High Wind in Washington, December 19, 2016

Area Forecast Discussion
National Weather Service Seattle WA
900 PM PST Sun Dec 18 2016...corrected headlines at bottom

.SYNOPSIS...A vigorous front will move through the area later
tonight and Monday morning. The cold air that has been over the
region will quickly give way to a mild maritime air tonight.
Another strong system will reach Western Washington Monday night
and Tuesday. Yet another front will reach the area Thursday.

&&

.SHORT TERM...The coastal radar indicates some showers near the
coast but the main pre-frontal precipitation shield arriving from
the northwest is already pushing onto the north coast. Steady light
rain should overspread Western Washington by around midnight. The
mid/lower level air mass over the interior is still fairly dry and
cold. Virtual temperatures on the Sand Point profiler are 2-3C up to
at least 2,000 ft. However, as rain spreads inland and precipitation
rates pick up, snow levels could fall to near the surface in some
areas. Whatcom county and the Kitsap Peninsula/lower Hood Canal is
the primary concern tonight with models showing up to
.2 inches through 12z (4am) which could fall as snow, then a mix
before rain Monday morning. A quick inch or two of wet snow is
certainly possible in those areas but the short duration before it
switches to rain does not seem to warrant an advisory at this time.
Will need to monitor these areas overnight as an isothermal profile
could maintain snow a bit longer than expected if precipitation
rates are high.  Elsewhere, rain or mixed precipitation should
become all rain fairly rapidly as southerly pressure gradients
continue to increase. Some of the hilltops above a few hundred feet
could see some wet snow or mix in at times tonight but is unlikely
to amount to anything. The Cascades will be the big winner as far as
snowfall. Passes could see 6 to 10 inches through midday Monday, and
over a foot could fall above 5000 feet. Will keep the winter weather
advisory going through midday Monday.

Southeasterly winds are picking up over the coastal waters and just
now starting to increase around the east entrance to the Strait and
Admiralty Inlet. Models continue to increase gradients too rapidly,
but PDX-BLI and OLM-BLI have shot up the past hour so the flow is
finally kicking in. Strongest winds will occur just ahead of and
along the front which will push across Western Washington early
Monday morning. The wind advisory looks ok, although stronger winds
in the Hood Canal zone will probably be limited to the far north
portion. It will be windy elsewhere, but sub-advisory gusts of 35 to
40 mph forecast seem reasonable.

Any break in rain and mountain snow will be short lived Monday as
the next progressive low will be approaching from the west. Models
have struggled with potential surface low deepening but the new 00z
runs seem to be trending toward a weaker low and less pressure rises
to the south. The NAM-12 is still the weaker and further south
solution with the ECMWF and GFS20 further north with more gradient
across the area. Wind now looks more advisory level for much of the
area but even this is questionable given the much weaker NAM-12
gradients. Besides the wind potential, a warm front will bring rain
and more mountain snow through early Tuesday. A break or lull in
rain/showers may develop later Tuesday and Tuesday night through
Wednesday. Mercer

.LONG TERM...Previous discussion...A front will likely move through
Western Washington Thursday morning followed by a period of cool
showery onshore flow into Friday. Light offshore flow and weak high
pressure Friday night and Saturday then give way to another front
Saturday night and low pressure and more precip following that on
Sunday.

&&

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

&&

.AVIATION...Westerly flow aloft. Moisture will increase overnight
through early Monday as a frontal system in the offshore waters
approaches the area. The front will move through the interior around
15Z Monday. Precipitation will come to an end 18Z- 03Z between
Monday morning's front and the one that will pass the region Tuesday
morning. Precipitation will increase from the southwest again after
03Z Tuesday as a warm front approaches the area.

Conditions are generally VFR across the region this evening.
Conditions will rapidly deteriorate to MVFR as moisture spreads
inland between 06Z and 08Z. Precipitation may briefly begin as snow
at KPAE and especially KBLI where some cooler and drier air remains.
But the cold air will quickly scour out between 09Z and 12Z as south
to southeast winds at the surface and aloft increases. Most
terminals will see a 3-5 hour period of wind 17015-20kt with gusts
to 27-35kt between 10Z and 15Z.

Behind the front expect precipitation to end with the exception to
scattered showers on the west slopes of the Cascades. Ceilings will
remain MVFR Monday afternoon. Albrecht

KSEA...VFR ceilings will deteriorate to MVFR and rain will begin
around 09Z Monday. While precipitation may briefly begin with a few
snowflakes mixed in, precipitation is increasingly appearing that it
will be all in the form of rain as temperatures and dewpoint
temperatures rise with the approaching front and increasing
southerly flow. Southeast wind 6-9 kt will turn southerly and
increase to 12-18 kt with gusts to 28 kt between 10Z and 15Z. wind
will diminish during the afternoon hours and will back to more
easterly by 00Z Tuesday. Albrecht

&&

.MARINE...A strong frontal system over the offshore waters will
move rapidly eastward, exiting the inland waters early Monday
morning. A few hour period of gales is expected late tonight and
early Monday ahead of, and with the front. Gale warnings are up for
all waters except the Central Strait of Juan de Fuca through Monday
morning.

Post frontal onshore flow Monday morning will give a brief period of
small craft advisory winds to the central portions of the strait.
Otherwise winds will rapidly diminish by midday Monday as we move
into a lull between Monday morning's front and the next system
upstream.

A 1000 mb low will develop to 995 mb early Tuesday morning, then
move east across south-central British Columbia by midday Tuesday.
The warm front associated with this low will move northeast across
the area Monday night, then the cold front will push rapidly inland
Tuesday morning. Models continue to show large south to north
pressure gradients with this front and as it moves through.
Gale force winds are expected over most of the waters except the
central strait. The gales over portions of the coastal waters could
be on the high end of the gale range.

Wave models continue to show seas building to 22 to 25 feet to the
south of the low moving onshore to the north, and moving onto the
coast Tuesday and Tuesday night.

Another front will move across the waters Wednesday night.
Albrecht

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from midnight tonight to noon PST Monday
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties.

     Wind Advisory until 10 AM PST Monday for Admiralty Inlet Area-
     Everett and Vicinity-Hood Canal Area-North Coast-San Juan
     County-Western Skagit County-Western Whatcom County.

PZ...Small Craft Advisory for rough bar from 4 AM Monday to 4 AM PST
     Wednesday for Grays Harbor Bar.

     Gale Warning until 9 AM PST Monday 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-Puget Sound and Hood Canal-West Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 4 AM to 3 PM PST Monday for Central
     U.S. Waters Strait Of Juan De Fuca.

&&

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html