Heavy Snow in Washington, November 27 to 28, 2016

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Sun Nov 27 2016

.SYNOPSIS...Light showers this morning will transition to rain and
mountain snow this afternoon as a vigorous cold front approaches.
Snow will become heavy at times this evening, affecting travel
through the Cascade passes. Showers will decrease Monday, and an
upper ridge should bring dry weather Tuesday. Another front will
arrive Wednesday. Dry weather is possible Thursday and Friday, ahead
of a system due for the weekend.

&&

.SHORT TERM...Radar is showing an uptick in spotty rain or showers,
mainly toward the coast and around Everett northward. The leading
edge of the 500 mb cold pool is just reaching the coast, probably
increasing instability shower activity. Lower level flow around the
925-850 mb level is increasing out of the southwest and will become
strong by midday. A warm frontal wave will approach the coast this
afternoon, driven by a strong 175kt Jet. However, models are
generally showing the Jet to sag south, directing the strongest flow
into Oregon and extreme southern Washington. This should minimize
warm advection into our area with snow levels likely to remain near
3000 feet possibly fluctuating between 3000-4000 feet from
Snoqualmie pass south this afternoon. Rain and mountain snow will
start off light ahead of the surface low and associated cold front
nearing the coast around 00z (4pm) this afternoon. A couple inches
of snow could fall in the passes late this afternoon, with a brief
mix possible in Snoqualmie pass at the start. Snow will become heavy
at times in the Olympics late this afternoon and in the Cascades by
early this evening with snow levels falling to 2500 feet, below all
passes during the heaviest precipitation. Rain will also increase
over the rest of the lowlands.

Models show up to about a half inch of rain over the lowlands, which
will transition to showers after midnight. Some shadowing around
Puget Sound could result in lower rain totals. The convergence zone
is likely to become active behind the front this evening. This could
produce locally higher rain amounts from Everett to Seattle. More
importantly, the strong post frontal flow and -30 to -32C air at 500
mb will enhance orographic snow along the west slopes of the
Cascades/Olympics. Latest models have shifted the strongest flow a
bit southward compared to previous runs. Deepest moisture and strong
flow combination may shift the heaviest snowfall over the Oregon
Cascades and extreme southern Washington Cascades. Snow is still
expected to be heavy at times over all the mountains in Washington
above 2500 feet but probably at the high end of advisory amounts, 6
to 11 inches with highest amounts in Stevens Pass, Mount Baker, and
Hurricane ridge. Travel through all passes will be impacted late
this afternoon and tonight. Windy conditions may also cause some
blowing snow and reduced visibility. It is advised to call 5-1-1 to
check travel conditions before departing, carry chains, and be
prepared for delays.

Precipitation will tapper to showers sooner over lowlands, likely
after midnight. Orographic enhanced snowfall in the mountains will
continue through the night but also start to ease Monday morning.
A short wave ridge will traverse the region Monday night and Tuesday
with a period of mostly dry weather. Expect colder morning low
temperatures and some patchy fog.

The next system arrives Tuesday night and Wednesday with more rain
and mountain snow. This system appears weaker than the one expected
tonight, but measurable rain and some additional snow accumulations
are a good bet for the mountains. Mercer

.LONG TERM...No update from previous discussion...Wednesday brings a
return to normalcy with a system passing through W WA that looks
very similar to what is expected for this afternoon and tonight.
Model progs remain consistent in showing that this system appears
weaker than what is expected in the near term. And like that
system...there appears to be a follow up ridge behind it expected
for Thursday and continuing into Friday. If the GFS is to be
believed...the dry conditions may extend into Saturday for some
locations as it brings the next system in more to the north. The
ECMWF brings this follow-up further south...impacting more of the
area. Current forecast splits the difference with generally low-end
chance POPs for the CWA.  SMR

&&

.AVIATION...Northwesterly flow aloft this morning will become more
westerly as a front approaches the coast. Flow aloft will become
northwesterly again tonight after the front passes. At the surface,
moderate southerly flow today will become strongly onshore tonight.
The air mass is stable and moist. Rain will develop this afternoon.

Ceilings are mostly in the 3-4K ft range with a few lower ones.
Conditions will generally fall to MVFR 2K ft ceilings this afternoon
as the front moves in, and stay MVFR through the night.

KSEA...Ceilings 2K ft should rise to 3-4K ft this morning, then fall
to 2K ft again as rain develops in the afternoon. Southerly wind 8-
12 KT today will increase to 15g25 KT tonight. CHB

&&

.MARINE...A front will reach the coast this afternoon and move
through the inland waters this evening. Southerly winds will
increase to small craft advisory levels ahead of the front this
afternoon. Post frontal winds are forecast to be strongly onshore,
with gales on the coast and down the strait tonight into Monday. A
gale warning is in effect for those waters, with small craft
advisories elsewhere.

In the wake of the front tonight, westerly swells of 15 feet or so
are possible on the coast Monday night, with 20 foot swells over the
offshore waters. Another front will arrive around Wednesday. This
one has been weaker in recent model runs. CHB

&&

.HYDROLOGY...Only minor update...The Skokomish River at Potlatch has
fallen to near the minor flood stage threshold of 17 feet as of 800
am this morning. Some slowing of the downward trend may occur but
more than likely the river will stay below flood stage by this
afternoon. The warning remains in effect but will watch trends the
next few hours. It is possible the warning could be ended earlier
than expected, but refer to the latest flood statement for current
information.

The Chehalis at Porter crested during the early morning hours, but
remains near the 21 foot flood stage with only minor flooding. The
flood warning will likely remain in effect tonight as the flood
crest works its way downstream into Grays Harbor. The Chehalis surge
plain may experience minor flooding around the time of high tide at
11 PM tonight.

Landslides remain a concern. USGS landslide indices show a continued
threat from the recent heavy rain. A minor landslide was reported
near Quilcene on Saturday. A special weather statement addresses
this.

The systems Sunday night and Wednesday appear to be fast moving.
Though more rain will fall, it is not expected to produce flooding.
A cooler and drier period is forecast later in the week. After the
current flooding episode has ended, more flooding is unlikley over
the next seven days. dtm

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 3 PM this afternoon to 7 AM PST
     Monday for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit
     Counties-Olympics.

     Flood warnings in effect for portions of Western Washington.
     See the latest flood statement for details.

PZ...Gale warning coast and strait.
     Small craft advisory other waters.
     Small craft advisory for rough Grays Harbor bar.

&&

www.weather.gov/seattle

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