Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 101757
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
957 AM PST Sat Dec 10 2016

.SYNOPSIS...Showers are expected again today through Sunday. High
pressure building over British Columbia will spread cold air
southward across the area on Monday. A wintry mix of
precipitation is possible as the cold air mass spreads south on
Sunday night and Monday morning. Dry northerly flow aloft next
week will maintain a cold dry weather pattern from Tuesday through
the end of next week.

&&

.SHORT TERM...The main forecast challenge through Monday relates
to timing, placement, and amounts of snow.

Radar currently shows a broad swath of showers spreading in from
the ocean onto the Olympic Peninsula. These showers will continue
to spread eastward this afternoon and evening. For the most part,
snow levels will rise to near 2000 feet this afternoon and
tonight. The Bellingham area is currently the lowland exception
with its colder northeast breeze at the surface. Even there, once
a southeast wind kicks in, precip will switch to rain.

As the showers pick up later today, it will come as another round
of heavy snow for the mountains. Looks like snow will pick up
after mid-afternoon over the Cascades and peak in intensity this
evening. With over 6 inches of snow at Stevens and Snoqualmie
Passes in a 12-hour period, have recently hoisted a Winter Weather
Advisory for the Cascades. The Olympic Mountains were a close
call, but the recently updated grids eek Hurricane Ridge out just
under advisory criteria. Advisory or not, it will be a winter
wonderland up at Hurricane Ridge this afternoon. Onshore flow will
continue to produce some showers on Sunday morning, but coverage
and precip amounts should be a step down from the more organized
shower activity tonight.

Arctic high pressure will spread southward through the interior of
B.C. tonight. Once the Bellingham-Williams Lake BC pressure
gradient strengthens to -12 mb, cold Fraser Outflow will begin
near Bellingham. This should happen during the day on Sunday. 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 marking the leading edge of the cold
continental air coming out of B.C.. 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 with Thursday night`s snowy system. Snow on Sunday
night and Monday will be more focused along boundaries and 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. The latest GFS dries the air mass out quickly enough to show
little or no lowland snow, while the NAM and ECMWF linger enough
precip long enough in the cold air to produce a few stripes of 1-3
inch snowfall amounts over the lowlands. The hard part this far in
advance is figuring out where those "stripes" of snow will set up.
Will probably take until tomorrow to figure out that part with any
degree of confidence. Haner

.LONG TERM...The long term forecast looks fairly simple. A couple
of deep upper level troughs over the center of the country will
keep any precip either south or east of the CWA for the duration
of next week. The good news is no rain or snow and might even see
the sun a few days. The bad news is its going to be cold with
daytime highs in the mid 30s and overnight lows in the mid to
lower 20s. SMR

&&

.AVIATION...an upper level trough will persist over the region
through Sunday. A front approaching the coast will push through the
interior this afternoon. Moderate southwest flow aloft becoming
westerly sunday. Moist and generally stable. Light rain developing
through midday, becoming showery late in the afternoon and
decreasing this evening. Mixed snow/rain possible at CLM and
snow/freezing rain possible at BLI, then all rain showers this
afternoon. Cigs will vary but generally MVFR expected through
tonight.

KSEA...Light rain becoming showers this afternoon. A front will pass
around midday. southeast wind 10 to 14 kt w/ gusts 22 kt, becoming
south 10 to 12 kt this afternoon. VFR becoming mostly MVFR this
morning through tonight.

&&

.MARINE...A front near the coast will push across the inland waters
this afternoon. The 999 mb surface low of Vancouver Island will fill
and move across the northern inland waters this evening. 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.

&&

.HYDROLOGY...River flooding is not expected for the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 2 PM this afternoon to noon PST
     Sunday for Cascades of Pierce and Lewis Counties-Cascades
     of Snohomish and King Counties-Cascades of Whatcom and
     Skagit Counties.

PZ...Gale Warning until 10 PM PST this evening for Coastal Waters
     From Cape Flattery To James Island 10 To 60 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.

     Small Craft Advisory for rough bar until 10 AM 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 Cape Flattery To James Island Out 10 Nm-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.

     Small Craft Advisory until 6 PM PST this evening for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

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