Heavy Snow in Washington, February 17 to 18, 2018

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

.SYNOPSIS...Windy conditions and heavy mountain snow are expected
Saturday as a deepening surface low tracks southeast through the
region. A modified arctic cold front will move southward through
Western Washington Sunday, bringing the possibility of lowland snow
and strong winds. Dry conditions are expected from late Sunday
through Tuesday with cold temperatures remaining in place. Another
system could slide in from the northwest around next Wednesday or
Thursday.

&&

.SHORT TERM...Entering into a very narrow 5-hour period with no
headlines in effect...living up to the cliched calm before the
storm. Current radar shows some shower activity...mainly relegated
to an area north of Admiralty Inlet and Everett and appears somewhat
convergence zone-ish in nature. There is another...far weaker
looking...convergence zone that seems to be fizzling out along the
King/Snohomish county line. So take a deep breath now because things
are about to get interesting.

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. Deepening
lows typically come with more lift and greater impact.
This...combined with a large swath of warm advection lift preceding
the low and northwest flow aloft also being great for enhancing
orographic lift will result in impressive snowfall totals over the
Cascades, including the main highway passes. The inherited Winter
Storm Warning looks to cover this well and will remain unaltered
this evening. 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.

Winds will be another factor as southwest gradients will really pick
up by 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. MOS data persists in bringing wind speeds
up to around 30 knots throughout the south Sound and Seattle/Tacoma
area...with gusts up to 45-50 mph entirely possible. 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. All of
that being said...wind advisories are up for the aforementioned
areas and again...given consistent data...there appears to be no
reason for any alteration to these headlines this evening.

In the wake of the exiting low, a cold front will enter Western
Washington 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. As 00Z data continues
to come in...will begin to weigh the need for additional headlines
to cover this feature in time for their inclusion in the morning
forecast package. This combination in addition to cold air aloft
will result in low-level convergence near the frontal boundary and
some instability which 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 may see more while others may see none.

The Port Angeles area appears to be 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 resulting in what should be a widespread hard freeze.
Haner/SMR

.LONG TERM...From Previous Discussion...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. Rain ahead of an approaching deepening low pressure system
is expected to reach W WA tonight and into Saturday morning with
showers and windy conditions expected afterward.

Cigs a mixed bag over the area this evening as all flight rules are
represented. VFR conditions most likely over the northern portions
of the area around BLI then turning more MVFR to IFR further south
with convergence zone showers present over portions of King and
Snohomish counties. Solid IFR conditions along the coast in advance
of the approaching system. Not expecting much in the way of
improvement for at least the next 12 hours. As winds ratchet up
Saturday afternoon...cigs may lift somewhat due to an increase in
onshore flow.

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/SMR

KSEA...Cigs this evening through Saturday morning look to be right
on the dividing line between MVFR and IFR...and will likely oscillate
between the two. Southerly winds 5-10 kts tonight. Southwest
sustained 20-30 knots with gusts to 40 knots will develop about 18Z
Saturday and continue into Saturday evening. SMR

&&

.MARINE...Winds will diminish this evening between systems. A
strengthening 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 remain
in place 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. This extended period of strong
winds may require current headlines be extended...but will wait to
evaluate that prospect for the early morning package once 00Z data
has come in completely.

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

&&

.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 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.

&&

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