Heavy Snow in Washington, January 10 to 11, 2017

Area Forecast Discussion...Updated
National Weather Service Portland Oregon
1025 AM PST Tue Jan 10 2017

.SYNOPSIS...A low pressure system will move onto the south Washington
coast this morning with a warm front over the central portion of
Oregon. The warm front will lift north this afternoon as a developing
surface low approaches the coast. This slow will draw cold air from
eastern Washington and Oregon west of the Cascades with snow
developing in much of the north and central Willamette valley this
evening through Wednesday morning. Colder but dryer conditions are
expected  over Western Oregon and Southwest Washington Wednesday
night through Friday.

&&

.SHORT TERM...Today through Friday...Warm front over central portion
of Western Oregon with a weakening deformation zone over southwest
Washington this morning. The deformation zone will die out as the
surface low along the south Washington coast moves inland today. Snow
levels will remain low over the Cascades today with the heaviest
precipitation in the Lane County Cascades where Winter Storm Warning
looks good.

First the changes to watches and warnings...advisory for the South
Washington Coast and the Central Oregon Coast will be cancelled.
Advisory for the Central Coast range will be limited to north of
highway 34. The Columbia River Gorge and Upper Hood River Valley will
be upgraded to a Winter Storm Warning.	All other Advisories will
remain.

Water Vapor and IR imagery show strong upper trough approaching from
the southwest. This sets up a complex winter precipitation pattern
for the area.  12Z GFS and NAM bring developing surface low a bit
farther north making landfall along Central Oregon Coast then moving
northeast through the Willamette Valley tonight. I prefer the
12Z NAM forecast, although similar to the GFS, in bringing the cold
air wrap around into the northern Willamette Valley beginning mid
afternoon today.  The 12Z NAM BUFR sounding for Portland and
Troutdale show a dramatic wind shift in the lower levels at 21Z
today. GFS is similar but delays the colder air a few hours.  As the
low approaches and deepens the gradient increases after 00z drawing
colder air from the north at the surface to 850mb down the east
slopes of the Washington Cascade and through the northern Cascade
passes and Columbia Gorge this evening. The snow level will come
crashing down in the north this evening...not that it is all that
high to begin with. This will bring all snow to areas north of about
Salem. Precipitation in the Northern Willamette Valley, Gorge,
Southwest Washington interior will be driven by deformation zone
processes with some significant banding possible.

Snow should begin during the early evening in the N Willamette Valley
and Clark county and I5 Corridor in SW Washington with rain changing
to snow in Salem around Midnight. The cold air will be slow to move
down the Willamette Valley late tonight and Wednesday morning so the
Southern Willamette Valley will likely remain all rain through
Wednesday morning. By the time the cold air reaches Eugene the low
will be shifting east and precipitation will be limited.

South of the low the snow levels will rise today and this evening
with the Lane County Foothills, Central Coast and Southern Willamette
Valley all warming this evening.

This is a somewhat low confidence forecast as the exact track of the
low will dictate where the rain/snow line is and the areas of
deformation will determine the heaviest snow areas. If the low moved
farther south there will be much less snow in the North valley and
Clark County and I5 Corridor of Southwest WA.

Right now it doesn'tlook like the far north Oregon coast and the
South Washington Coast will get much snow at all. Areas around
Tillamook could receive the most snow.

Snow amounts in the Portland/Vancouver Metro area is a very
difficult forecast and will give a preliminary estimate of 1 to 4
inches. However, if the track of the low is just right it is possible
to see higher amounts...especially along the upslope areas of the
west hills and east slopes of the north Coast Range, like around
Forest Grove. If the low tracks farther south...then 1 inch to
dusting is possible.

The Gorge and Upper Hood river valley could get hit hard again with
strong east winds gusting in developing tonight in the gorge with
gusting to 50 mph with considerable falling and drifting snow. Upper
Hood River valley will be in prime upslope conditions tonight
through Wednesday morning. The models indicate the deformation zone
will near the Gorge and we will issue a Winter Storm Warning for the
Gorge and the Upper Hood River Valley.

