Heavy Snow and High Wind in Oregon, February 5 to 6, 2017

Area Forecast Discussion
National Weather Service Portland OR
629 AM PST Sun Feb  5 2017

.SYNOPSIS...A complex and potentially high-impact winter weather
event is beginning to take shape across SW Washington and NW Oregon.
A stalled frontal zone draped across the area is becoming entrained
with increasing moisture from the subtropics. Precipitation rates are
increasing, which is gradually bringing down the snow levels this
morning. This process will likely result in the snow levels lowering
to 1000 feet or lower at times...in fact this is already occurring in
the North Oregon Coast Range. The threat of valley or sea level snow
continues, with the potential for several inches north of a Salem to
Newport line. The most likely area to see significant lowland snow
appears to be the North Oregon Coast, Lower Columbia, and I-5
Corridor in Cowlitz County. The Willamette Valley north of Salem will
likely see some snow as well, just not as much. As low pressure moves
onshore into Oregon later today and across the Cascades,
precipitation will turn more showery and low-level flow will turn
onshore...pushing the snow level up to around 1000 feet or so.
Another system may approach from the southwest Tuesday bringing the
potential for some mixed precipitation to the inland valleys...
particularly near the Columbia Gorge. Another potent frontal system
is possible around Thursday.

&&

.SHORT TERM...Today through Monday...A complex weather scenario
continues to take shape across the Pac NW and adjacent waters this
morning, with a frontal zone stretched across NW Oregon. Moisture is
increasing along the front, as well as the lift due to increasing low
and mid-level frontogenesis. Very little has changed with the big
picture, as a cold upper low continues to rotate off the WA Coast and
a southern stream system approaches the US West Coast from the
Pacific. While the southern stream system is pulling moisture
northward from the tropical/subtropical latitudes, precipitable water
values are not all that impressive...on the order of 0.7 to 0.8
inches in general. Deeper moisture exists closer to the southern low,
with PW values closer to 1.00 to 1.25 inches. This deeper moisture is
assisting in some decent convection near the southern low, the debris
of which will likely continue to feed moderate to occasionally heavy
precipitation across NW Oregon and SW Washington. While the upper
trough associated with the southern stream system is expected to move
into Northern California Sun night, there is good consensus between
models at this point that the surface low will move onshore into
Oregon. However, the devil is in the details, and latest guidance has
trended slowly northward with where this low makes landfall. This
increases the chance that our frontal zone will nudged northward
today as the low approaches, which could keep much of the Willamette
Valley in the warm sector if the 00z ECMWF verifies. The 00z and 06z
NAM/GFS continue to suggest the low moving onshore slightly farther
south, which would tend to keep the warm sector suppressed to the
south a little longer. The position of this quasi-stationary front
will dictate a lot in terms of which areas in the lowlands get snow
(near and north of the front), and which just see a cold or even mild
rain (south of the front).

Based on radar trends and surface observations, the frontal zone
appears to be between Aurora and Salem. This front will probably
serve as the path for the surface low while it approaches the coast,
so it may afford a hint regarding how the low may track. So...at this
point, we are favoring areas north of Salem for the possibility of
lowland snow, while areas south of Salem...such as Eugene and
Florence...have a better chance of staying just rain.

Model QPF with this system has been impressive for the last several
runs, with 2-5 inches of liquid equivalent for much of the district.
Model soundings have wavered a bit on whether or not rainfall rates
will be sufficient to pull the snow level all the way down to sea
level or the valley floors...backing off slightly in the past couple
runs. However, looking at NAM and GFS model cross sections, 850-700
mb frontogenesis looks impressive Salem northward with strong lift
through the dendritic growth zone (-12 to -20 deg C). This is perfect
for producing copious amounts of precipitation. Meanwhile, strong
high pressure over British Columbia keeps cold air fairly stubborn
over Washington while the low moves into Oregon. Persistent upper
troughing over BC/WA is also helping to keep cold air a bit more
stubborn than it normally would be with a 990-995 mb low moving
onshore.

While snow levels would normally only lower to 2000 feet or so in
this situation, the combination of cold air and heavy precipitation
rates is already causing the snow level to gradually lower this
morning. The latent heat of melting snowflakes...a similar phase
change to evaporative cooling...can also act to cool the column
isothermally to near the freezing point. This is what many model
soundings have shown over the past 24-36 hours for this afternoon and
evening...though models have recently backed off slightly. Bottom
line is that it is very difficult to tell just how much of this
cooling will occur until we see the precipitation evolve this
morning...but Timber Junction at 780 feet recently changed over to
snow and Sunset Summit at 1400 feet in the Coast Range is starting to
see snow covering the road. So the forecast thinking appears to be
working, although the biggest lowland snow threat may have been
shifted about 30-50 miles north over the past 24 hours.

