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Area Forecast Discussion (AFD)

Issued by Seattle/Tacoma, WA (SEW)

                            
002 
FXUS66 KSEW 211657
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
857 AM PST Tue Nov 21 2017

.SYNOPSIS...A strong warm front, fed by a plume of warm 
subtropical moisture will bring heavy rain to Western Washington 
today and tonight, with the snow level rising to near 8000 feet. 
After a relative break in the weather on Wednesday, another wet 
frontal system will move through the region Wednesday night and 
Thanksgiving. There will be another break on Friday, followed by 
more wet weather this weekend and into Monday.

&&

.SHORT TERM...RADAR has been lit up this morning by rain from the
incoming warm front. Strong SW flow aloft has created a tiny rain
shadow NE of the Olympics around Port Townsend. IR imagery shows
the tap into subtropical moisture E of Hawaii. The heaviest rain
is still expected late this morning through this afternoon.

Precipitation is expected to diminish, but not end, this evening
as a slight shift in the flow aloft directs the weakening moist 
plume over Oregon. The subtropical air mass will push snow levels
up to around 8000 feet this afternoon and 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, as
an upper trough digs into our far offshore waters outside of 135W.
There will be more rain on Wednesday, especially at the coast and
over the mountains, but it will be a relative break in the action.
Precipitation in the mountains will amount to another half inch to
inch, with the snow level still near 8000 feet. The warm air mass
will push temperatures up to around 10 degrees above normal on 
Wednesday, with highs near 60.

The next frontal system will develop offshore on Wednesday, ahead
of the upper trough, and progress across Western Washington
Wednesday night and Thanksgiving. The snow level will be around
7000 feet as the event starts, then lower to around 4000 feet 
behind the front as precipitation decreases Thursday night. This 
system doesn't look quite as wet, but it should bring another 1.5 
to 3.5 inches of precipitation to the mountains with the heaviest 
amounts over the Olympics. McDonnal/Kam

.LONG TERM...Previous discussion from the 325 AM AFD...The models
agree fairly well on the big picture in the long term period, 
Friday through Monday. They show a short- lived, relative break in
the weather on Friday, then two frontal systems in quick 
succession on Saturday and Sunday, followed by cooler moist 
westerly flow on Monday. Confidence in model details isn't very 
high, but I will increase POPs somewhat for Saturday through 
Sunday night. McDonnal

&&

.AVIATION...Strong southwest flow aloft will continue today and 
tonight. A warm front will move across the area from south to north 
today. Rain today, tapering tonight. The air mass will be stable 
through tonight. At the surface, strong easterly gradients will ease 
tonight and become more southeasterly. LLWS is possible at most 
terminals today.

Conditions are generally MVFR today with pockets of IFR. Low end 
MVFR conditions will continue behind the warm front tonight, but 
rainfall rates and low level wind shear will decrease from the 
south.

KSEA...Discussion above applies. Northeast or east winds 4-8 kt will 
become southeast 8-12 kt after 00Z. CHB

&&

.MARINE...A warm front will move south to north across the area 
today. Strong offshore flow will give gales to the coast and west 
entrance, with small craft advisory conditions elsewhere.

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 
possible with this system. 

Latest model solutions show a developing low offshore moving by to
the west of the waters on Saturday. This will give windy
conditions to the area on Saturday. CHB

&&

.HYDROLOGY...A warm front and the plume of subtropical moisture
with it will bring 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, which only fell below flood stage Monday 
evening, will almost certainly flood again late today or tonight. 
In addition the Northwest River Forecast Center models show 
several 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.

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

&&


.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from 4 PM PST this afternoon 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 from 4 PM this afternoon to 4 
     PM PST Wednesday for Grays Harbor Bar.

     Gale Warning until 4 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-West Entrance U.S. Waters Strait Of 
     Juan De Fuca.

     Small Craft Advisory until 10 PM PST this evening for 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 this afternoon for Central 
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 4 AM PST Wednesday 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