Heavy Snow in Oregon, January 13, 2020

Area Forecast Discussion...Corrected
National Weather Service Portland OR
356 AM PST Mon Jan 13 2020

.SYNOPSIS...A series of weather systems moving down into the
Pacific Northwest this week will bring progressively colder air down
into the region while keeping rain and snow likely. The chances for
snow at low elevations will increase through the early part of the
week, especially across the northern part of the forecast area. A
stronger low pressure system approaching the coast Wednesday night
will bring the best chances for widespread snow Wednesday into
Thursday before a milder pattern returns late in the week.

&&

.SHORT TERM...Today through Wednesday...Water vapor imagery and
upper level analyses show a broad upper level trough dominating much
of the western United States and Canada, with a baroclinic zone and
potent jet stream lined up from the Gulf of Alaska to southern Oregon
and the Great Basin. Meanwhile, an arctic front remains hung up near
Seattle, keeping most of the accumulating snow off the valley floors
in SW Washington/NW Oregon for the time being. That said, snow has
been accumulating at a decent clip above 1000 feet. Onshore flow and
steep low-level lapse rates have maintained a decent amount of shower
activity overnight, and even prompted the issuance of a Winter
Weather Advisory for the Willapa Hills and North Oregon Coast Range.
Showers should decrease over the next few hours as lapse rates become
less steep and onshore flow weakens, but another couple inches are
possible above 1000 feet by daybreak.

A series of fast-moving disturbances are moving along the baroclinic
zone, with the next shortwave disturbance upstream presently near
47N/135W as of 11Z. This next disturbance is aiming for the
central/southern Oregon coast, and is showing some signs of
strengthening on water vapor while it approaches. Most of the latest
guidance appears to be initializing this system well, including the
06z NAM, GFS, and 00z ECMWF operational runs. While there are some
subtle differences in strength and track of the related surface low,
most models have settled on a near-1005 mb low moving onshore
somewhere between Coos Bay and Newport early this afternoon.

With cold air tantalizingly close and low pressure moving onshore
near the southern portion of our forecast area today, this will be a
nailbiter of a forecast when it comes to whether or not the valley
floors see accumulating snow. Assuming the strength and track of the
low remain unchanged from current forecasts, it appears the lowest
elevations will largely escape accumulating snow today. However, the
06z NAM soundings for Eugene come very close to changing over to snow
as precipitation increases. This appears to be largely driven by
latent heat processes associated with the melting of snowflakes in
the column of air above, which depends significantly on precipitation
rates being heavy enough to allow this process to cause appreciable
cooling of the column. While it is not out of the question that this
could occur to the point where accumulating snow occurs across a
portion of the Willamette Valley today, it appears too conditional on
too many factors to really bank on in the forecast for any particular
location. Ironically, considering the arctic air to our north,
central and southern portions of the forecast area may stand the best
chance for this cooling of the column to occur today, as they will
likely be closer to the best dynamic support for lift associated with
the incoming low and thus the heaviest precip rates. Will certainly
need to monitor this situation closely while it unfolds today, as a
stronger low would stand a better chance to pull enough cold air
southward for better snow chances on the north side of the low track.
Either way, this system will eventually pull colder air southward on
its back side tonight while crossing the Cascades.

Models have trended upward with the QPF associated with today's
system, and it will be plenty cold enough for snow above 1500-2000
feet across the vast majority of our forecast area. Thus we will need
to upgrade the Winter Weather Advisories to warnings for our Oregon
Cascades and foothills, with the emphasis being on areas south of Mt
Jefferson. With 50th percentile NBM QPF now up to 1 inch or more in
the Cascades, these areas stand to see 8 to 16 inches of new snow
from this system. The Coast Range is trickier, as there may be more
of a rise in snow level due to their closer proximity to the ocean
and continued onshore flow in the low levels. Will split the
difference and issue a Winter Weather Advisory for our Oregon Coast
Range zones for today.

A trailing system moves onshore later tonight/early Tue, a little
further to the south of today's system. As mentioned above, the air
mass will be a little colder, increasing the chance for snow to the
lowest elevations. However this system may move onshore a little too
far south for much more than a dusting in the low elevations and a
couple inches for the higher terrain, mainly south. Otherwise,
onshore flow and scattered rain/snow showers will prevail Tuesday,
not likely amounting to much. Meanwhile, colder air will continue to
seep southward, with 850 mb temps lowering to the -8 to -10 deg C
range per most guidance by Tue evening.

