Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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068
FXUS66 KSEW 212247
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
247 PM PST Tue Nov 21 2017

.SYNOPSIS...A strong warm front, fed by a plume of warm
subtropical moisture will continue to bring rain to Western
Washington this evening. The snow level will be near 8000 feet.
After a lull in the rain on Wednesday, another wet frontal system
will move through the region Wednesday night through Thanksgiving
morning. After another break on Friday, another system will reach
the area Saturday afternoon and night. This active weather pattern
will continue into early next week.

&&

.SHORT TERM...RADAR remains lit up this afternoon by rain from
a wet warm front over the area. IR imagery shows a narrow tap
into subtropical moisture E of Hawaii. The heaviest rain should be
falling this afternoon with the RADAR showing large areas of
moderate rain over the mountains. A back edge to the precip is
apparent on the coastal RADAR, so rainfall rates over the Olympics
and coast should be decreasing by 4 PM.

Precipitation is expected to diminish, but not end completely,
across W WA this evening, as a slight shift in the flow aloft
directs the weakening narrow moisture plume over Oregon. The
subtropical air mass should already have snow levels up to around
8000 feet this afternoon and they will remain near that level
tonight. Models have been fairly consistent with regard to
precipitation amounts; storm totals for today and tonight should
be roughly 2 to 5.5 inches over the Olympics and Mount Rainier,
with 1 to 4 inches over the Cascades north of Mount Rainier.

Warm moist southwest flow aloft will continue Wednesday but with a
relative lull in the rain. Only light rain at times is expected
over the lowlands with maybe a half to less than and inch over
the mountains. The strong warm advection will push Wednesdays high
temperatures up to the upper 50s and lower 60s.

The large upper level longwave trough currently well offshore
along roughly 145W will split on Wednesday with the northern part
of the trough heading east toward W WA. Another wet front will
develop ahead of the trough as it approaches the coast Wednesday
night. Another period of moderate to heavy rain is expected. The
heaviest rain will fall over the Olympics Wednesday night with the
heaviest rain over the Cascades expected early Thursday morning.
Mountain rainfall totals with this second system will be in the 1
to 3.5 inch range. The interior lowlands The rapidly weakening
upper level trough will move across W WA Thursday afternoon for a
showery period. The trough will bring a cooler air mass to the
area with mountain snow levels lowering to around 5000 feet late
Thursday morning, then to around 4000 feet Thursday evening.

Friday will be another lull day with cooler W flow aloft over the
area. A weak system moving E across southern B.C. will probably
bring some light rain to mainly the N part. Kam

.LONG TERM...The GFS and ECMWF are actually in fair agreement
with the large scale pattern through this weekend. Another large
upper level trough will deepen offshore along roughly 140W Friday
night and Saturday, which will re-invigorate the SW flow over the
PacNW. This will allow another shortwave trough embedded in the SW
flow to move NE across W WA Saturday afternoon and evening. The
weakening main upper level trough should follow on Sunday. The
models show a warm front arriving on Monday, but day 6 is a little
far out for much confidence in this active pattern. Kam

&&

.AVIATION...Moist southwest flow aloft will continue tonight and
Wednesday. A warm front will exit the region to the north tonight,
but there will still be rain at times. The air mass will be stable.
At the surface, strong easterly gradients will ease tonight and
become more southeasterly. The southeasterly gradients will increase
somewhat on Wednesday.

There has been some improvement in conditions as the warm front has
moved north. Terminals around Puget Sound are mostly VFR, but in
other places ceilings are MVFR or even IFR. The improving trend will
continue into the evening, but after 06Z or so ceilings everywhere
will fall to MVFR with pockets of IFR. VFR conditions are likely on
Wednesday.

KSEA...Discussion above applies. Southeast wind 8-12 kt. CHB

&&

.MARINE...A warm front will exit the region to the north this
evening, but a few more hours of gale force winds are likely on the
coast. Small craft advisory conditions will continue elsewhere
tonight.

Another frontal system west of the waters will give strong southerly
flow Wednesday and Wednesday night. Windy southerly flow will
continue on Thursday as a trough brushes the area. Gales are likely
on the coast and over the north inland waters with this system.
Another system will arrive around Saturday, with gales again
possible. CHB

&&

.HYDROLOGY...A warm front and the plume of subtropical moisture
with it is bringing 2 to 5.5 inches of rain to the Olympics and
Mount Rainier today and tonight, and 1 to 4 inches of rain to the
Cascades north of Mount Rainier. The snow level will be near 8000
feet. A second frontal system will bring another 1 to 4 inches on
Wednesday night and Thursday, with the snow level falling from
7000 feet to 4000 feet as the system moves through.

A flood watch is in effect for most of the forecast area. The
Skokomish River is already under a flood warning. In addition the
Northwest River Forecast Center models show a number of rivers
cresting near or just above flood stage including the Cowlitz,
Nisqually, White, Snoqualmie, Stillaguamish, Skagit, and Nooksack.
This doesn`t mean the possibility of flooding is limited to these
rivers. Flooding on these rivers could begin tonight or Wednesday.
It looks like the threat is for a relatively minor flooding episode,
though some forecast crests were near moderate category.

In addition, the threat of landslides will increase across the
lowlands of Western Washington. The USGS landslide guidance shows
one or two stations near or just above the threshold of concern, and
of course the additional rain today and tonight and again Wednesday
and Thursday will drive that higher. McDonnal/JBB

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch through Thursday afternoon for Admiralty Inlet Area-
     Bellevue and Vicinity-Cascades of Pierce and Lewis Counties-
     Cascades of Snohomish and King Counties-Cascades of Whatcom
     and Skagit Counties-Central Coast-East Puget Sound Lowlands-
     Eastern Strait of Juan de Fuca-Everett and Vicinity-Hood
     Canal Area-Lower Chehalis Valley Area-North Coast-Olympics-
     Seattle and Vicinity-Southwest Interior-Tacoma Area-Western
     Skagit County-Western Strait of Juan De Fuca-Western
     Whatcom County.

PZ...Small Craft Advisory for rough bar until 4 PM PST Wednesday for
     Grays Harbor Bar.

     Gale Watch from Wednesday afternoon through late Wednesday night
     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-East Entrance U.S.
     Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory until 4 PM PST Wednesday 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 until midnight PST tonight for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca-Northern Inland Waters Including The
     San Juan Islands.

     Small Craft Advisory until 4 AM PST Wednesday for Admiralty
     Inlet-West Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$

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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