Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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316
FXUS66 KSEW 171115
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
315 AM PST Tue Jan 17 2017

.SYNOPSIS...A weakening frontal system will move into Western
Washington this morning. A stronger system will arrive tonight
bringing wet and windy conditions. Cold air trapped in the central
Cascade passes could produce freezing rain through tonight. A cool
upper level trough will settle over the area for the second half of
the week with showery and occasionally breezy weather.

&&

.SHORT TERM...A weakening frontal system associated with a surface
low moving toward Haida Gwaii will push some light precipitation
into Western Washington this morning. The overall air mass has
warmed significantly over the past 24 hours with free air freezing
levels of 9000 to 10000 feet. The exception to this is cold air
trapped in some Cascade passes...especially places like Snoqualmie
Pass and Stampede...where easterly gradients continue to import
chilly air from the east side of the state. We have trimmed back the
winter weather warnings and advisories to just a winter weather
advisory for the Cascades of King and Snohomish counties with an
emphasis on freezing rain in the passes. Heavier precipitation
arrives this evening through Wednesday morning. Flood watches remain
in effect for rivers flowing off the Olympics and North Cascades.
For more details on this...see the Hydrology discussion.

A secondary surface low will pass over the offshore waters this
afternoon en route to the north end of Vancouver Island with yet a
third surface low on its heels Wednesday morning. This will serve to
tighten east to southeast gradients later this morning continuing
through midday Wednesday. Wind advisories remain in affect for areas
prone to southeast winds...i.e. the coast...north interior...and
Admiralty Inlet area.

A cool upper trough will settle over the region Wednesday evening
into Thursday with lowering snow levels and cool showery conditions.

.LONG TERM...
The models generally agree on bringing a broad upper level trough
into the offshore waters by late Friday. They then begin to diverge
in solutions over the weekend in regards to where and how quickly to
bring the trough onshore. The GFS is considerably faster than the
Euro. Despite the differences...a strong jet stream is poised to
remain to our south into the weekend. Coastal areas and the
Southwest Interior may see an uptick in precipitation over the
weekend, but Oregon and California are likely to see the bulk of the
precipitation in the extended period. 27

&&

.AVIATION...A weakening frontal system will move onshore this
morning with southwest flow aloft. A stronger front will reach the
area late today. The air mass is stable with mid and high level
moisture. The low levels will become moist today.

KSEA...Mid clouds will lower to VFR low clouds this morning as rain
moves in. Ceilings will likely remain VFR most of the day then lower
to MVFR late today as heavier rain arrives. South wind 5-10 knots
will increase to 10-15 knots Today. Schneider

&&

.MARINE...A series of fronts will move through the waters this
week. Off and on gale force winds are expected for the Coastal
Waters, entrances to the Strait, and Northern Inland waters today
through Wednesday as a couple of frontal systems move through.
There will be a bit of a lull in between weather systems Thursday,
then additional fronts will affect the waters Friday and Saturday.
Schneider

&&

.HYDROLOGY...A pair of weather systems will move through Western
Washington in the next couple of days. The first system today will
not produce hydrologically significant rainfall amounts. The second
system tonight into Wednesday will be stronger and much wetter as it
taps into moist southwesterly flow aloft extending back toward the
Hawaiian Islands. Snow levels tonight through Wednesday will
generally be in the 6000-7000 foot range. Model 850 mb winds are
mostly 35 to 50 knots with a few hours of 50 to 60 knot winds over
the Olympics.
Peak rainfall amounts at this time look to be in the 00z Wednesday
to 00z Thursday time frame with 4 to 6 inches in the Olympics...3 to
5 inches in the North Cascades and 2 to 4 inches in the Central and
Southern Cascades.

Rivers coming off of the Olympics have the best chance to flood in
this scenario with the flood prone Skokomish the most likely of the
bunch. Other rivers that could reach flood stage will be the rivers
coming off the North Cascades in Whatcom County.

Rivers in Western Washington are currently running at some of the
lowest levels seen since last summer. This reduces the chance of
flooding across the area. Felton


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 4 PM PST this afternoon for
     Cascades of Snohomish and King Counties.

     Flood Watch from this evening through Wednesday evening for
     Admiralty Inlet Area-Cascades of Whatcom and Skagit Counties-
     Central Coast-Eastern Strait of Juan de Fuca-Hood Canal Area-
     Lower Chehalis Valley Area-North Coast-Olympics-Western Strait
     of Juan De Fuca-Western Whatcom County.

     Wind Advisory until 3 PM PST Wednesday for Admiralty Inlet Area-
     Central Coast-North Coast-San Juan County-Western Skagit
     County-Western Whatcom County.

PZ...Small Craft Advisory for rough bar until 2 AM PST Wednesday for
     Grays Harbor Bar.

     Gale Warning until noon PST Wednesday 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 2 AM PST Wednesday 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 10 AM PST Wednesday for Central U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 6 AM this morning to 4 PM PST
     Wednesday for 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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