Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 211106
AFDSEW

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

.SYNOPSIS...A warm front will lift northeast over Western Washington
today. Rain will be heavy at times, especially coast and mountains
with some river flooding likely this weekend. Snow in the mountains
today will change to rain in the passes by afternoon as warmer air
arrives. Winds will be breezy to locally windy, with advisory level
winds at the coast and much of the north interior near the water.
Rain tapers to showers later Sunday. Showers end Monday with dry
weather into Tuesday. After some light rain with a front on
Wednesday, dry weather should return for the end of the week.

&&

.SHORT TERM...Water vapor satellite loop shows the warm front
advancing toward our region this morning. The associated atmospheric
river of moisture extends well out across the Pacific. Models are
more aggressive in lifting the warm front into the area today with
heavy rain at times likely in the warm sector through tonight. Most
models still show the heaviest rain amounts in the central/south
Cascades and Olympics through Sunday with 3 to 5 inches of rain
expected, with locally higher amounts. The lowlands should see 1 to
2 inches, with highest amounts from around King county south. River
flooding is likely to develop on numerous rivers and some localized
urban flooding due to clogged storm drains is also possible. See the
hydrology section below for further details and always refer to the
latest flood statement for current information.

The models have generally trended toward lifting the warm front much
further north than previous solutions. Widespread breezy winds are
likely to pick up across the entire area behind the warm front,
especially late this afternoon and evening. Models indicate that
locally stronger winds are likely to develop over much of the
coastal north interior today where southeast winds tend to be
higher. Local power outages are possible with sustained winds up to
30 or 35 mph with gusts to 45 mph today. At the coast, stronger
winds are favored behind the warm front with southwest winds
increasing to 20 to 35 mph with gusts 45 to 50 mph this afternoon
into the evening. Wind advisories are posted for the above mentioned
areas. Isolated power outages are still possible over the rest of
the interior but wind gusts are expected to peak at 35 to 40 mph
late this afternoon and evening, below advisory criteria.

Will need to monitor mountain snowfall today as the warm front moves
in. Most models show snow levels rising rapidly this afternoon.
Several inches of snow could fall down to around 4000 feet before a
transition to rain occurs. A few inches of snow are possible at
higher passes like Stevens and White pass, and up to 5 inches at
Mount Baker and Paradise. Decided against a winter weather advisory
given the expected rapid rise in snow levels.

Rain tapers to showers from the northwest on Sunday. River flooding
is likely to continue on numerous area rivers. By Monday, residual
showers will end with drier and slightly warmer weather expected.

.LONG TERM...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 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 this morning with rain continuing
through 06Z Sunday. Conditions are expected to lower rapidly around
12Z-18Z with MVFR conditions and areas of IFR.

KSEA...A vigorous frontal system will maintain poor conditions
through tonight. MVFR conditions are expected by 14Z along with
occasional IFR conditions. 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 will move across W WA today
producing southerly gales over the coastal waters, the east entrance
to the Strait of Juan De Fuca and the North Inland waters. Small
Craft advisory winds are expected over all other waters. The gales
should begin diminishing this 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

&&

.HYDROLOGY...No big changes to the forecast this morning. Updated
rainfall forecasts similar to the previous forecasts with just a
little more precipitation added to the south slopes of the
Olympics. The time frame for the heaviest rainfall continues to be
Saturday evening into early Sunday morning. The river model has
rivers from the Skykomish south reaching minor to moderate flood
levels. The rivers with the best chances to get to moderate flood
stage include but are not limited to 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.

Rain this past week and expected into this weekend has and will
continue to increase the landslide threat in shallow soil layers
especially in the Puget Sound area and the Southwest Interior. Felton

&&

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

     Wind Advisory central and north coast from Noon today through
     10 PM this evening.

     Wind Advisory for Whatcom county until 5 PM this afternoon.

     Wind Advisory for Admiralty Inlet and San Juan county until
     Noon today.

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

     Gale Warning until 5 PM PDT 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.

     Small Craft Advisory from 3 PM this afternoon to 9 PM PDT this
     evening for East Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Gale Warning until 3 PM PDT this afternoon 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 this afternoon to 5 AM PDT Sunday
     for Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 5 AM PDT Sunday for West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PDT this evening for Admiralty
     Inlet-Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 4 PM this afternoon 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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