Precipitation will decrease tomorrow afternoon in all areas as the
low exits and is replaced by colder air.  East winds will persist
Thursday and Friday, gradually decreasing. The air mass will slowly
dry Wednesday night and be quite dry Thursday. but with light winds
there could be some freezing fog developing over the Willamette
Valley. Schneider

.LONG TERM...No Changes. Previous discussion follows...Thursday night
through Monday...The extended forecast period looks abnormally normal
by this winter's standards. The positive-tilt upper ridge leans over
further Friday, likely leading to increasing clouds and a chance of
rain sometime during the weekend as a fairly modest cold front moves
in from the Pacific. Low-level cold air trapped east of the Cascades
will have moderated slightly by then, but there will probably be
enough left over to bring the threat of snow...sleet...or freezing
rain to the Columbia Gorge and Hood River Valley. Longer range models
are hinting toward a milder regime early next week...which is
supported by the CPC 6-10 day temperature forecast suggesting a
slightly higher chance of above normal temps. This may facilitate a
warmer rain event with higher snow levels early next week, as the 18z
GFS/12z ECMWF and 12z GEM all hint toward an atmospheric river event
by next Tuesday. Something to keep an eye on, but a lot can change
between now and 7-8 days from now.  Weagle

&&

.AVIATION...Showers spreading inland this morning will bring
predominantly MVFR conditions with occasional IFR and VFR conditions
to most TAF sites. Brief high pressure this afternoon should
allow many sites to trend predominantly VFR between 18Z Tuesday
and 22Z. However, a surface low pressure moving onto the central
Oregon coast this evening will result in rain and MVFR conditions
spreading back into the region from south to north this afternoon
and into the evening. Cold air being drawn out of the Columbia
River Gorge should result in snow levels lowering to the valley
floor between 03-09z Wednesday for sites north of Salem with snow
becoming increasingly likely. Cold air won't arrive further south
until later tonight but precip should turn to snow when it does
arrive. The coast is a bigger question with snow more likely north
than south, but fairly low accumulations.

KPDX AND APPROACHES...Rain showers beginning to exit the area this
morning and could see an improvement to VFR conditions between
about 18Z and 22Z. Rain and likely MVFR conditions then return
this afternoon along with strengthening easterly winds. Precip
likely turning to snow between 03-06Z Wednesday with accumulation
expected. Most of the snow is expected to fall overnight tonight,
tapering off through the day Wednesday. Bowen

&&

.MARINE...Rain showers are moving out of the area this morning as
local low pressure weakens. Winds should begin to shift more
easterly this afternoon as a stronger surface low pressure
approaches the region and moves ashore along the central Oregon
coast tonight. This should result in gusty east winds continuing
across the northern waters. There is a chance the central Oregon
waters could see a few gusts to 25 kt, but looks like gusts will
be infrequent and localized. Seas will continue a general downward
trend today, dropping below 10 ft this evening and then to around
6 ft Wednesday.

Weak high pressure should then shift over the northeast Pacific
for the second half of the work week and bring weak north to
northeasterly winds and lower seas to the waters. A series of
fronts will then likely produce periods of gusty southerly winds
and higher seas over the weekend and early next week. Bowen/Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 4 PM PST Wednesday for Central
     Columbia River Gorge-Upper Hood River Valley-Western
     Columbia River Gorge.

     Winter Weather Advisory until 1 PM PST Wednesday for Northern
     Oregon Cascade Foothills-Northern Oregon Cascades.

     Winter Storm Warning until 4 PM PST Wednesday for Cascades in
     Lane County.

     Winter Weather Advisory from 7 PM this evening to 1 PM PST
     Wednesday for Central Willamette Valley-Greater Portland
     Metro Area-Lower Columbia.

     Winter Weather Advisory from 10 PM this evening to noon PST
     Wednesday for Central Coast Range of Western Oregon.

     Winter Weather Advisory from 7 PM this evening to noon PST
     Wednesday for Coast Range of Northwest Oregon.

     Winter Weather Advisory from 4 AM to 4 PM PST Wednesday for
     South Willamette Valley.

     Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Wednesday for North Oregon Coast.

WA...Winter Storm Warning until 4 PM PST Wednesday for Central
     Columbia River Gorge-Western Columbia River Gorge.

     Winter Weather Advisory until 1 PM PST Wednesday for South
     Washington Cascade Foothills-South Washington Cascades.

     Winter Weather Advisory from 7 PM this evening to 1 PM PST
     Wednesday for Greater Vancouver Area-I-5 Corridor in
     Cowlitz County.

     Winter Weather Advisory from 7 PM this evening to noon PST
     Wednesday for Willapa Hills.

PZ...Small Craft Advisory for winds until 9 AM PST Wednesday for
     Coastal Waters from Cape Shoalwater WA to Cascade Head OR
     out 60 nm.

     Small Craft Advisory for hazardous seas until 9 PM PST this
     evening for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 5 PM
     PST this afternoon.

&&

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. The area is
commonly referred to as the forecast area.