Heavy Snow in Oregon, February 20, 2018

Area Forecast Discussion
National Weather Service Portland OR
310 AM PST Tue Feb 20 2018

.SYNOPSIS...Cool weather stays over the region for this week under
a north flow aloft. A low pressure system and a series of upper level
troughs will result in a low-elevation snow event Tuesday night.
The next shortwave trough will provide another chance for low-
elevation snow Wednesday night. Additional troughs will continue a
threat for rain and snow this weekend into early next week.

&&

.SHORT TERM...Today through Thursday...A northerly flow aloft will
continue for the next few days, with a relatively cold air mass
characterized by 850 mb temps fluctuating between -4C and -10C.
Several shortwaves will move south through the region, bringing
chances for snow, mixed with rain only at the lowest elevations. The
models do show some critical differences in the details, and of
course, its the details that count when forecasting snow.

A pair of shortwaves, the first off the southern BC coast early this
morning, and the second over ne BC, will move south today and
tonight. The first tracks south off the coast today, ehile the
second track south across the forecast area tonight. With a weak
surface low tracking south off the coast today, the ne half of the
forecast area will keep a weak offshore flow today allowing the cold
air mass to remain in place, while the sw half of the forecast area
sees a little more of a southerly onshore component to the surface
winds, suggesting a little moderation to the air mass. Either way,
temperatures will rise above freezing at lower levels today, similar
to yesterday. The offshore system will begin to spread mid level
moisture, and to some extent some low level moisture inland today.
This will bring increasing chance for precipitation today, with the
best chances along the coast associated with the first shortwave.
The next shortwave approaches this evening, along with a weak
surface low tracking south across western WA towards the north
Oregon Cascades. Prospects for lift will increase, aided by low to
mid level deformation between the two systems. Timing and location
of the best lift varies some between models, but in general will
tend to prefer the more similar NAM and EC for forecast timing of
the more significant snowfall potential late this afternoon into
tonight, beginning over the lower Columbia and I-5 corridor this
afternoon and shifting south through the western Oregon through the
evening and midnight hours. With afternoon highs generalyl expected
to be in the upper 30s to around 40, expect to see precipitation at
low elevations inland begin as a mix of rain and snow, but turning
to predominantly snow as precipitation rates increase. At the coast,
snow levels will be low, allowing at least some mixing of rain and
snow, but do not expect widespread accumulations with more marginal
temps and some ocean influence. Overall the current headlines look
reasonable. with the caveat that some areas generally on the
periphery of the valley zones may see a little higher amounts where
there is some low level orographic lifting aiding the general lift.

Models generally agree on a bit of a break between systems Wed,
before another shortwave drops down into the region Wed night.
Another weak coastal ow is expected to develop with the influx of
more cold air over the warmer ocean waters, that will enhance low
level onshore flow and as a result low level moisture. Expect this
to result in increased pops Wed night, highest along the coast and
over the nw part of the forecast area. compared to other models, GFS
looks too quick to dig the wave further south. Prefer to go more
with a EC/CMC/NAM blend for Thu, which will slow down the exit of
snow showers from the region until the end of the day Thu. Model
dynamics coupled with low and mid level moisture as well as
lingering cold air mass suggest another threat for accumulating low
level snow wed night into early Thu. Will need to get through todays
system before we consider any headlines for this next event.

.LONG TERM...Thursday night through Monday...A high amplitude ridge
will remain over the Pacific centered near/around 140W for much of
the second half of this week, while a deep trough persists across
the western CONUS during this time. Dry north to northeast flow
aloft gradually shifts on Friday to a more moist northwest flow
pattern in response to a deepening shortwave trough embedded in the
flow along the northern coast of British Columbia. Expect snow
levels to generally remain around 1,500 feet for Friday and early
Saturday as the air mass will gradually be warming late this week.

