Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 162358
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
358 PM PST Fri Feb 16 2018

.SYNOPSIS...Windy conditions and heavy mountain snow are expected
Saturday as a deepening surface low tracks southeast down
Vancouver Island and then across to northeast Washington. On
Sunday, a modified arctic cold front will move southward through
the area, bringing the possibility of lowland snow, as well as
strong wind. It will be followed from late Sunday through Tuesday
with dry weather but some of the coldest air so far this winter.
Another system could slide in from the northwest around next
Wednesday or Thursday.

&&

.SHORT TERM...Active and potentially impactful weather coming up
this weekend, first on Saturday in the form of wind and heavy
mountain snow, then on Sunday with the potential for lowland snow
and strong northeast-north Fraser Outflow wind. Coldest air mass
of the winter is expected from Sunday night through Tuesday
morning.

On Saturday, a deepening low pressure center will move southeast
down Vancouver Island, then track east near Vancouver B.C. and
continue deepening as it moves to northeast Washington. At it
closest approach, the central pressure of the low will be near 998
mb. Strengthening or deepening lows typically come with more lift
and greater impact. The low will be preceded by a large swath of
warm advection lift, while northwest flow aloft is a great
direction for enhancing orographic lift and snowfall totals over
the Cascades, including the main highway passes. Some of the
heaviest snow totals in the passes come in this type of pattern.
A Winter Storm Warning has been issued for the Cascades from late
tonight through Saturday night. Nearly two feet of snow is
forecast at the passes in 24 hours. Avalanche danger will be
elevated given the the heavy amount of snow and strong wind in
exposed locales.

Southwest gradients will really pick up as the low makes it
closest approach around mid-day Saturday. Models bring the HQM-SEA
gradient to around +4 to +4.5 mb, and the UIL-BLI gradient should
reach near +5 mb around mid-day Saturday. 18z GFS MOS peaks the
sustained wind at Shelton, Tacoma Narrows, Tacoma McChord, Sea-
Tac, and West Point (Discovery Park) at 33, 28, 30, 27, and 34
knots respectively around mid-day Saturday. With 925 mb winds of
40-50 knots and cold advection during this period, wind gusts of
45-50 mph are totally reasonable for the South and Central Sound
regions. Further north, a strong westerly surge is expected
through the Strait during the afternoon. NW flow aloft favors such
surges impinging on land areas on the U.S. side of the Strait and
the Admiralty Inlet area. Given all these things, have gone with
a round of Wind Advisories for tomorrow. Would not be surprised if
one or two zones pick up a few stronger gusts.

After the low exits east on Saturday night, a cold front will
spread out of B.C. into Western Washington on Sunday morning,
aided by strong Fraser Outflow wind. The Bellingham to Williams
Lake gradient could exceed -20 mb on Sunday morning, making this a
strong outflow event. May eventually need a High Wind Warning for
Western Whatcom County on Sunday. This pattern will cause a sharp
modified arctic cold front to slip southward. The air aloft will
be cold, so low-level convergence near the frontal boundary and
some instability could support snow showers as the front moves
south. On average up and down the I-5 corridor, the official
forecast has about 1 inch of snow centered around Sunday morning.
In reality, some places will see more, and others will get none.
GFS ensemble mean has about 1 inch of snow at SEA early Sunday.

The Port Angeles area is most likely to get snow, given
northeast wind upsloping along the north-facing side of the
Olympic Mtns. The official forecast has 1 inch of snow near the
water there, but 3-6 inches above 500 feet along the Strait.

With all of the cold, dry continental air pouring out from the
interior of B.C., the air mass will dry out quickly later Sunday,
and the last showers will end over the Cascades on Sunday night.
Coldest air mass of the winter is expected, with Monday night
being the coldest. Should be a widespread hard freeze.

.LONG TERM...Northwest flow aloft will continue from Tuesday
through next Friday, keeping temps below normal. A system will
dive southeast around Wednesday or Thursday. The air mass will
moderate a bit as clouds and precip spread in, and weak onshore
flow develops. Still, a rain/snow mix is forecast over the
lowlands. QPF is modest, so only minor snow accumulation is
expected on higher hills near the Cascades. Haner

&&

.AVIATION...Strong northwest flow aloft will continue through
Saturday. A deepening low pressure system in the Gulf of Alaska
will move rapidly southeastward and along the Canadian Border
Saturday afternoon. Rain will spread east ahead of the low tonight
into Saturday morning. Showers and windy conditions are expected
Saturday afternoon.

Conditions are generally on the low end of MVFR this afternoon
except at the higher elevation terminals where they are IFR.
Ceilings will likely stay about where they are tonight into
Saturday morning. Expect some improvement Saturday afternoon as
onshore flow increases behind the low.

Winds are expected to rise to 25 to 30 knots with some gusts to 40
knots Saturday afternoon and evening. At this time the strongest
winds are likely to be near the Strait of Juan de Fuca in westerly
flow and southwesterly from about KSEA to KTCM. Strong and gusty
westerly winds can also be expected along the coast Saturday.
Albrecht

KSEA...Low end MVFR ceilings tonight and Saturday morning with
some light improvement Saturday afternoon. Southerly winds 12-15
knots with gusts to 22 knots through early this evening. Winds
easing back down to 5 to 10 knots later this evening. Southwest
sustained 20-30 knots with gusts to 40 knots will develop about
18Z Saturday and continue into Saturday evening. Albrecht

&&

.MARINE...Winds will diminish this evening between systems. A
996 mb low will move southeast across northern portions of the
area Saturday afternoon. Expect widespread gales to develop from
west to east starting early Saturday morning. Gale warnings have
been issued for all waters, beginning over the coastal waters
early Saturday morning and spreading into the inland waters late
Saturday morning.

Strong northeast Fraser outflow wind will spread out across the
waters starting late Saturday night and continuing into Sunday
night. Expect gales over most of the waters with the outflow,
including Puget Sound and Hood Canal. Mesoscale models hint at the
possibility of storm force winds over portions of the Northern
Inland waters on Sunday.

Gradients will ease Sunday night and become light by later Monday
into Tuesday. Albrecht

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory from 7 AM to 7 PM PST Saturday for Bellevue and
     Vicinity-Bremerton and Vicinity-Hood Canal Area-Seattle and
     Vicinity-Southwest Interior-Tacoma Area.

     Winter Weather Advisory until 7 PM PST this evening for Cascades
     of Pierce and Lewis Counties-Cascades of Snohomish and King
     Counties-Cascades of Whatcom and Skagit Counties.

     Winter Storm Warning from midnight tonight to 6 AM PST Sunday
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties.

     Wind Advisory from 6 AM to 6 PM PST Saturday for Central Coast-
     North Coast.

     Winter Weather Advisory from midnight tonight to 6 PM PST
     Saturday for Olympics.

     Wind Advisory from 9 AM to 6 PM PST Saturday for San Juan County.

     Wind Advisory from 11 AM to 11 PM PST Saturday for Admiralty
     Inlet Area-Eastern Strait of Juan de Fuca.

     Wind Advisory from 8 AM to 6 PM PST Saturday for Western Strait
     of Juan De Fuca.

PZ...Small Craft Advisory for rough bar from noon Saturday to 3 AM
     PST Sunday for Grays Harbor Bar.

     Gale Warning from 4 AM Saturday to 3 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.

     Gale Warning from 10 AM Saturday to 3 AM PST Sunday for
     Admiralty Inlet-Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands-Puget Sound
     and Hood Canal-West Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Small Craft Advisory until 3 AM PST Saturday for Puget Sound and
     Hood Canal.

&&

$$

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