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Forecast Discussion for Seattle/Tacoma, WA

000
FXUS66 KSEW 210542
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
915 PM PST Sat Jan 20 2018

.SYNOPSIS...A strong front will move through the area tonight and
Sunday morning. The system will produce strong winds, rain, mountain
snow in addition to other hazards. Another strong front should cross
Western Washington Tuesday night and Wednesday. A cool showery upper
trough will reach the area Thursday.

&&

.SHORT TERM...Windy conditions are developing as expected this
evening, with peak wind gusts of 50-55 mph over the north interior
approaching high wind criteria. Southeast pressure gradients will
continue to tighten further tonight and locations prone to southeast
flow could have gusts up to 60 mph tonight as the front approaches.
The wind warning remains in effect for the Admiralty Inlet area
northward and also for the north/central WA coast. Winds around far
north Puget Sound including Everett to the eastern Strait should
also get some strong winds as a deep meso low develops northeast of
the Olympics. The north part of Hood Canal could also see some windy
conditions. An advisory covers these area with gusts up to 50 mph.
Breezy to locally windy conditions elsewhere but damaging winds are
not expected from King county south. The front will push through
Western Washington later Sunday morning and winds will ease across
the area.

Rain has reached the coast and is pushing into the Olympics this
evening. Heavy rain is likely to develop along the south slopes of
the Olympics with 2 to 3 inches of rain possible by Sunday morning.
This raises a flood threat for the Skokomish river, refer to the
hydro section below and the latest flood bulletins. As of 9 pm, a
flood watch remains in effect for Mason county.

Snow levels remain low, around 2500 to 3000 feet. Precipitation may
be slow to reach the Cascades tonight, but eventually heavy snow
will fall at times through Monday morning. Ahead of the front,
heaviest snow will fall in the Olympics and north Cascades which do
better in south flow. Total snowfall in those areas could reach a
couple feet due to heavy snow ahead of and behind the front. A
winter storm warning is in effect for the Olympics and Cascades of
Whatcom and Skagit counties. Mount Baker will likely see the highest
snowfall of 2 feet or possibly more. Snow will also become heavy at
times from the Cascades of Snohomish, King, Pierce, and Lewis
counties, especially on Sunday. The southeast flow pattern is not as
favorable for heavy snow amounts ahead of the front but cold onshore
flow Sunday will eventually produce some periods of heavy snow. Up
to 1 foot could fall by Monday morning. Passes will likely see 6 to
10 inches with an advisory in effect.

A cold, showery, and unstable air mass will be over the region
Sunday. Models indicate enough instability for a chance of
thunderstorms near the coast. Some isolated thunder is possible
inland but does not warrant adding to the forecast at this time.
Most likely just a strike or two will occur randomly over the
interior.

Showers continue into Monday but much less coverage. Some partial
sun is possible. The next system will approach the region Monday
night. Mercer

High surf will affect the Washington coast and minor flooding is
expected for the Admiralty Inlet coastline areas. Strong winds over
the coastal waters will build seas to around 20 feet later tonight
into midday Sunday. A High Surf Advisory has been issued for the
Pacific coastal beach locations in Western Washington. Waves will
not be as energetic or as large as the ones that hit the coast on
Thursday. But the beaches will be dangerous and local beach erosion
is expected.

The combination of a high astronomical tide, tides running about 1
foot above predicted values, and waves from a strong southeast wind
will result in some minor coastal flooding around the time of high
tide Sunday morning in the Admiralty Inlet area. Southeast facing
shorelines will be most vulnerable to the minor coastal flooding,
including Port Townsend. A Coastal Flood Advisory is in effect for
the Admiralty Inlet area from 6 AM PST until 10 AM PST on Sunday.
Albrecht

.LONG TERM...Previous discussion...A vigorous frontal system is
likely midweek, followed by a cool showery upper trough. It is a
little early to sort out the details, but the pattern certainly
looks good for continuing to build the snowpack.

