High Wind in Washington, December 12, 2016

Area Forecast Discussion
National Weather Service Seattle WA
416 AM PST Mon Dec 12 2016

.SYNOPSIS...Showers will generally decrease today and tonight as
cold air spreads south across Western Washington. Local lowland
snowfall of an inch or so will be mainly limited to the northwest
interior. Dry cold weather will prevail Tuesday and Wednesday. A
chance of snow will return Thursday and Friday as a weather system
passes to the south. Temperatures will moderate next weekend.

&&

.SHORT TERM...Westerly flow continues aloft across Western
Washington early this morning, with a weak shortwave moving inland
from the coastal waters. Scattered showers continue across the
forecast area. Surface high pressure over British Columbia is
pushing cold air at low levels into the northwest interior -- ie
Western Whatcom county and the San Juans -- by way of the mouth of
the Fraser River. Precipitation at Bellingham changed to snow a
little before midnight, and at 3 am the temperature was 31 degrees
with northeast wind gusting to 30 mph. Across the remainder of the
Western Washington lowlands, the 3 am temperatures were in the mid
30s to lower 40s.

The weak shortwave will move across the area and east of the
Cascades late today, with a flat weak upper ridge moving from the
offshore waters into the Pacific Northwest late today and tonight.
So showers will generally decrease today and tonight, but there
will probably be a west-east convergence band that gradually moves
southward and weakens through this evening. Low level cold air
will continue to spread west out the Strait of Juan de Fuca and
southward across the interior lowlands. But the tendency for
showers to decrease means that many areas, especially from roughly
Seattle southward, will have little precipitation by the time it
gets cold enough for precipitation to fall as snow.

So the best chance for snow accumulations today -- mainly a trace
to 1 inch -- is from around Everett northward, especially on
higher terrain away from the water. Local accumulations up to 2
inches are likely in Western Whatcom county, and the winter
weather advisory there will remain in effect this morning. Seattle
and the rest of the south Puget Sound region could see local
accumulations of a trace to an inch of snow late today or this
evening, mainly on higher terrain away from the water.

As for the Fraser River outflow, some areas north of Bellingham
have northeast winds of 25 mph sustained with 40 mph gusts. The
Williams Lake BC to Bellingham pressure difference appears to have
peaked at 13.7 mb at 2 am. It will be windy there for the morning
hours, but below the criteria for a wind advisory so we will end
that.

In the mountains, showers will continue today and tonight but
generally decreasing. There will probably be another 2 to 6 inches
of snow, with the snow level 1500 to 2000 ft today falling to 500
to 1000 ft tonight. The winter storm warning will end as scheduled
at 4 am this morning.

Drier northwest flow aloft will develop Tuesday, with dry cold
northeast flow continuing at low levels. It will be a dry day with
highs in the 30s to lower 40s, but it could take a while for the
cloud cover to clear out especially in the south part. It should
be cold and mostly clear Tuesday night with overnight lows below
freezing across the forecast area. Wednesday should be mostly
sunny and cool with highs in the 30s. McDonnal

.LONG TERM...On Wednesday morning an upper low will be well
offshore of Washington. It will dig south or southeast Wednesday
and Wednesday night, causing the upper ridge over the Pacific
Northwest to build a bit. A frontal system will develop in the
southwest flow between the low and the ridge, probably moving onto
the southern Oregon or northern California coast late Wednesday or
Wednesday night. The precipitation associated with this system
will spread northward, probably moving into our forecast area late
Wednesday or more likely Thursday or Thursday night. The air mass
over the area should be cold enough for lowland snow. If you've
been reading our forecast discussions recently, then you know that
model agreement and run-to-run consistency have been poor. So
forecast confidence in how this will play out is necessarily low.
The chance for snow will probably linger into Friday, but it looks
like drier northwest flow will develop as the system moves east.
A rather weak upper shortwave in the northwest flow could move
across the area this weekend. McDonnal

&&

.AVIATION...Light northwest flow aloft will prevail over the
region today. A weak modified arctic front currently just north of
the British Columbia/Washington border will move slowly south,
reaching the south Puget Sound Region late today or early this
evening. Expect northerly flow behind the front. The precip should
transition over to snow showers or showers of rain mixed with snow
with the frontal passage. The precip will end behind the front due
to drying of the air mass.

KSEA...CIGs should generally be in the MVFR category through this
evening; although, CIGs will probably lift into the VFR category
for brief periods through early this afternoon. Southerly winds
will become northerly late today. Expect a few gusts near 20 knots
with or in the vicinity of showers this morning.

Meanwhile, there is a chance of rain mixed with snow late this
afternoon, and a short period of light snow showers this evening.
Not expecting the snowfall to accumulate (a light dusting at
best).

&&

.MARINE...
A 1012 mb low northwest about 64 nm northwest of Cape Flattery
will remain quasi-stationary while slowly weakening. This will
allow weakening onshore flow to become offshore with time.

Meanwhile, a weak modified arctic front near the British
Columbia/Washington border will move slowly south during the day
today, and is not expected to reach southwest Washington until
sometime tonight. Light to moderate flow will prevail behind the
front and then persist through midweek.

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 10 AM PST this morning for Western
     Whatcom County.

PZ...Small Craft Advisory until noon PST today for 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 until 6 PM PST this evening for Central
     U.S. Waters Strait Of Juan De Fuca-Coastal Waters From Cape
     Flattery To James Island 10 To 60 Nm-Coastal Waters From
     Cape Flattery To James Island Out 10 Nm-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 PM PST this afternoon for East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning until 3 PM PST this afternoon for Northern Inland
     Waters Including The San Juan Islands.

&&

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