Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 211646
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
945 AM PDT Sat Oct 21 2017

.SYNOPSIS...Rain will be heavy at times today, especially in
the mountains. River flooding is likely. Snow in the mountains
today will change to rain in the passes by afternoon as a warm
front arrives. Winds will be breezy to locally windy, especially
tonight. Showers on Sunday will end by Monday. Tuesday will be dry.
After some light rain Wednesday, dry weather should return.

&&

.SHORT TERM...A band of rain moved through the area early this
morning, but currently the Puget Sound area is in a bit of a lull in
precipitation. Heavier rain is evident on the coast and in Oregon.
This heavy rain will move into the rest of Western Washington this
evening, and will continue overnight. The heavy rain will end from
north to south Sunday morning, though showers will continue on
Sunday, especially in the mountains.

The snow level, currently below 4000 feet, will rise to 7000-8000
feet tonight during the heavy rain. Webcams show snow at Snoqualmie
pass this morning, at around 3000 feet. The snow level will fall to
5000 feet or so on Sunday. Light snow will fall in the mountains
today until the warm front passes, which for most places will
probably be in the afternoon. At 9 AM the warm front appeared to lie
between Hoquiam (47 degrees, northeast wind) and Astoria (54
degrees, south wind). At best this timing makes for a messy forecast
in the mountains. The warm front should turn all precipitation to
rain and will wash away most or all of the snow that will have
fallen in the mountains yesterday and today. This is clearly a flood
pattern.

There is likely to be some wind with the warm front as well. Have
increased wind over most areas, especially this evening when 20-30
mph winds will probably be widespread. The wind advisory for the
coast and north interior has been simplified somewhat, and is now in
effect today through midnight tonight. MOS indicates the windiest
period is likely to be this evening.

Rain will turn to showers from the northwest on Sunday. By Monday,
residual showers will end with drier and slightly warmer weather
expected. High temperatures will be 55-60 today and Sunday, with
mostly low 60s Monday. Burke

.LONG TERM...Previous discussion...Dry weather continues into
Tuesday. Models remain in pretty good agreement in bringing a fairly
weak front southeast across the area Tuesday night or Wednesday.
This will be a progressive system will only light rain amounts
expected. This will not have an impact on area rivers. The GFS/ECMWF
agree that a strong upper ridge will build over the Pacific
Northwest Thursday and persist through Friday, and likely beyond.
Expect dry weather but with some areas of fog likely with light
winds over the area. Mercer

&&

.AVIATION...Strong southwest flow aloft over Western Washington this
morning will veer to westerly tonight as a vigorous Pacific frontal
system moves across the forecast area. The air mass will be moist
and stable.

MVFR to low-end VFR conditions prevail across the forecast area at 9
am with a relative lull in precipitation. Rainfall will increase
during the next few hours and continue through tonight, with mainly
MVFR to local IFR conditions due to ceilings and visibility. In
addition there will be areas of surface wind at or above 25 knots,
mainly tonight.

KSEA...Southeast wind 6-13 kt, then south to southwest 16-24 kt
gusting 30 kt beginning around 6 pm. Wind at 3000 ft will be SW 45-
60 kt from this afternoon through tonight. Mainly MVFR conditions,
possibly lowering to IFR at times tonight in heavier rain. McDonnal

&&

.MARINE...A vigorous Pacific frontal system will move through the
area today and tonight with gales over the coastal waters, East
Entrance Strait of Juan de Fuca and Northern Inland Waters. The gale
warnings that were in effect have been extended through midnight
tonight. Small craft advisory winds will prevail over the remaining
zones. Onshore flow will follow the front on Sunday, and the flow
will turn weakly offshore Monday and Tuesday. Another front will
move across the region on Wednesday. McDonnal

&&

.HYDROLOGY...Latest mesoscale models spread the area of heavy
precipitation into the northern Cascades. Minor flooding now seems
possible on the Nooksack and Stillaguamish Rivers, as well as many
of the rivers from the Skykomish southward. The rivers with the best
chances to get to moderate flood stage include the Snoqualmie,
Skykomish, Snohomish, Cowlitz and Carbon. The Skokomish river in
Mason county is also likely to flood. In general, flooding will
occur on Sunday but could start as early as late Saturday evening.
The flood watch remains in effect for all of Western Washington.

The burn scar from the recent Norse Peak fire presents a special
concern. Increased runoff is probable in the burn area and there
is some chance of debris flows. The Goat Creek valley and the
Crystal Mountain ski area are at risk of flash flooding, which
could reach all the way to State Route 410. A flash flood watch is
in effect for this small area.

Despite recent heavy rains, there has not been enough accumulated
precipitation to saturate soils. Even after the forecast rain this
weekend, the general threat of landslides and mudslides will not be
significantly raised above the normal level. Burke

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from 5 PM PDT this afternoon through late Sunday
     night for Admiralty Inlet Area-Bellevue and Vicinity-
     Bremerton 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.

     Flash Flood Watch through Sunday morning for Cascades of Pierce
     and Lewis Counties-East Puget Sound Lowlands-Tacoma Area.

     Wind Advisory until midnight PDT tonight for Admiralty Inlet
     Area-San Juan County-Western Whatcom County.

     Wind Advisory until midnight PDT tonight for Central Coast-North
     Coast.

PZ...Small Craft Advisory for rough bar until 6 AM PDT Sunday for
     Grays Harbor Bar.

     Gale Warning until midnight PDT tonight 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 6 AM PDT Sunday for Puget Sound and
     Hood Canal-West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 AM PDT Sunday for Central U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PDT tonight 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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