Heavy Snow in Washington, December 1 to 2, 2017

Area Forecast Discussion
National Weather Service Seattle WA
300 AM PST Fri Dec 01 2017

.SYNOPSIS...A large upper level low pressure system over the
northeast Pacific will send a couple of weather systems across
Western Washington bringing more rain today and Saturday. The low
pressure aloft is expected to move onshore late Saturday and inland
Sunday. Strong high pressure aloft is forecast to build over the
region next week for dry weather with areas of night and morning fog.

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.SHORT TERM...Amplification of the upper flow pattern across the
northern Pacific is underway. A deep upper low in the SW Bering Sea
was helping pump up the upper ridge near 160W early this morning. As
a result, the Gulf of Alaska upper trough was translating eastward.

Two shortwaves embedded in the flow around the upper trough this
morning. The first had moved inside 130W and in the left exit region
of a 120 kt+ jet and difluent flow aloft, really blossomed with an
expanding baroclinic zone and cooling cloud tops. The progs agree on
bringing this system onshore today and inland this evening for rain
spreading SE through the area through the day. There is enough
instability and lift with this system for a threat of tstms along
the coast and offshore. Will continue the winter weather advisory
for the Olympics and N Cascades for today. Snow amounts further
south to be lighter as this weather system weakens, likely in the 2
to 5 inch range with snow levels around 2500 feet.

IR satellite imagery showed the following shortwave near 145W in the
bottom of the upper trough early this morning. This shortwave
approaches the coast late tonight with showers spreading inland
Saturday. This system has the coolest air mass,and again instability
and difluent flow aloft for a threat of tstms along the coast and
offshore, collaborated with NWS Portland.

By Sunday, the upper trough moves further inland and northerly flow
aloft develops across Western Washington as an upper ridge builds
offshore. Showers should taper off Sunday with some limited
clearing. The day will be a cool one with highs only in the lower to
mid 40s. Buehner

.LONG TERM...As the flow across the north Pacific continues to
amplify, the upper ridge builds over Western North America Monday
into mid week for a dry period. Low level flow turns offshore
Tuesday resulting in clearing skies. Warming aloft with freezing
levels rising above 9000 feet should gradually create a temperature
inversion with a growing trend of areas of night/morning fog and
eroding air quality. Some foggy areas may not break out in the
afternoons by Thursday. Chilly nights with some areas dropping below
freezing under clearing skies. With higher surface pressure
developing east of the Cascades, will likely have some east winds
thru the passes as well. High temperatures may have a wide range
later next week - mid and upper 40s in the clear and wind mixed
areas while sheltered foggy spots struggle in the 30s. Buehner

&&

.AVIATION...A front will weaken and push through Western Washington
today. An upper low will move over the area tonight and Saturday.
Moderate southwest flow aloft. Air mass stable and becoming moist at
levels this morning.

Cigs have improved around much of Puget Sound early this morning
with mostly VFR cigs but areas of fog has developed in some lower
lying locations including the south and west part of the Sound and
southwest interior (including PWT, OLM). As the front pushes inland
this morning more mixing should cause fog to mostly lift, but MVFR
cigs will become more widespread as light rain reaches most of the
interior mid to late morning into the afternoon hours.

KSEA...South wind 7-10 kt, becoming southwest 10-12 kt gusting 20 kt
this afternoon and evening before easing. Stratus has scattered out
and lifted temporarily with increasing mid/high clouds early this
morning. Rain along the north coast should push inland this morning
and reach the terminal around 18Z with MVFR cigs prevailing. The
front will move through around 22z and MVFR cigs will probably
continue into tonight. dtm

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.MARINE...A front along the north coast will weaken as it pushes
southeast across the waters today. Small craft winds are expected in
most waters but will be spotty at times with periods of lighter
winds. combined seas will remain high around 14 to 16 ft over the
coastal waters and around 12 to 14 ft at the west entrance through
tonight.

A weaker system will move across the forecast area Saturday. A
strong ridge will build over the area Sunday through Tuesday.
dtm

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.HYDROLOGY...No river flooding is expected for the next 7 days.

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.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory all waters except the central Strait.

     Small Craft Advisory coastal waters and west entrance tonight.

     Small Craft Advisory for rough bar through Saturday morning.

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