Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 181122
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
322 AM PST Wed Jan 18 2017

.SYNOPSIS...A frontal system will bring rain and windy conditions
through today. Cold air trapped in the central Cascade passes will
produce a combination of freezing rain and sleet today. A cool upper
level trough will settle over the region tonight through the weekend
giving lower snow levels and more showery weather along with
occasionally breezy conditions.

&&

.SHORT TERM...Mild and wet conditions continue across western
Washington early this morning with 24 hour precip totals ranging from
1 to 2 inches in the lowlands outside of the Olympic rain shadow to 3
to 5 inches in the Olympics. Periods of moderate to locally heavy
rain are expected to continue into the afternoon. For flood and
river flooding related impacts, see the hydrology section of the
discussion. Winds along the coast and in the north interior have
eased somewhat overnight, but are expected to pick up again later
this morning as another front associated with a surface low headed
for the north end of Vancouver Island pushes into the region.
Easterly winds continue to import cold eastern Washington air
into the passes of the central Cascades where temperatures remain in
the upper teens. A wintry mix of freezing rain and sleet is likely to
continue to affect Snoqualmie Pass and Stevens Pass through this
afternoon and a Winter Storm Warning remains in affect for those
areas.

Following the frontal passage later today, a cool upper level trough
will settle over the region Thursday into Friday for periods of
showers, lower snow levels, and near seasonal temperatures.

.LONG TERM...
The models are general agreement with the idea of a broad upper
level trough moving into the offshore waters by the weekend with a
series of disturbances rotating around it and bringing periods of
precipitation. The 00Z runs of both the GFS and the Euro agree that
Sunday could be the wetter day this weekend. By the early next
weeks, the models attempt to build a reasonably high amplitude
shortware ridge into the Pacific Northwest. So we might finally get
a couple of dry days. 27

&&

.AVIATION...A frontal system will move onshore today with
southwest flow aloft. An upper trough will move over Western
Washington tonight. The air mass is moist and stable, becoming
slightly unstable over the Coastal Waters today.

KSEA...Low clouds will persist through tonight with ceilings
sometimes VFR and sometimes MVFR. Southerly wind 5-10 knots will
increase to 10-15 knots today, then ease tonight. Schneider

&&

.MARINE...A series of frontal systems will move through the
waters this week. Gales force winds over the Coastal Waters,
entrances to the Strait, Northern Inland waters and Admiralty
Inlet will ease later today. There will be a bit of a lull in
between weather systems Thursday before additional strong fronts
affect the waters Friday through Sunday.

The combination of high astronomical tides, low atmospheric
pressure, and waves from strong southerly winds could lead to
minor tidal overflow later this morning for the Northern Inland
Waters. West swell around 20 feet could affect the Coastal Waters
on Saturday. Schneider

&&

.HYDROLOGY...Heavy rain will continue over the mountains this
morning with moist strong southwesterly flow aloft over the area.
Rainfall amounts over the last 12 hours in the Olympics have been
in the 2 to 3.5 inch range and in the Cascades in the 1 to 2.5 inch
range. Model 850 mb winds in the 40-60 knot range this morning ease
to 20 to 40 knots this afternoon. Another 1 to 2.5 inches of rain is
forecast over the Olympics today with 1 to 2 inches over the
Cascades. Highest amounts in the Cascades are expected to be around
Mount Baker.

Rainfall amounts will taper off significantly tonight into Thursday
morning.

A flood warning has already been issued for the Skokomish river in
Mason county. A flood watch continues for the Olympic Peninsula
(Grays Harbor and Mason Counties northward), for Whatcom County and
for King and Pierce Counties. Other than the Skokomish, rivers most
likely to flood this afternoon or this evening include but are not
limited to the Nooksack in Whatcom county and the White river in
Pierce and King county.  Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch through this evening for Admiralty Inlet Area-
     Bellevue 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-Hood Canal Area-Lower
     Chehalis Valley Area-North Coast-Olympics-Seattle and
     Vicinity-Southwest Interior-Tacoma Area-Western Strait of
     Juan De Fuca-Western Whatcom County.

     High Wind Warning until 3 PM PST this afternoon for Admiralty
     Inlet Area-San Juan County-Western Skagit County-Western
     Whatcom County.

     Coastal Flood Advisory from 7 AM this morning to noon PST today
     for Admiralty Inlet Area-San Juan County-Western Skagit
     County-Western Whatcom County.

     High Wind Warning until 10 AM PST this morning for Central Coast-
     North Coast.

PZ...Small Craft Advisory for rough bar until midnight PST tonight
     for Grays Harbor Bar.

     Gale Warning until 3 PM PST 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.

     Gale Warning until 1 PM PST this afternoon for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands-West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 PM PST this afternoon for Central
     U.S. Waters Strait Of Juan De Fuca-Puget Sound and Hood
     Canal.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html



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