However, the previously mentioned disturbance will spread some
moisture across the region late Friday night and into Saturday.
There's some indication in the ensemble data that cold air will
surge into the region later Saturday and early Saturday night behind
a front that could bring another threat of snow to the region.
Critical thickness and temperatures aloft would support some snow,
but the timing of the moisture and cold air may or may not end up
arriving in phase to bring snow down to the lowest elevations.
Latest ensemble guidance suggests anything from a relatively mild
rain (i.e. surface temperatures in the mid 40s) to a cold rain (rain
with temperatures in the 33-36 degree range) to snow at the valley
floor remaining within the realm of possibility. Therefore, have
trended the forecast to a more middle-of-the-road compromise until
the guidance can get a better handle on the timing of nature of the
key factors.

The relative low confidence in the temperature and precipitation
type forecast remains through the start of next week where little
change to the overall synoptic pattern will leave the pattern in
some form of northerly flow aloft in the upstream side of a broad
upper trough where various embedded impulses will have the potential
to keep the weather active and more winter-like into at least the
start of next week.   Cullen
&&

.AVIATION...Moisture overrunning the cold air currently
in place will result in VFR conditions this morning
deteriorating into MVFR and IFR thresholds from northwest to
southeast across the area between 15z Tuesday and 06z Wednesday.
Light snow should spread onto the coast this morning and may
reach inland taf sites by 18z Tuesday. Warmer temperatures should
turn snow to a rain/snow mix this afternoon, but a more intense
band of precipitation will drop southward late this afternoon and
evening and likely help usher in lower snow levels and
accumulating snow (1-3" with locally higher amounts possible) in
the Willamette Valley after 00z Wednesday. Expect a mix of IFR
and MVFR conditions with any light snow this morning and IFR and
LIFR conditions with the band of snow that drops southward this
evening and overnight.

PDX AND APPROACHES...Moisture overrunning the cold air currently
in place will result in VFR conditions this morning
deteriorating into MVFR and IFR thresholds later today and
tonight. Light snow remains possible around 15-18z Tuesday, but
temperatures should warm enough that little to no accumulation is
expected and precipitation could switch to a light rain/snow mix.
However, precipitation will likely intensify around 00-03z
Wednesday, which will likely bring a period of LIFR and IFR
conditions and sticking snow with 1 to 3" possible. /Neuman

&&

.MARINE...A weak low pressure will drop southeastward offshore
today and tonight and bring a period of gusty southerly winds to
the waters today, which will be followed by northeasterly winds
tonight. There is a chance southerly wind gusts of 25 kt will
occur within 20 nm of the coast, but still not confident. Will
continue to monitor conditions early this morning before making a
decision.

Another weak low pressure will bring gusty northwesterly to
northerly winds Wednesday night and early Thursday. Surface high
pressure will then traverse the waters on Friday and bring a
temporary lull in the winds before a couple stronger storm
systems bring stronger winds and higher seas over the weekend and
early next week. Seas should generally remain well under 10 ft
through Friday before climbing into the teens Friday night or
Saturday. There is a chance seas could approach 20 ft Sunday,
but considerable uncertain remains. /Neuman

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 4 PM this afternoon to 7 AM PST
     Wednesday for Cascade Foothills in Lane County-Central
     Columbia River Gorge-Northern Oregon Cascade Foothills-
     Upper Hood River Valley-Western Columbia River Gorge.

     Winter Weather Advisory from 2 PM this afternoon to 4 AM PST
     Wednesday for Coast Range of Northwest Oregon-Lower
     Columbia.

     Winter Weather Advisory from 6 PM this evening to 7 AM PST
     Wednesday for Central Coast Range of Western Oregon-South
     Willamette Valley.

     Winter Weather Advisory from 4 PM this afternoon to 6 AM PST
     Wednesday for Central Willamette Valley-Greater Portland
     Metro Area.

WA...Winter Weather Advisory from 4 PM this afternoon to 7 AM PST
     Wednesday for Central Columbia River Gorge-Western
     Columbia River Gorge.

     Winter Weather Advisory from 2 PM this afternoon to 4 AM PST
     Wednesday for Greater Vancouver Area-I-5 Corridor in
     Cowlitz County-South Washington Cascade Foothills-Willapa
     Hills.

PZ...None.

&&

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