The air mass will likely be cold enough to support snow to the lowest
elevations for much of the forecast area on Wed. However 00Z
deterministic models currently in general agreement Wed will start
out dry as a low center near 44N 132W at 12Z Wed tracks toward the
northeast. A front will approach the coast in the afternoon with
precipitation spreading inland. But GEFS ensemble members show
varying placement of the low by late Wed afternoon, some GEFS
ensemble members and 00Z deterministic models place the low offshore
of Astoria which would support rising snow levels for at least the
southern half of the CWA. The GEFS ensemble mean has the low further
south, which would help to maintain low snow levels.  QPF amounts
appear modest at this time as the front is splitting/weakening as the
main jet energy heads south of the area.   Weagle/mh

.LONG TERM...Wednesday night through Saturday...Confidence in snow
levels and snow amounts for the low elevations remain low Wed night
onward.  00Z deterministic models show a faster northward
progression of the low towards south Vancouver Island. The GEFS
ensemble mean is slower and should this be realized will likely see
snow accumulations for most of the lowlands, possibly at the coast
too. QPF remains modest with 0.10" to 0.25" for the interior
lowlands and coast for a 1" to 3" snow accumulations.  Have nudged
QPF to mix of CONS and WPC guidance.

Models have changed a bit for Thu with the upper and surface low
tracking more along the coast toward Vancouver Island rather than
right over.  Most of the GEFS ensemble members in agreement with
this idea.  This should usher in a more milder air mass for rising
snow levels.  But low level easterly flow through the Gorge will
likely maintain a mix of snow, freezing rain, and sleet Thu and Thu
night through the Gorge and possibly for the Portland/Vancouver
metro area and surrounding suburbs.

Fri and Sat appearing more mild with low pressure systems heading
toward Vancouver Island for rain in the lowland and continued snow
in the mountains.  Lingering wintry mix of freezing rain, sleet, and
snow still possbile in the Gorge and Upper Hood River Valley.

&&

.AVIATION...Mostly VFR early this morning under unstable
northwest flow with the localized brief MVFR with showers. Snow
showers above 1000 feet, and possibly down to 500 feet early this
morning. Flight conditions likely to become more MVFR after 15z,
especially across the south half of the forecast area. SW
Washington may end up more VFR Mon as the main energy from the
incoming frontal system heads into SW Oregon. There is the
potential for -SHRASN over northern portions of the forecast area
late tonight into Tuesday morning.

KPDX AND APPROACHES...VFR in scattered showers will continue
through sunrise, but brief periods of MVFR will be possible in
heavier showers. MVFR becomes more predominate after 14Z. Colder
air moves into the area today for increasing chances for light
snow. Think the best chance for snow showers for the terminal is
late tonight into early Tuesday morning. ~TJ/Weishaar

&&

.MARINE...Have issued a small craft advisory for the waters today
through Wednesday morning. Wind speeds will fall below small
craft criteria later this morning, but the seas will remain above
10 feet for the next several days. The seas will subside today ,
and are expected to subside to around 10 to 12 feet early this
afternoon. A low skirting by south of the waters may produce a
brief period of small craft or possibly low end gales south of
Florence this afternoon, but the best chance for these stronger
winds remain south of marine zones 255 and 275.

Northwest winds will likely reach small craft criteria on
Tuesday as high pressure briefly builds in behind a couple of
relatively weak lows. South winds develop Tuesday night ahead of
a larger and deeper low with Gales likely on Wednesday. Gales are
likely with this storm and have issued a Gale Watch. The main
uncertainty with this event is in regard to the timing of the
gales as the models are still struggling on working out the track
of this low. The seas will be a mixture of large steep wind
waves, a fresh southwest swell, and a weaker northwest swell
Wednesday and Thursday with the significant wave heights likely
building above 20 feet Thursday morning. ~TJ

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until noon PST today for Greater
     Portland Metro Area-Lower Columbia-Western Columbia River
     Gorge.

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

     Winter Storm Warning until 10 PM PST this evening for Cascade
     Foothills in Lane County-Cascades in Lane County-Northern
     Oregon Cascade Foothills-Northern Oregon Cascades.

     Winter Weather Advisory until 4 PM PST this afternoon for
     Central Coast Range of Western Oregon-Coast Range of
     Northwest Oregon.

WA...Winter Weather Advisory until noon PST today for Greater
     Vancouver Area-I-5 Corridor in Cowlitz County-Western
     Columbia River Gorge.

     Winter Weather Advisory until 6 PM PST this evening for Central
     Columbia River Gorge-South Washington Cascade Foothills-
     South Washington Cascades.

PZ...Gale Watch from Wednesday morning through Thursday morning for
     coastal waters from Cape Shoalwater WA to Florence OR out
     60 NM.

     Small Craft Advisory until 10 AM PST Wednesday for coastal
     waters from Cape Shoalwater WA to Florence OR out 60 NM.

     Small Craft Advisory until noon PST today for Columbia River
     Bar.

     Small Craft Advisory from 4 PM this afternoon to 4 AM PST
     Tuesday for Columbia River Bar.

&&

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This discussion is for Northwest oregon and Southwest Washington
from the Cascade crest to 60 nm offshore. This area is commonly
referred to as the forecast area.