Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 210427
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
930 PM PDT Fri Oct 20 2017

.SYNOPSIS...A passing warm front will bring showers to the area
tonight with some mountain snow. A strong storm system will impact
the region over the weekend. Rain will increase Saturday and become
heavy at times, especially over the coast and mountains. There may
be snow accumulation in the Cascades Saturday before snow levels
rise. Drier weather will develop Monday and Tuesday.

&&

.SHORT TERM...Activity on the radar this evening looks to be in a
state of transition. Most of the earlier activity has quieted
down...save for a Puget Sound Convergence Zone still kicking up
showers over much of Snohomish county...but starting to see showers
creep in along the coast and over western portions of the CWA. PSCZ
activity does stretch into the Cascades and with the currently
coastal showers expected to move inland overnight...inherited Winter
Weather Advisory for snow up in the Cascades looks good to remain in
place through 6 AM PDT.

More widespread rainfall is expected to overtake the area late
tonight/early Saturday morning with things ratcheting up a notch by
the afternoon...as the heavies rainfall is expected to kick in and
remain into the evening hours. A cold front will reach the area
Sunday morning and rain will turn to showers. Rainfall amounts are
currently forecast to be 2-4 inches in the North Cascades and
Olympics, and 4-6 inches in the Central and South Cascades, with
some higher bullseyes. The lowlands will get an inch or so in the
north and two inches or more in the south. Flooding is likely on
area rivers; most flooding will be on Sunday and could last into
Sunday night. Thus...inherited flood watch will remain in place.

Showers will taper considerably Sunday night and will likely be
completely over on Monday. Skies will start to clear on Monday. High
temperatures will be 55-60 Saturday, near 60 Sunday, and in the low
60s Monday. Burke/SMR

.LONG TERM...From Previous Discussion...Tuesday continues to look
like a sunny and warm day. Heights will be near 590dm. Highs are
currently forecast to be in the 60s and could go higher. A weak
weather system will move through the region Wednesday, giving a
chance of showers. After that, dry northerly flow sets up, with
sunny skies and highs 55-60 on both Thursday and Friday. Burke

&&

.HYDROLOGY...From Previous Discussion...New QPF from a blend of most
recent mesoscale model runs boosted river crests a bit in the river
model. The river model has rivers from the Skykomish south reaching
minor to moderate flood levels.  The Skokomish River in the Olympics
also seems more likely to flood with the new QPF. The Skykomish,
Snohomish, Snoqualmie, Carbon, and Cowlitz rivers are the rivers
most likely to crest above the moderate flood level. In general,
flooding will occur on Sunday from heavy rain Saturday night. 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 probably 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 westerly flow aloft will continue over Western
Washington beyond 00Z Sunday. A vigorous warm front embedded in
the flow will spread rain over W WA after 09Z with rain
continuing through 06Z Sunday. Mainly VFR conditions at 04Z will
lower to MVFR with areas of IFR by 16Z-18Z.

KSEA...A vigorous warm front will maintain poor conditions
Saturday and Saturday night. VFR conditions at 04Z will lower to
MVFR 16Z-18Z with occasional IFR conditions expected. The strong
system will produce southerly winds 40-50 KT at 5000 feet starting
around 15Z and continuing through the day. Winds at 10000 feet
will be SW 45-60 KT. Surface winds will S-SE 8-14 KT becoming
gusty to 22 KT after 17Z. Kam

&&

.MARINE...A strong frontal system approaching the coast tonight
will produce southerly gales over the coastal waters, the east
entrance to the Strait of Juan De Fuca and the part of the North
inland waters near the strait. Small Craft advisory winds are
expected over all other waters. The gales should begin diminishing
Saturday afternoon but SCA winds will continue into Saturday
night.

Onshore flow behind the frontal system on Sunday will ease and
eventually turn weakly offshore on Monday and Tuesday as a surface
ridge builds over the region. A weak front will move through the
area on Wednesday. Kam

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from Saturday 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 from Saturday morning through Sunday morning
     for Cascades of Pierce and Lewis Counties-East Puget Sound
     Lowlands-Tacoma Area.

     Winter Weather Advisory until 6 AM PDT Saturday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

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

     Small Craft Advisory until 2 AM PDT Saturday 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 from 2 AM to noon PDT Saturday 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 from 3 PM to 9 PM PDT Saturday for East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 3 AM to 3 PM PDT Saturday for East Entrance
     U.S. Waters Strait Of Juan De Fuca-Northern Inland Waters
     Including The San Juan Islands.

     Small Craft Advisory from 3 PM to 11 PM PDT Saturday for
     Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 6 PM PDT Saturday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 AM to 6 PM PDT Saturday for
     Admiralty Inlet-Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PDT tonight for Puget Sound
     and Hood Canal.

     Small Craft Advisory from 2 PM Saturday to 5 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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