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Area forecast discussion
National Weather Service Seattle Washington
400 PM PST Sat Dec 10 2016

Synopsis...a passing upper trough will produce cold and showery
weather through Sunday. Later Sunday into Monday, a cold front
will move south through western Washington, ushering in a much
colder air mass with its origins well to the north. This front
could produce a few showers and spotty lowland snow accumulations.
A cold and dry air mass will then stay in place into next weekend.

&&

Short term...the main forecast challenge through Monday relates
to timing, placement, and amounts of snow.

We are currently on the cold side of the jet stream, which is
pointed into northern California. Here on the cold side, a
shortwave will make its way across western Washington tonight,
with plenty of convective showers and a turn to onshore flow. This
is an excellent pattern for bringing heavy snow to the mountains,
and cold temperatures will help to increase snow ratios and
accumulations as well. Suffice it to say that a trek across the
passes will be very difficult tonight and Sunday morning. Though
showers will back off a little on Sunday, they will continue to
add to already- hefty accumulations through Sunday night. Snow
levels will average around 1500 feet through Sunday, then start to
lower further on Sunday night.

Arctic high pressure will spread southward through the interior of
b.C. Tonight. Once the Bellingham-Williams Lake British Columbia pressure
gradient strengthens to -12 mb around Sunday afternoon, cold
Fraser outflow winds will begin near Bellingham. Have hoisted a
high wind watch for the Bellingham area and San Juan Islands.
Fraser outflow will usher in cold Continental air from northwest
Canada and Alaska. The leading edge of the cold air will spread
southward and coastward from Sunday evening through Monday. In
addition, on Sunday night and Monday morning, models bring a
weakening surface low east-southeast across the forecast area,
which could enhance lift and precip along a southward-moving
boundary. This will pose the potential for some lowland snow,
particularly up and down the interior lowlands. This type of
pattern typically produces spottier snowfall accumulations, in
contrast to the widespread accumulations that came with Thursday
night's and friday's snowy system. Snow on Sunday night and
Monday will mainly result from flare-ups of convective showers
along the southward-moving cold front and perhaps in bands
related to the progressive and weakening low center. Also unlike
the more recent snow with its gradual and painfully slow
transition from snow to rain, the next chance of lowland snow will
come with cooling temperatures and a possible change from rain to
snow. About the only detail-related consensus so far is that the
Bellingham area and the San Juans could see an inch or two of snow
on Sunday night. Elsewhere, few spotty stripes of 1-3 inch
snowfall amounts should occur over The Lowlands, separated by
areas that get no accumulation. Basically, the snowfall will be
driven by smaller-scale atmospheric features that will be hard to
really nail down with certainty until a few hours ahead of time.

The air mass will really dry out later Monday and Monday night as
cold Continental air overspreads the entire forecast area. By
Tuesday, the main story will be how cold it is. Any snow
accumulation left on the ground will freeze hard during the
nighttime hours. Haner

Long term...an upper ridge over Alaska and an upper trough near
Hudson Bay will continue to funnel cold and dry air down the
Fraser River canyon on Wednesday, so the air mass over western
Washington will not be able to modify or warm. Beyond Wednesday,
most confident part of the forecast is that temperatures will
remain well below normal.

What is uncertain is how the models deal with southern stream
energy or the possibility of impulses moving southeast from the
Alaska Panhandle. While the 12z and 18z GFS holds back any precip
until at least next weekend, the 12z European model (ecmwf) and Canadian bring a
low center southeast off the coast. Precip would overrun the cold
air in place over the Pacific northwest and bring yet another
round of lowland snow. So it bears watching to see what model
consensus eventually develops. For now, the official forecast
reflects a low chance of snow on Thursday night and Friday.
Haner

&&

Aviation...upper level low pressure will dominate the region
through Sunday. Moderate southwest flow aloft becoming westerly
Sunday. Moist and generally stable. Showers. Mostly MVFR cigs with a
mix of VFR and MVFR Sunday ahead of strong Fraser outflow.

Fraser outflow developing by Sunday evening over Whatcom County will
spill out into much of the north interior later Sunday night and
Monday. A modified Arctic front may develop Sunday night and push
south through Puget Sound sometime on Monday. A wintry mix or snow
may develop at the bli and kclm terminals late in the day or evening
Sunday, eventually reaching pae after midnight. Monitor forecasts
closely as details are fine tuned and some potential for snow exists
Monday at other terminals.

Ksea...showers...decreasing later tonight. Mainly MVFR with periods
of VFR Sunday. South wind 5 to 10 kt.

&&

Marine...a 1000 mb low will cross the northern inland waters
tonight. Expect increased onshore or westerly flow behind it.
Onshore flow will weaken on Sunday. Anticipate a weak modified
Arctic front to move south across the northern inland waters Sunday
night. Offshore flow will develop later Monday through the week.

&&

Hydrology...river flooding is not expected for the next 7 days.

&&

Sew watches/warnings/advisories...
Washington...Winter Storm Warning until 4 am PST Monday for Cascades of
Pierce and Lewis counties-Cascades of Snohomish and King
counties-Cascades of Whatcom and Skagit counties.

High wind watch from Sunday afternoon through Monday afternoon
for San Juan County-western Whatcom County.

Winter Weather Advisory until noon PST Sunday for Olympics.

Pz...Small Craft Advisory for rough bar until 4 PM PST Sunday for
Grays Harbor bar.

Small Craft Advisory until 6 am PST Sunday for Admiralty Inlet-
central U.S. Waters Strait of Juan de Fuca-coastal waters
from James Island to Point Grenville out 10 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.

Gale Warning until 4 am 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 Point Grenville to Cape Shoalwater 10 to 60 nm.

Gale watch from Sunday afternoon through Monday afternoon for
northern inland waters including the San Juan Islands.

Small Craft Advisory until 4 am PST Sunday for West Entrance
U.S. Waters Strait of Juan de Fuca.

&&

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