Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 201124
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
324 AM PST Mon Nov 20 2017

.SYNOPSIS...A front will move slowly east today, with rainfall
decreasing across Western Washington this afternoon. A strong warm
front will bring lots of rain Tuesday and Tuesday night, with the
snow level above 8000 feet. Wednesday will be less wet, then
another front moves through Thanksgiving Day. Friday and Saturday
look relatively dry, with a front arriving late in the weekend.

&&

.SHORT TERM...A southwest-northeast oriented front continues to
move slowly eastward across Western Washington early this morning.
The front has deep moisture and a warm air mass associated with it,
which is being carried into the forecast area by strong southwest
winds aloft. Not surprisingly, it has dropped 2.5 to 4+ inches of
rain over the southwest facing slopes of the Olympic Mountains.
The slow eastward progression of the warm air and easterly flow
across the Cascades kept precipitation there in the form of snow
until early this morning, with 10 inches of snow at Paradise and
13 inches of snow at Mount Baker since Sunday afternoon. Radar
showed rain still falling over much of the area at 3 am, with
lowland temperatures in the 40s to lower 50s and winds still
gusting into the 20 to 30 mph range in a few spots.

The front will continue its slow eastward progression today. The
parent upper low of the frontal system is offshore of Vancouver
Island near 49N/128W early this morning, and a shortwave is
rounding the base of the trough; this feature will move northeast
across the area through midday today, which should help to
prolong precipitation along the back side of the main frontal
band. Precipitation will generally decrease across the forecast
area this afternoon, though, as the upper low weakens into an open
trough and moves into southwest British Columbia. The Puget Sound
convergence zone could form this afternoon as the upper trough
moves inland and low level flow veers to westerly.

A minor upper trough will progress quickly from the offshore
waters across Western Washington tonight. The leading edge of the
next round of rain - also a region of deep warm moisture - will
spread quickly into the forecast area late tonight and early
Tuesday morning. This system will bring significant precipitation
to the area Tuesday and Tuesday night, with the heaviest
precipitation likely in the afternoon and evening as the warm
front lifts north across the area. It will drive the snow level
quickly to 8000 to 9000 feet, with storm total precipitation
totals of 2 to 5 inches likely over the Olympics and at Mount
Rainier, and roughly 1 to 3.5 inches over the Cascades further
north. Breezy conditions are likely especially at the coast and in
the north interior.

Warm moist southwest flow aloft will continue Wednesday. The snow
level will be around 9000 feet, but precipitation amounts look
lighter. This will be just a short break, though, before another
wet system arrives later Wednesday night. McDonnal

.LONG TERM...The next frontal system will bring another round of
rain to Western Washington late Wednesday night and Thanksgiving
Day. The rainfall doesn`t look as heavy with this feature as with
the Tuesday and Tuesday night system, but the snow level will
still be 6000 to 7000 feet. Fairly moist southwest flow aloft will
continue over the region Friday and Saturday; it doesn`t look dry
for these couple of days, but rather less wet. Models agree
generally that another frontal system - also with fairly high snow
levels - will arrive Saturday night or Sunday. McDonnal

&&

.HYDROLOGY...In the very short term, the only river that faces a
threat of flooding is the Skokomish River in Mason County. It
should crest late this morning close to flood stage - maybe a
little above but maybe just below. At this point, I will sit on
the flood watch and issue a short-term warning if needed.

The threat of flooding will become more widespread, though, as
warm wet frontal systems move through Tuesday/Tuesday night and
again Wednesday night/Thursday. The first system looks more
significant with 2 to 5 inches of rain over the Olympics and Mount
Rainier and 1 to 4 inches further north over the Cascades, and
with the snow level 8000 to 9000 feet. The second system won`t
bring as much rain, but the snow level will be 6000 to 7000 feet.
We issued a flood potential outlook yesterday, and I will update
it this morning.

The Skokomish will almost certainly flood Tuesday or Tuesday night.
Northwest River Forecast Center guidance also shows several other
forecast area rivers cresting near or just above flood stage
including the Cowlitz, Nisqually, Snoqualmie, Skagit, Nooksack,
and Dungeness.

Landslides could also become a threat with the lowland rains
Tuesday and Tuesday night. The USGS landslide guidance shows one
or two stations near or just above the threshold of concern, and
of course the additional rain with that front will drive that
higher. McDonnal

&&

.AVIATION...A cold front will move slowly southeast across the
area early this morning. In its wake, light to moderate northwest
flow at the lower levels will develop. Flow will become
increasingly offshore later tonight as a strong warm front
approaches the area from the southwest.

An upper level low west of Vancouver Island early this morning
will move eastward along the Canadian border today. Moderate
southwest flow aloft this morning will become westerly as the
upper low passes by to the north. The air mass is moist and
stable. It will gradually dry later this afternoon through this
evening, then mid and upper level moisture will rapidly increase
late tonight.

The cold front extends from about KBLI to KHQM at 11Z. Radars
show a band of rain extending about 75 miles north of the frontal
system as it pushes slowly southeastward. Ceilings and
visibilities are MVFR to locally IFR in the rain band and in
rain to the southeast of the front. Expect conditions to improve
to VFR by later this afternoon as showers become increasingly
confined to the terrain and to a Puget Sound Convergence zone from
around KPWT to north of KSEA. Conditions will become VFR this
evening with clouds above about 050. Albrecht

KSEA...CIGS between 020 and 025 will fall to around 015 this
morning as the front moves slowly southeast across the terminal.
Conditions will only slowly improve this afternoon as convergence
zone activity hangs up north of or near the terminal. There may be
periods of light opposing winds around the terminal area
associated with the convergence zone. Ceilings will slowly lift
and become mainly VFR later this afternoon through tonight. Winds
at the terminal will be mainly light southeasterly today and will
shift to light northerly tonight. Albrecht

&&

.MARINE...A cold front from the Northern Inland Waters
southwestward into the South Washington Coastal Waters will move
slowly southeastward this morning. Behind the front, expect light
to moderate northwesterly winds with Small Craft Advisory wind
conditions limited to the central and eastern portions of the
Strait of Juan de Fuca. Small craft advisories will be maintained
over the coastal waters for today as steep short period swell 14
to 16 feet gradually subsides.

A weak high pressure center will pass across the waters this
evening. Then a strong warm front will push northward through the
waters on Tuesday. Gale force easterly winds are possible on
Tuesday at the West Entrance to the Strait of Juan de Fuca and
the northern portions of the coastal waters.

The warm front will be followed by southerly flow on Wednesday.
Strong low pressure over the northeast Pacific will maintain
southerly flow on Thursday and Friday. Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch until 10 AM PST this morning for Hood Canal Area-
     Lower Chehalis Valley Area-Olympics.

     Winter Storm Warning until 10 AM PST this morning for Cascades
     of Whatcom and Skagit Counties.

PZ...Small Craft Advisory for hazardous seas until 3 PM PST this
     afternoon 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.

     Small Craft Advisory for rough bar until 3 PM PST this afternoon
     for Grays Harbor Bar.

     Gale Watch from late tonight through Tuesday afternoon for
     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 Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory for hazardous seas until 9 AM PST this
     morning for West Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Small Craft Advisory until 7 AM PST this morning for Admiralty
     Inlet.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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