood Canal.

&&

$$

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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