Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 212209
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 PM PDT Sat Oct 21 2017

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

&&

.SHORT TERM...The warm front is making slow progress into Western
Washington. Paradise at 5000 feet and White Pass at 4500 feet were
both 34 degrees at 2 PM.  Heavy rain is apparent on the radar on
the coast and in the far south. This heavy rain will move into the
rest of Western Washington this evening, and will then continue
overnight. Snow in the Cascades will change to rain at some point
this evening, in the south part first. The heavy rain will end from
north to south Sunday morning, though showers will continue on
Sunday, especially in the mountains.

The snow level will rise to 7000-8000 feet tonight during the heavy
rain, then fall to 5000 feet or so on Sunday. Light snow will fall
in the mountains until the warm front passes, which for most places
will probably be late in the afternoon or early in the evening. The
latest 20km GFS model run has 850mb temperatures rising above
freezing around Snoqualmie Pass by 03Z, and above freezing in the
North Cascades by 06Z. At 3 PM the warm front appeared to lie around
a KHQM-KOLM line. The warm front should turn all precipitation in
the mountains to rain, and it 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. The
forecast for most areas this evening is 20-30 mph winds. Wind
will be a bit stronger on the coast and over the north interior,
where a wind advisory remains in effect 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. Tuesday continues to look sunny and warm, with heights
around 590dm. Have added patchy fog to the nights and mornings for
both Monday and Tuesday. High temperatures will be 55-60 Sunday,
mostly low 60s Monday, with some mid 60s Tuesday. Burke

.LONG TERM...Models remain in good agreement in bringing a weak
front southeast across the area Wednesday. This will be a fast
moving system, will only light rain amounts expected, and 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. burke

&&

.HYDROLOGY...Little change in the thinking from this morning, other
than perhaps a bit of a delay in the onset of any flooding. The
river model now shows minor flooding 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 tonight. 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

&&

.AVIATION...Strong southwest flow aloft over Western Washington
this afternoon will veer to westerly tonight as a vigorous
Pacific frontal system moves across the forecast area. The air
mass will be moist and stable. Westerly flow will ease a bit
behind the system on Sunday, and the air mass will dry somewhat
and become weakly unstable.

Mainly MVFR and local IFR conditions continue across Western
Washington this afternoon, with widespread rain. This will
continue this evening. In addition there will be areas of surface
wind at or above 25 knots, mainly tonight. There could be
marginal improvement late tonight as rain decreases, and then more
significant improvement to generally VFR conditions Sunday
afternoon.

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 continue moving
through the area tonight. Gales are likely over the coastal
waters, East Entrance Strait of Juan de Fuca and Northern Inland
Waters this evening. Small craft advisory winds will prevail over
the remaining zones.

Onshore flow will follow the front late tonight and Sunday, with
small craft advisory westerlies in the Strait of Juan de Fuca. The
flow will turn weakly offshore Monday and Tuesday. Another front
will move across the region on Wednesday, followed by northerly
flow on Thursday. McDonnal

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch 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-Central Coast-North Coast-San Juan County-Western
     Whatcom County.

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

     Small Craft Advisory from midnight tonight to 3 AM PDT 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.

     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 from midnight tonight to noon PDT Sunday
     for East Entrance U.S. Waters Strait Of Juan De Fuca-
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 3 AM PDT Monday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

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

     Small Craft Advisory until midnight PDT tonight for Admiralty
     Inlet.

     Small Craft Advisory until 6 AM PDT Sunday for Puget Sound and
     Hood Canal.

&&

$$

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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