&&

.HYDROLOGY...The Skokomish river in Mason county is likely to rise
above flood stage around midday Sunday with minor flooding expected.
Heavy rain, possibly 2-3 inches, is expected in the lower elevations
of the Olympics by Sunday morning due to strong southerly flow.
Otherwise River flooding is not expected for the next 7 days.

Rainfall through Sunday will probably push the USGS landslide
indices close to thresholds for shallow landslides.

&&

.AVIATION...A strong cold front will move through the coastal
waters late tonight into early Sunday morning and through the
interior of Western Washington by midday Sunday. The air mass is
moist and stable, and will become unstable behind the front on
Sunday. Strong south to southeast winds 25 to 35 knots with gusts
to 50 knots can be expected in the interior north of KPAE and
along the Pacific Coast. Winds aloft will become strong out of the
south ahead of the front tonight with 5000-10000 foot winds
peaking around 70 knots with the front. A few hours of heavier
rain can be expected Sunday morning ahead of the front. Easterly
pressure gradients ahead of the front will generally hold ceilings
above 2500 feet through early Sunday, except lower near Hood
Canal. Some low level wind shear can be expected around KHQM and
from south of KPAE to KOLM late tonight and Sunday morning as
winds aloft increase. Albrecht

KSEA...Mainly low end VFR conditions tonight and MVFR conditions
Sunday morning. S-SW winds aloft will strengthen overnight,
meeting the criteria to include low-level wind shear in the TAF
tonight until the front passes Sunday morning. Strong winds aloft
expected Sunday morning with winds peaking 15z- 18z Sunday
morning. At FL045 will peak at SW 65 knots, and 2000-foot winds
will peak southerly around 50 knots. Haner/Albrecht

&&

.MARINE...A strong cold front will move across the coastal waters
late tonight into early Sunday morning and through the inland
waters Sunday morning through midday. Southeast pressure gradients
are tightening and winds are increasing in advance of the front.
Expect storm force winds to 50 knots over the coastal waters and
at the eastern entrance to the Strait of Juan de Fuca later
tonight until the front passes. The other waters will see gale
force south to southeast winds ahead of and with the front. Winds
are expected to ease abruptly behind the front.

Weaker southerly onshore flow will follow from Sunday afternoon
through Monday morning. A warm front on Tuesday will cause east to
southeast gradients to become strong. Beyond Tuesday, a series of
fronts will continue to bring active weather to the waters
through the week ahead. Haner/Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 9 AM PST Monday for Cascades of
     Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties.

     Winter Storm Warning until 9 AM PST Monday for Cascades of
     Whatcom and Skagit Counties-Olympics.

     Flood Watch from 3 AM PST Sunday through Sunday evening for Hood
     Canal Area-Lower Chehalis Valley Area-Olympics.

     Wind Advisory until noon PST Sunday for Everett and Vicinity-
     Hood Canal Area.

     High Wind Warning until noon PST Sunday for Admiralty Inlet Area-
     San Juan County-Western Skagit County-Western Whatcom
     County.

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

     High Surf Advisory from midnight tonight to 3 PM PST Sunday for
     Central Coast-North Coast.

     Coastal Flood Advisory from 6 AM to 10 AM PST Sunday for
     Admiralty Inlet Area.

     Wind Advisory until 10 AM PST Sunday for Eastern Strait of Juan
     de Fuca.

PZ...Small Craft Advisory for rough bar until 9 PM PST Monday for
     Grays Harbor Bar.

     Storm Warning until 9 AM PST Sunday 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.

     Storm Warning until noon PST Sunday for East Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Gale Warning until noon PST Sunday for Admiralty Inlet-Northern
     Inland Waters Including The San Juan Islands.

     Gale Warning until 9 AM PST Sunday for West Entrance U.S. Waters
     Strait Of Juan De Fuca.

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

     Gale Warning until 10 AM PST Sunday for Puget Sound and Hood
     Canal.

&&

$$

www.weather.gov/seattle

NWS Seattle/Tacoma, WA (SEW) Office Area Forecast Discussion

Discussion Script by Ken True at Saratoga Weather - WFO Script by SE Lincoln Weather