Heavy Snow in Washington, December 3 to 4, 2016

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Sat Dec 3 2016

.SYNOPSIS...Scattered showers will prevail today ahead of the next
weather system. A vigorous cold front will arrive tonight, followed
by a cold upper trough Sunday through Tuesday. Snow levels will fall
to around 500 feet with snow possible down to sea-level at times
Monday and Monday night. Cold and mainly dry weather develops
Tuesday. A system arriving Wednesday night and Thursday could bring
some low elevation snow before warm air brings rising snow levels.

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.SHORT TERM...Radar shows a few scattered showers around western
Washington this morning, mainly near the coast and over the
mountains. Low level instability is weak and the air mass is
marginally moist below 700 mb so most measurable precipitation today
will be along west facing slopes of the Olympics and Cascades due to
some westerly orographic lift. There will be breaks in the stratus
around Puget Sound but high level clouds will be increasing with
highs in the upper 40s.

Satellite imagery shows the confluent westerly upper level flow
pattern over the eastern Pacific directed at the Pacific Northwest.
500 mb temperatures around -40c associated with the upper low over
the Gulf of Alaska will drive southeastward today as moisture along
the baroclinic boundary heads toward Washington. The associated
surface front will out run the coldest air aloft tonight, pushing
across the area later this evening. However, moist westerly flow
will continue overnight into Sunday morning as the colder air aloft
arrives. Precipitation will begin to fill in late this afternoon
across the area with rain and mountain snow over the entire area
tonight. Snow levels will fluctuate before crashing below all pass
levels after midnight. Lower passes like Snoqualmie may see rain and
snow mixed limiting snow amounts there until changing to all snow
later tonight. Paradise, Mount Baker and higher ski resorts should
be mostly snow and even Stevens pass will probably stay snow.
Moisture will shift south of the Olympics and north cascades sooner,
so those areas may only get advisory levels snow amounts, less than
18 inches in 24 hours. The warning looks good for the Cascades of
King/Snohomish county, with Stevens pass likely to get over a foot.
Snoqualmie will probably see half that amount. May need to upgrade
the advisory for the Cascades of Pierce/Lewis counties as Paradise
seems likely to see over a foot in 12 hours or 18 inches in 24
hours.

Rain and mountain snow changes to showers late Sunday morning and
afternoon with unstable conditions during the day. The coldest air
aloft infiltrates the area Sunday night with 500 mb near -38C over
Washington just ahead of a stacked closed upper low and weak surface
low off the central/south B.C. coast. Models are in good agreement
of taking a lee side low off Vancouver Island south to off the north
Washington coast by Monday morning. A surface trough extends south
over Western Washington generating precipitation across the area as
snow level fall near 500 feet. 1000-850 mb thickness values fall to
near 1290 m which would usually be cold enough for snow down to sea-
level in offshore or light flow. However, the other main closed low
will still be off the B.C. coast with light south flow below 850 mb.
This will likely cause precipitation to fall as wet snow or a
rain/snow mix Monday morning below 500 feet. The warm ground will
also make it difficult for snow to accumulate except locally on some
grassy areas with heavier showers. An inch or two of wet snow could
fall on hill tops above 500 feet but even that will be mostly on
grassy surfaces, so not much impact expected as far as roads are
concerned. The exception is north of Seattle where convergence zone
activity is possible and the air a bit colder. Light accumulations
could be possible from Snohomish county east of I-5 Monday.

The main surface low consolidates and moves over southwest
Washington late Monday afternoon. A 1040-1045 mb surface high over
B.C. associated with a much colder low level air mass will cause the
flow to turn more northeasterly. Fraser outflow could develop and
increase Monday night, although models differ on how much outflow
develops. This could bring a reinforcing shot of cold air and lower
dew points into the lowlands which could squeeze out any residual
moisture in form of snow down to sea-level. As far as the greater
Seattle-Tacoma metro areas, Monday night night be the best chance
for light accumulations. This could impact the Monday evening or
Tuesday morning commutes, but uncertainty on the chance of lowland
snow remains low at this time. Forecasts should be monitored as we
fine tune details closer to the potential period of snow. Low
temperatures by Tuesday morning do look to fall below freezing,
upper 20s in the metro areas and low 20s for outlying areas. Black
ice will be a potential hazard by Tuesday morning. Mercer

.LONG TERM...Previous discussion...Extended models in good agreement
early on with the upper level trough digging south Monday night with
the precipitation decreasing significantly as the flow aloft turns
northerly.

Upper level ridge building offshore Tuesday with dry northerly
flow over Western Washington. The air mass will remain cold so
even with the sunshine highs will only be in the 30s and lower
40s.

Upper level ridge moving over Western Washington on Wednesday for
another cool and dry day. Differences in the models how up late in
the period with the GFS faster and warmer with the system on
Thursday versus the ECMWF. MOS forecast highs are up to 10 degrees
cooler on the ECMWF on Thursday versus the GFS forecasts. The
issue late next week will be the transition out of the cool air
mass. The GFS moderates the air mass quickly with the system on
Thursday with 850 mb winds southwesterly at 30 to 45 knots leading
to a relatively short period of a snow to rain transition and
maybe none at all with the warm air scouring out the cool air
quickly. The slower and cooler ECMWF is weaker with the system
leading to a better chance of the onset of the precipitation being
in the form of snow in some of the lower elevations. Right now
the forecast is more in line with the GFS solution. Both models
have the warmer air mass over the area on Friday for a return to
normal temperatures. Felton

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 6 PM this evening to 2 PM PST
     Sunday for Cascades of Pierce and Lewis Counties.

     Winter Storm Warning from 2 PM this afternoon to 2 PM PST Sunday
     for Cascades of Snohomish and King Counties.

     Winter Weather Advisory from 2 PM this afternoon to 2 PM PST
     Sunday for Cascades of Whatcom and Skagit Counties.

     Winter Weather Advisory from 2 PM this afternoon to 6 AM PST
     Sunday for Olympics.

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www.weather.gov/seattle

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