Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 110522
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
922 PM PST Sat Dec 10 2016

.SYNOPSIS...A passing upper trough will produce cold and showery
weather through Sunday. Later Sunday or Sunday night 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...Well, it looks like the forecast challenges
mentioned by the earlier forecast shift only grow on this one with
models showing poor run-to-run and model-to-model consistency in
the all important fine details. Previous model runs and their
ensembles generally insisted on cold and dry weather dominating
this week. Ensembles from early this morning and before averaged
on an inch or less snowfall accumulation in most of the lowlands
as the arctic air moved in sometime between late Sunday afternoon
and Monday morning. The 18Z GFS and NAM along with the 12Z
Canadian and ECMWF solutions started introducing a bit more
moisture into the mix for late Sunday night and Monday in the
central portions of Western Washington during the transition (but
the 18Z GFS was on the wettest end of the ensemble mix).

The 00Z NAM shows a period from 12Z-18Z Monday where some mixed
precipitation could move through the central Puget Sound area but
with marginal temperatures. The 00Z GFS is much wetter. It is
also cold enough to provide the potential for a significant snow
event for the north slopes of the Olympics (like around and east
of Port Angeles) late Sunday night, for the area around Snohomish
and North King county Monday morning and for the central sound
area including Seatac Monday afternoon and early evening. The
arctic air in the latest solutions is overall slower to move
southward.

Is the 00Z GFS for real? Well, it is really hard to say. Satellite
imagery over the eastern Pacific shows a rather complex pattern
with numerous swirls rotating around a mean upper center along
135-140W. Very cold air is spilling offshore in southeastern
Alaska. ASCAT data earlier showed the low level wind fields for
the ECMWF and GFS to appear similar and rather well initialized
(not completely perfect with the satellite data but good). It
appears that the very small model differences could be
contributing to some instability in the solutions. Another source
for model differences could be due to low pressure development
off Vancouver Island due to cold air outflow to the northwest. Any
solution at this point appears plausible.

With all that said, I would like to see another run or two with
this solution before biting off and issuing any watches or
warnings. We do have a high wind watch for the San Juans and
Western Whatcom county starting tomorrow afternoon and lasting
into Monday afternoon. This may be a tad fast, but it appears
reasonable given the developing Williams Lake to Bellingham
pressure gradients starting Sunday afternoon. We have Winter
Storm Warnings out for the West Slopes of the Cascades through 4
AM Monday. This looks to verify nicely tonight into Sunday and
possibly into Sunday night.

Current forecasts look good for now. No updates are planned at
this time. Albrecht

.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 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. The 00z GFS is even more robust in bringing
moist southern stream air northward into Western Washington
showing several inches of snow for Thursday afternoon and Thursday
night. 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. Albrecht/Haner

&&

.AVIATION from the previous discussion...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...Mainly MVFR CIGS with periods of VFR. Winds will remain
sly. Mercer/05

&&

.MARINE...
Onshore flow will weaken during the day Sunday. Anticipate a
modified arctic front to move south across the northern inland
waters Sunday night, and then slowly across the rest of the area
on Monday. This will result in increased northerly flow.

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...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 Advisories are in effect for all waters except
 the Admiralty Inlet.

  Small Craft Advisory for rough bar in effect until 4 PM PST
 Sunday for the Grays Harbor Bar.

&&

$$

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


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