So...given that snow now looks unlikely for Eugene, we canceled the
Winter Storm Watch for the South Willamette Valley and adjacent
Cascade foothills. Given the marginality of this event, and the
likelihood that there will be periods of lighter precip interspersed
with the heavier periods - which could change snow back to rain - we
decided to cut snowfall forecasts a bit for the Portland Metro area.
Current forecast is for Advisory amounts of 1-4 inches of snow for
the Willamette Valley north of about Salem or McMinnville. As
mentioned, areas slightly further north... such as Kelso or Castle
Rock... are closer to the cold air and thus require less work to
bring the snow level down to the surface. This is also the case for
the North Oregon Coast, since they are slightly further NW of the
incoming low than Portland/Salem. This brings the chance of higher
snowfall amounts to those areas...perhaps with warning amounts more
on the order of 4 to 8 inches. Abundant moisture and healthy
orographics will likely hammer much of the Coast Range and Cascades
north of Santiam Pass with over a foot of snow...possibly 2 feet plus
in the S WA Cascades.

Then again, this lowland snow situation is so delicate that it could
still easily fall apart if QPF ends up being less than guidance for
some reason. Areas above 1500 feet elevation are likely to see
several inches of snow, but the big question is of course whether or
not the more populated lower elevation areas are affected.
Unfortunately, the honest answer to that question is that we still do
not know the extent of how the lowlands will be affected. With 1-2
inches of QPF, there is certainly explosive potential for very high
snow amounts...but this is on the tail end of the possible outcomes.
Feel about as confident in our forecast as we possibly can...given
all the unknowns. Once this system starts to show its hand, with
things like what is being observed at Timber Jct and Sunset Summit on
Hwy 26, we will become a lot more confident in the details for
tonight and Monday.

By the way, there is also the potential for flooding if this system
remains warm enough. The best chance for this appears to be in the
warm sector, which seems to be favoring areas south of Salem and
Newport. We have a Flood Watch out to address this...though it still
encompasses a larger area than that. We may be able to refine those
details further with the afternoon package as details become better
known.	Weagle

.LONG TERM...Monday Night Through Wednesday...The stalled frontal
system over the region Sun and early Mon will be dwindling as Mon
progresses. Expect some showers to linger into Mon night as an upper
level trough axis moves through. There will then be a period of
drier conditions late Mon night into Tue morning and possibly some
scattering in the cloud cover as light offshore flow develops near
the surface. However, another surface low will form offshore along
the baroclinic zone draped over the NE Pac on Tue and approach the
coast from the SW. The low will lift a warm front and associated
precip into Oregon. There is considerable uncertainty on the track
of this low, and the exact track has significant impact on the fcst
for us. The 00Z GFS and ECMWF bring the low onshore along the
northern California coast, with most of the warm frontal rain
confined to N Calif and S Oregon, with some light precip reaching
into the southern portions of our CWA. Meanwhile, the NAM has
continued the trend of bringing the low further north, along the
central Oregon coast. This track would bring significant precip to
most of the CWA. Furthermore, with light offshore flow developing
north of the low, we would again be dealing with the potential for
low elevation snow. The KPDX NAM fcst sounding is solidly snow Tue
afternoon and evening. The NAM appears to be somewhat of an outlier
based on the SREF and GEFS ensembles, so will keep a blend favoring
the GFS and ECMWF for now. However, the fcst has a mention of a
rain/snow mix in the fcst for the northern half of the valley late
Tue. Will need to keep a close eye on model trends over the next few
cycles.

Another low pres system will approach Wed into Thu. There is some
uncertainty on the timing of this feature, with precip potentially
increasing during the day Wed, or possibly holding off until Wed
night. This low will move north toward the B.C. coast, so it will
bring a moderating SW wind to the Pac NW. It appears to have
subtropical moisture associated with it and should bring a good shot
of rain to the region. Snow levels will likely rise above the passes
with the initial warm front late Wed.

Wednesday Night through Sunday...A warm front is expected to bring
moderate rain Wed night into Thu. We may also need to watch for high
wind potential along the coast with this system. The cold front is
modeled to move through later Thu into early Fri. An upper level
trough moves through Fri, bringing another shot of rain and mountain
snow as the snow levels drop back below the Cascade passes. Some
showers likely linger into Sat. But the trend will be toward drier
weather over the weekend as an upper level trough begins to build
over the Pac NW. Pyle

&&

.AVIATION...A mix of MVFR and VFR CIGS continue with steady -RA
and occasional RA. Expect MVFR to become more prevalent through
the morning hours as rain rates intensify. A low will move
through the area, likely tracking between KEUG and KSLE this
evening with areas north of the track seeing -RASN and eventually
some -SN in the evening and overnight hours before switching back
to -RA after a trailing front passes through. Amounts will be
quite variable depending on when snow starts in relation to the
precipitation rates, but heaviest snow appears like it will be
north of a KTMK to KPDX line and likely spare KAST for the most
part.

