Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 192333

Area Forecast Discussion
National Weather Service Portland OR
332 PM PST Mon Feb 19 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...cold dry air will move into
the area tonight under north flow aloft. Clearing this afternoon and
tonight should allow for radiation cooling, and for the overnight
temperatures to lower into the teens for the Cascade foothills,
the low to mid 20s for the interior valleys, and the upper 20s to low
30s for the coast.

The dry and cold conditions tonight will prime the environment for a
low-elevation snow event on Tuesday. A surface low drops south just
offshore the Washington and Oregon coast late tonight through Tuesday
morning. Some moisture associated with this low will bring light
precipitation to the region Tuesday morning and early afternoon. This
precipitation will reach the coast early, then move east and inland
through the late morning. Snow levels will generally be between 200
and 500 feet, yet snow will likely mix down to the valley floor.
South winds ahead of the low will provide slightly higher snow levels
(400-800 feet) for the coast and do not expect much if any snow
accumulations for the coast. Snow accumulations in general will be
light Tuesday morning and expect less than an inch of snow for the
higher terrain and less than a half of an inch for the interior

A secondary upper level short wave trough will drop south from the
north in the afternoon and provide a better opportunity for
accumulating snow across SW Washington and NW Oregon. Models are
fairly consistent that a baroclinic band will set up either across
the Portland metro area or just north of the Portland metro area.
Snow levels will remain low as the precipitation is enhanced over the
area and most locations should have measurable snow. There is high
confidence that measurable snow will occur at the lower elevations,
with a moderate confidence on the location of the heaviest
precipitation and the timing of the snow.

In general, the best chance for the accumulating snow will be in the
late afternoon through the evening north of Salem, and the late
evening through late at night south of Salem. There is a higher
confidence for snow north of Salem. Snow accumulations on the coast
is questionable. If the moisture band stalls over the area, it is
possible that isothermal cooling could lower the snow levels down to
the beaches under the band, but that is a lot of `ifs` to have
accumulating snow for elevations below 500 feet along the coast, and
have opted to forecast what the most likely scenario is rather than
the most impactful. With that said, Winter Weather Advisories have
been issued for all zones except the Cascades and the coast.

The snow will decrease from south to north late Tuesday night into
Wednesday morning. Dry cold air will move in from the north late
Tuesday night into Wednesday morning, and low temperatures will again
be in the teens to low 20s inland and the low 30s for the coast.

Then there will be another opportunity for more low-elevation snow
Wednesday night and Thursday as another upper level short wave trough
drops south across the area. Current models suggest that snow
accumulations will be less than an inch Wednesday night into
Thursday, but the latest ECMWF is forecasting slightly more
precipitation. ~TJ

.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...Cigs are scattering out from NW to SE today as weak
high pres briefly settles over the region and dry northerly flow
strengthens. Expect this trend to continue through the evening
and early overnight hrs, with VFR conditions in control. A low
pressure system dropping south along the WA coast overnight will
move through the OR offshore waters during the day Tue. This will
bring increasing high clouds overnight. Cigs will lower into the
3000-5000 ft range Tue, with light precipitation overspreading
the region. Precipitation likely will fall as light snow in most
locations Tue morning, then a rain/snow mix in the afternoon.
A light dusting of snow is possible during the morning hours at
any of the TAF sites, but should be no more than that.
Predominantly VFR conditions to start the day will likely
deteriorate to a VFR/MVFR mix of possibly predominant MVFR
conditions by late morning or early afternoon. Pyle

PDX AND APPROACHES...Clouds to scatter out later this afternoon
and evening. High clouds will increase overnight, but VFR
prevails into Tue morning. Cigs will lower through the day Tue,
with light snow possible in the 12Z-18Z timeframe. Little to no
accumulation expected. A light rain/snow mix will likely fall at
times from 18Z-00Z Wed. VFR conditions through much of the
morning will likely deteriorate to a mix of VFR and MVFR by
around 18Z. Pyle


.MARINE...Winds have been steadily weakening today as high
pressure briefly rebuilds over the coastal waters. Have allowed
the Small Craft Advisory for winds to expire, and expect winds to
continue weakening tonight. The NW winds will shift to the SW
late tonight as a low pres system approaches the WA offshore
waters. This low will move south through the OR offshore waters
during the day Tue, and the fcst models suggest that there may be
a narrow area of southerly winds gusting 20 to 25 kt may occur
during the late morning and afternoon within 30 nm of the coast.
A small craft advisory may be required. Winds will shift to the
NE Tue night into Wed as the low pushes south coasts the CA
coastal waters. Winds may gust to 25 kt at times Tue night and
early Wed morning, mainly over the outer waters.

Yet another low pressure system is modeled to move down the coast
Wed night and Thu morning, This brings the potential for more
low-end small craft advisory NW winds Thu. Additional low
pressure system will bring more chances for gusty winds heading
into the weekend.

Seas have been gradually subsiding today as the NW swell from the
weekend system begins to decay. The latest buoy obs are still in
the 10-12 ft range, so will continue the small craft advisory
for seas into the early evening. Seas should drop below 10 ft
this evening, then remain below 10 ft through the remainder of
the week. Pyle


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

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

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

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

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

PZ...Small Craft Advisory for hazardous seas until 7 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 9 PM
     PST this evening.


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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. is the U.S. government's official web portal to all federal, state and local government web resources and services.