Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 172331 CCA
AFDSEW

Area Forecast Discussion...CORRECTED
National Weather Service Seattle WA
331 PM PST Sun Dec 17 2017

.SYNOPSIS...Moist westerly flow will continue through Monday for
rain and mountain snow. A strengthening low center will pass
across the area on Tuesday morning, and a strong cold front will
bring wind and heavy mountain snow. Lingering showers will
continue Wednesday morning, then an upper ridge will bring dry
weather on Thursday. A shortwave trough may clip the area Thursday
night into Friday for a chance of rain and snow. The ridge will
bring dry weather through the weekend and into Christmas Day.

&&

.SHORT TERM...A frontal system will stall over Western Washington
tonight and Monday with strong, moist westerly flow over the
region. The main impact is heavy snow in the Cascades, mainly from
Mount Baker to Stevens Pass. Snow levels are around 4,000 to
5,000 feet. Snoqualmie Pass should see mainly rain although could
see a few inches on Monday as the snow levels drops to around
3,000 feet.

Meanwhile, in the lowlands, expect rain at times through the
period although Seattle is getting rain shadowed. South winds will
remain gusty with this front too and the interior will see gusts
to 30-40 MPH. Showers will taper off Monday afternoon as this
front weakens.

The next system will impact the area Monday night into Tuesday as
another strong frontal system approaches and moves inland. The
main impacts are again possible heavy snow in the mountains. A
Winter Storm Watch is in effect for the Olympics and north
Cascades where the heaviest snowfall totals are likely - over a
foot. Winds will be gusty with the frontal passage, especially the
coast and south sound where winds will gust to 40 MPH. A cool
upper level trough will cross the area Tuesday night into
Wednesday and snow levels will be low, around 1,000 feet. This
means we could see a few light snow showers lingering Wednesday
morning, affecting areas in the East Puget Sound Lowlands near the
Cascades. Other areas to watch will be the Strait of Juan de Fuca
and the NW Interior as snow levels will be low here too.

Dry, northerly flow will prevail on Wednesday as a strong upper
level ridge builds over the NE Pacific. 33

.LONG TERM...A new wrinkle in the pattern brings a threat of
lowland snow Thursday night and Friday. Models show the ridge
offshore retrograding while a shortwave trough tracks S/SE into
Western Washington. The cool setup is already here with the dry
offshore flow, temperatures in the lower 30s and snow levels near
the surface. Light accumulations are possible but exact details
are still unclear since this system is still several days out.
This system exits Friday afternoon with the threat of showers
ending.

Otherwise, the high amplitude ridge offshore will continue to
bring dry and cool weather through next weekend and into Christmas
Day. Temperatures are trending below normal with lows in the 20s
and highs only in the 30s. 33

&&

.AVIATION...A warm front will remain stall over the area through at
least Monday morning. Strong southwest flow aloft becoming light
westerly on Monday between weather systems. Air mass moist and
stable. Widespread MVFR cigs with light rain at times. The KSEA/KBFI
to KPAE corridor may partially rain shadow this evening before rain
fills in. Local IFR also possible later tonight/Monday morning but
with breezy conditions, mixing should help minimize IFR coverage
across the Wrn WA. Southerly winds up to 20-25 kt w/ gusts to 30 kt,
highest around Puget Sound and Admiralty Inlet (KSEA/KBFI/KPWT/KPAE)
easing late tonight.

KSEA...Rain shadowing and increasing wind has allowed cigs to
improve slightly this afternoon. However, rain is expected to fill
in with MVFR prevailing most of tonight into Monday morning. S winds
13-18 kt with gusts up to 30 kt, gradually easing late
tonight. DTM

&&

.MARINE...A warm front will bring small craft advisory strength
winds to all waters tonight. Winds will turn westerly in the Strait
this evening with a switch to westerly small craft in the
central/east entrance. Strongest winds around 30 kt, close to gale,
will likely occur this evening in Puget Sound to Admiralty Inlet.

There will be relative lull in between weather systems Monday
afternoon. A stronger frontal system will move through the area
Monday night and Tuesday. Small craft winds likely or possibly gales
for the southerly Coastal Waters with this system. A gale watch is
in effect for some coastal zone. Confidence is low regarding the
track of the surface low Tuesday, therefore forecasts should be
monitored.

Offshore flow will develop Wednesday and continue into Thursday. DTM

&&

.HYDROLOGY...River flooding is not expected the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 10 AM PST Monday for Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties.

     Winter Storm Watch from Monday afternoon through late Tuesday
     night for Cascades of Whatcom and Skagit Counties.

     Winter Storm Watch from Monday afternoon through Tuesday
     afternoon for Olympics.

PZ...Gale Watch from late Monday night through Tuesday afternoon for
     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 until 4 AM PST 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-Northern Inland Waters Including The
     San Juan Islands-West Entrance U.S. Waters Strait Of Juan
     De Fuca.

     Small Craft Advisory until 10 AM PST Monday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 7 PM this evening to 10 AM PST Monday
     for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 AM PST Monday for Admiralty Inlet-
     Puget Sound and Hood Canal.

&&

$$

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