FXUS66 KSEW 180551
Area Forecast Discussion
National Weather Service Seattle WA
951 PM PST Sun Dec 17 2017
.SYNOPSIS...A cold front over southwest British Columbia will move
across the northwest part of the state overnight and the rest of
the area Monday morning. Expect a warm front to move across the
southwest part of the state Monday afternoon. A strong cold front
will move across the region on Tuesday and usher in a colder air
mass. High pressure aloft will prevail on Wednesday for dry and
A cold front over southwest British Columbia will move across the
northwest part of the CWA overnight and the rest of the area
Monday morning. Expect a warm front to move across the southern
part of the forecast area Monday afternoon. The main impact these
fronts will have on our weather will be to keep conditions wet
and unsettled. Locally gusty winds can also be expected ahead of
and with the approaching cold front. So far, Mount Baker has
received about 10 inches of new snow. Total new snowfall
accumulations of 6 to 11 inches are likely on the north Cascades
(King County northward) by late Monday morning.
Meanwhile, on the lowlands, expect rain at times through the
period although parts of the Puget Sound region will remain in the
rain shadow due to strong westerly flow downsloping off the
Olympic range. It looks like convergence zone will develop Monday
morning, so this will give parts of the north Sound and, perhaps,
the central Sound a better chance of decent rainfall.
A stronger system will impact the area Monday night into Tuesday
as a strong frontal system approaches and then moves inland. The
main impacts will be the potential for heavy snow in the
mountains. A Winter Storm Watch is in effect for the Olympics and
north Cascades where amounts of 1-3 feet are possible.
Winds will be gusty with the frontal passage on Tuesday,
especially over the coast and south Puget Sound where winds will
gust to 40 MPH.
A cold upper level trough will move across the region Tuesday
night into Wednesday, resulting in snow levels falling to near
1,000 feet. This means there could see a few light snow showers
in the Cascade foothills Wednesday morning. Other areas to watch,
in the lowlands, will be the Strait of Juan de Fuca and western
An amplified upper level ridge centered near 138W will provide
the region with dry, northerly flow aloft on Wednesday. 33/05
.LONG TERM FROM THE PREV DISCUSSION...
A new wrinkle in the pattern brings a threat of lowland snow
Thursday night and Friday. Models show the ridge offshore
retrograding while a shortwave trough tracks S/SE into Western
Washington. The cool setup is already here with the dry offshore
flow, temperatures in the lower 30s and snow levels near the
surface. Light accumulations are possible but exact details are
still unclear since this system is still several days out. This
system exits Friday afternoon with the threat of showers ending.
Otherwise, the high amplitude ridge offshore will continue to
bring dry and cool weather through next weekend and into Christmas
Day. Temperatures are trending below normal with lows in the 20s
and highs only in the 30s. 33
.AVIATION...A front draped across the area tonight will sag south
Monday and then lift back north as a warm front Monday night.
Westerly flow aloft will continue. The air mass is moist and
KSEA...MVFR low clouds are expected to be prevalent tonight and
Monday. Gusty south wind 10-20 knots will ease Monday morning.
.MARINE...A front over the area tonight will bring small craft
advisory strength winds to most waters. Gales in Puget Sound are
expected to end after midnight as winds begin to ease. A westerly
surge is expected in the Strait of Juan de Fuca later tonight.
The front will sag south Monday with winds generally easing. A
second vigorous frontal system will move through the area Monday
night and Tuesday with a warm front lifting north through the area
Monday night and a cold front shifting inland Tuesday.
An associated deepening surface low will move across the area
Tuesday. This will likely bring gales to at least some waters.
Models differ on the track of the low which will affect which
areas get the strongest winds and makes the wind forecast a bit
tricky. For now, the expectation is the low will make landfall
along the north coast with the strongest winds to the south.
Offshore flow will develop Wednesday and continue Thursday and
HYDROLOGY...River flooding is not expected the next 7 days.
WA...Winter Weather Advisory until 10 AM PST Monday for Cascades of
Snohomish and King Counties-Cascades of Whatcom and Skagit
Counties above 3500 feet.
Winter Storm Watch from Monday evening through late Tuesday
night for Cascades of Whatcom and Skagit Counties above 2500
Winter Storm Watch from Monday evening through Tuesday afternoon
for Olympics above 3000 feet.
PZ...Gale Watch from late Monday night through Tuesday afternoon for
Coastal Waters From James Island To Point Grenville 10 To
60 Nm-Coastal Waters From James Island To Point Grenville
Out 10 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
To Cape Shoalwater Out 10 Nm.
Small Craft Advisory until 4 AM PST Monday for Coastal Waters
From Cape Flattery To James Island 10 To 60 Nm-Coastal
Waters From Cape Flattery To James Island Out 10 Nm-Coastal
Waters From James Island To Point Grenville 10 To 60 Nm-
Coastal Waters From James Island To Point Grenville Out 10
Nm-Coastal Waters From Point Grenville To Cape Shoalwater
10 To 60 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater Out 10 Nm-Northern Inland Waters Including The
San Juan Islands-West Entrance U.S. Waters Strait Of Juan
Small Craft Advisory until 10 AM PST Monday for Central U.S.
Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
Strait Of Juan De Fuca.
Small Craft Advisory from 2 AM to 10 AM PST Monday for Admiralty
Small Craft Advisory until 6 AM PST Monday for Puget Sound and
Gale Warning until midnight PST tonight for Puget Sound and Hood
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