KPDX AND APPROACHES...Mostly VFR in the area early this morning
but expect deteriorating conditions as rainfall rates increase
into the morning hours. Rain will begin to mix with snow
somewhere around 21Z Sun and switch to all snow around 03Z Mon.
Expect a switch back to -RASN or even straight to -RA after the
front passes through, likely before 12Z Monday. Bowen

&&

.MARINE...Winds turning to out of the west early this morning and
have decreased to 15 to 20 kt with gusts to 25 kt. Expect winds
to turn more northerly later today and decrease further. As a low
pressure system moves near and onshore around North Bend, OR,
winds south of about Waldport will turn back to out of the south
and pick up with gusts to about 30 kt possible this afternoon and
evening. Seas will hover around 10 ft into the morning hours and
then generally decrease. But the same area which sees increased
winds later today will likely also see seas increase back to
around 10 ft for a bit. Have adjusted Small Craft Advisories for
Winds and Seas to reflect this trend.

There will be a few periods of borderline small craft advisory
winds and seas for the first half of the work week, but main
focus will be increasing seas and winds Wed night through Thu
night. Models continue to waffle with seas, now keeping them
below 20 ft instead of near 30 ft as previous runs showed. Also
seeing the possibility of storm winds with this system, but at
this point gales appear to be the most likely scenario. Again,
expect plenty of changes to the forecast between now and then.
Bowen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 6 PM PST Monday for Northern Oregon
     Cascade Foothills-Northern Oregon Cascades-Upper Hood
     River Valley.

     Winter Weather Advisory from 10 AM this morning to 6 AM PST
     Monday for Central Willamette Valley-Greater Portland
     Metro Area-Western Columbia River Gorge.

     Winter Weather Advisory until 6 PM PST Monday for Cascades in
     Lane County-Central Columbia River Gorge.

     Flood Watch from 8 AM PST this morning through Monday morning
     for Cascade Foothills in Lane County-Central Coast Range
     of Western Oregon-Central Oregon Coast-Central Willamette
     Valley-Coast Range of Northwest Oregon-Greater Portland
     Metro Area-North Oregon Coast-Northern Oregon Cascade
     Foothills-South Willamette Valley.

     Winter Storm Warning from 10 AM this morning to 6 AM PST Monday
     for Lower Columbia-North Oregon Coast.

     Winter Weather Advisory from 10 AM this morning to 1 AM PST
     Monday for Central Oregon Coast.

     Winter Storm Warning from 8 AM this morning to 6 AM PST Monday
     for Central Coast Range of Western Oregon-Coast Range of
     Northwest Oregon.

WA...Winter Storm Warning until 6 PM PST Monday for South Washington
     Cascade Foothills-South Washington Cascades.

     Winter Weather Advisory from 10 AM this morning to 6 AM PST
     Monday for Greater Vancouver Area-Western Columbia River
     Gorge.

     Winter Weather Advisory until 6 PM PST Monday for Central
     Columbia River Gorge.

     Flood Watch from 8 AM PST this morning through Monday morning
     for Greater Vancouver Area.

     Winter Storm Warning from 10 AM this morning to 6 AM PST Monday
     for I-5 Corridor in Cowlitz County.

     Winter Weather Advisory from 10 AM this morning to 1 AM PST
     Monday for South Washington Coast.

     Winter Storm Warning from 8 AM this morning to 6 AM PST Monday
     for Willapa Hills.

PZ...Small Craft Advisory for winds until 8 AM PST this morning for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Small Craft Advisory for hazardous seas until 10 AM PST this
     morning for Coastal Waters from Cape Shoalwater WA to
     Cascade Head OR out 60 nm.

     Small Craft Advisory for winds from 2 PM this afternoon to
     midnight PST tonight for Coastal Waters from Cascade Head
     OR to Florence OR out 60 nm.

     Small Craft Advisory for hazardous seas until 4 AM PST Monday
     for Coastal Waters from Cascade Head OR to Florence OR out
     60 nm.

     Small Craft Advisory for Rough Columbia River Bar from 8 AM
     this morning to 3 PM PST this afternoon.

     Small Craft Advisory for Rough Columbia River Bar from 10 PM
     this evening to 2 AM PST Monday.

&&

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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.