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Area forecast discussion...updated
National Weather Service Seattle Washington
1032 PM PST Sat Feb 17 2018

Update...the latest mesoscale models indicated that locally
heavy snow will develop overnight on the olympic range due to
strengthening northerly upslope flow. This will impact the north
facing slopes. Therefore, the Winter Weather Advisory for the
olympic range was upgraded to a Winter Storm Warning.

Will also need to keep an eye on the land area adjacent to
eastern Strait of Juan de Fuca because the mesoscale models were
also showing fairly significant snowfall amounts later tonight.
The Winter Weather Advisory for this zone was updated to include
locally higher amounts.

&&

Previous discussion... /issued 750 PM PST Sat Feb 17 2018/

Synopsis...a modified Arctic cold front will move southward through
western Washington on the heels of exiting low pressure bringing the
possibility of lowland snow followed by strong north winds late
tonight and early Sunday. Dry conditions and cold temperatures are
expected for the start of the new week. Another chance for light
snow is possible as a system slides into the area next Wednesday.
Additional systems late in the week will keep conditions active and
cold.

Short term...as low pressure continues to exit...current obs
showing winds are subsiding...with near term wind headlines being
allowed to expire. Current radar shows much of the activity over the
Cascades is also starting to taper off...with the exception being
snow showers associated with a convergence zone over portions of
eastern Skagit and Whatcom counties as well as portions of
northeastern Snohomish. Although avalanche warning has
expired...caution is still urged...especially in the aforementioned
areas where snow is still occurring.

Continental polar air is expected to quickly spread south through
interior b.C. And through the Fraser later tonight. The bli-cywl
gradient remains forecast to peak near -20 mb on Sunday morning,
making this a strong Fraser outflow event. As such, strong north-
northeast winds will ensue, most notably near Bellingham, the San
Juans, and around to Admiralty Inlet. Wind headlines there for this
event will remain in place. Admiralty Inlet is not currently covered
by the headline...and will likely add them once headlines associated
with earlier today fall by the Wayside to avoid confusion. Other
locations in the area may see breezy conditions persist. The
windiness from the less common northerly direction will have greater
tree impacts than a similar south wind. A modified Arctic front will
spread south and southwest through western Washington early Sunday
morning, with localized frontal convergence and orographic lift
leading to some lowland snow accumulations. The average forecast
snowfall up and down the I-5 corridor will be about 1 to 2 inches.
However, all meso models continue show a distinct hole in the
snowfall pattern from Poulsbo over to Seattle, likely from precip
shadowing downwind of the Olympics in northwest flow aloft. Thus the
Bremerton and Seattle forecast zones will remain excluded from the
Winter Weather Advisory over The Lowlands. Some local enhancement of
snow is expected just away from the water near Port Angeles due to
north-northeast wind upsloping against the North Slope of the
Olympics, so the forecast there is for 2-4 inches of snow.

The front will be followed by the coldest air mass of this winter.
A hard freeze is expected areawide on Sunday night, with Monday
night likely being the coldest night of the upcoming cold spell.
At least the cold will come with drier weather. The last showers
will taper off over the Cascades on Sunday night or Monday, with
dry weather expected Monday and Monday night.

The NAM and GFS keep Tuesday dry as well. The European model (ecmwf) tries to
develop a warm advection nose of precip over western Washington on
Tue. Since only two of the GFS ensemble's 20 members show precip
at hqm on Tuesday, have kept pops on the low side. If any precip
does occur on Tue, it would still be cold enough for snow, even at
the coast.

Inherited forecast covers the above fairly well. While incoming
models will be watched to see if the aforementioned hole in the
snowfall for Seattle and Bremerton areas remains...at this time
there appears no need for any evening updates. Haner/smr

Long term...from previous discussion...a cool northwest flow
pattern will continue from Wed through Sat. A system diving down
from the northwest on Wed could very well spread light precip across
western Washington, with light snowfall accumulations. A lull in
precip is possible Thu.

More precip with more weak shortwaves on Fri and Sat. The 12z
European model (ecmwf) was colder than the 12z GFS, but the 18z GFS seems to be
coming around to a cooler solution. This would keep snow levels at
or below 1000 feet, a situation of the snow threat continuing on
higher lowland hills but less so at sea level. Haner

Aviation...cigs generally MVFR with isolated VFR conditions
present...mainly along the coast. Strong onshore surface flow
continues this evening before transitioning to a more north to
northeasterly flow overnight tonight. Showers continue to taper off
over most locations save for a convergence zone over portions of
Snohomish...Skagit and Whatcom counties. The expectation remains
that once surface flow turns northerly overnight...showers will
transition to snow showers and move south. Timing for this change
over still looks to be around daybreak with clearing from the
north taking place once snow showers have moved through. Smr

Ksea...terminal currently in a clear bubble on current
radar...making chances for showers in the near term pretty minimal
this evening and into tonight. Winds will taper slowly but still
remain gusty tonight and overnight. Scattered snow showers remain
possible with time of most concern falling between 12z and 14z...but
even then amounts...if any...are expected to be light. Another
limiting factor in snowfall will be rapid clearing expected to occur
Sunday morning. Smr

Marine...a modified Arctic front will move south through
western Washington before daybreak Sunday--in fact cold Fraser
outflow northeasterly gales should develop pretty quickly overnight--
spreading down into the Port Townsend to Port Angeles area well
before daybreak. The switch to northerly winds in Puget Sound will
probably be around daybreak for a raw brisk day. Gradients ease
Monday and Monday night and by Tuesday pressure gradients will be
quite light. Mm/smr

Hydrology...river flooding is not expected the next 7 days.

&&

Sew watches/warnings/advisories...
Washington...Winter Weather Advisory until 9 am PST Sunday for Admiralty
Inlet area-Bellevue and vicinity-central coast-east Puget
Sound lowlands-Everett and vicinity-Hood Canal area-lower
Chehalis valley area-north coast-southwest interior-Tacoma
area-western Skagit County.

Coastal flood advisory from 4 am to 8 am PST Sunday for
Bremerton and vicinity-Hood Canal area-Seattle and vicinity-
southwest interior-Tacoma area.

Winter Storm Warning until 6 am PST Sunday for Cascades of
Pierce and Lewis counties-Cascades of Snohomish and King
counties-Cascades of Whatcom and Skagit counties.

Winter Weather Advisory until 4 am PST Sunday for San Juan
County-western Whatcom County.

Wind Advisory until 6 PM PST Sunday for western Skagit County-
western Whatcom County.

Winter Storm Warning until 10 am PST Sunday for Olympics.

Wind Advisory until 6 PM PST Sunday for Admiralty Inlet area-San
Juan County.

Winter Weather Advisory until 8 am PST Sunday for eastern Strait
of Juan de Fuca-western Strait of Juan de Fuca.

Pz...Small Craft Advisory for rough bar until 4 PM PST Sunday for
Grays Harbor bar.

Gale Warning until 4 PM PST Sunday for Admiralty Inlet-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-East Entrance U.S. Waters
Strait of Juan de Fuca-northern inland waters including the
San Juan Islands.

Gale Warning until 3 am PST Sunday for central U.S. Waters
Strait of Juan de Fuca-Puget Sound and Hood Canal-West
Entrance U.S. Waters Strait of Juan de Fuca.

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