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Area forecast discussion
National Weather Service Portland or
924 PM PST Wed Feb 22 2017

Synopsis...a series of cool storm systems will bring cooler than
average temperatures, off and on showers and occasional snow to the
Coast Range and Cascade foothills into early next week. The best
chance for snow to dip to sea level and the valley floors will be on
Friday morning, but any accumulations should remain rather short
lived as temperatures warm Friday afternoon. Saturday looks to be
the driest day in the near future.



&&



Short term...tonight through Saturday...infrared satellite imagery
reveals plenty of cold core cumulus across the eastern Pacific and
Pacific northwest with the area under large scale north to
northwesterly flow aloft. As interior locations heated up this
afternoon, showers pushed eastward across the Coast Range and into
the Willamette Valley. Most showers were fairly benign, but snow,
graupel and small hail and even a rumble of thunder or two
accompanied the strongest showers. Photos out of Corvallis and
Keizer revealed enough graupel and small hail to turn roads white
and temporarily create some hazardous driving conditions. With the
loss of daytime heating this evening, showers across the interior
have been weakening and should gradually diminish overnight. Given
light winds and small dewpoint depressions, fog should once again
develop.

With another weak shortwave trough dropping southward out of Canada
Thursday and similar temperatures aloft, expect diurnally driven
showers to once again pop in the afternoon hours and weaken Thursday
evening especially in the Willamette Valley and Cascades. Split
grids into 3 hour segments to try and capture this trend with the 1-
7pm time frame likely being the most active period for showers.
Snow, graupel and small hail will likely once again accompany the
stronger showers with just about anyone, regardless of elevation, at
risk of seeing some white stuff mix in if they get hit by one of the
stronger showers. Accumulations will once again likely be brief and
very localized and primarily limited on a larger scale basis to
elevations above 1000-1500 ft.

Going forward, a shortwave trough currently located over northern
Alaska will push into the Yukon and northwest territories and dive
south-southwestward over British Columbia before entering the
northeast Pacific Thursday night and Friday. Models are in good
agreement a surface low pressure around 1016mb will develop just
south of Vancouver Island in response. It will then drop southward
off the Pacific northwest coast Friday. While models have definitely
trended towards this low pressure moving closer to the coast over
the last 48 hours, the bulk of the precipitation produced by the
surface low pressure still looks like it will generally remain
offshore. More importantly, a 500mb low pressure pinches off and
drops southward just offshore. Between cooling temperatures aloft,
strong mid level vorticity advection and diffluent flow to the south
and east of the 500mb circulation, plenty of showers should spread
into northwest Oregon and southwest Washington beginning as early as
Friday morning. Coincidentally, 850mb temperatures will likely be at
their lowest point in the next week Friday morning, likely near -7c,
which should allow snow to mix down to or very near sea level
wherever precipitation falls. Models have trended towards higher quantitative precipitation forecast
during this timeframe across northwest Oregon and southwest
Washington and seem to be keying on a subtle 850-700mb trough
swinging across the area so higher hills closer to 1000' stand a
good chance of an inch or two of snow. Closer to the valley bottom,
accumulations should be harder to come by, but certainly are
possible and just about anyone north of the mid Willamette Valley
could see a slushy inch all the way down to sea level (or valley
bottom) if they end up under one of the isolated heavier bands of
precipitation, but predicting who will and who won't be under one of
these bands at this point is near impossible...except to say that
anyone north of the central Oregon coast and mid Willamette Valley
stands at least a small chance of being lucky...or unlucky depending
upon your Point of View.

Showers may intensify Friday afternoon with daytime heating, but
with surface temperatures rising and the sun angle increasing
midday, accumulations should be shorter lived than Friday morning.
As the aforementioned 500mb and low level low pressure drop
southwest of the region, our flow will turn increasingly offshore,
which should allow the area to dry out Friday night. The good news
is that models are in good agreement that shortwave ridging should
bring a cool, but dry day to the area on Saturday. /Neuman



Long term...Saturday night through Wednesday...no changes...previous
discussion from Wednesday afternoon follows...the longwave upper
level trough will be shifting over the central U.S. Over the weekend.
However, one last shortwave will rotate around the backside of the
trough on sun. This will bring another round of precip. Snow levels
will remain on the lower side, likely around 1000-1500 ft, with
additional decent accumulations for the Coast Range and Cascades. The
upper level flow will be northwesterly and slightly cyclonic going
into early next week, which will likely allow for some lingering
shower activity, mainly over the higher terrain. However, the trend
will be toward drier weather as we get into the middle of next week.
A broad upper level ridge over the NE pac will build toward the pac
northwest. Pyle



&&

Aviation...cool air mass over the region, with light low level
flow. Upper low pres sits off the coast, with light northerly
flow aloft. This will keep some threat of showers over region
through Thu, mainly along the coast and offshore. Inland, will
see scattered to broken clouds around 5000 ft, with a few
showers. Still think that will see areas of fog/low clouds reform
after midnight, especially from Salem north through Portland and
up through the Cowlitz valley. But to the south of Salem, suspect
will see more low stratus with some fog. Either way, fog/low clouds
will be slow to improve on Thu am, but should give way to VFR by
noon. Still think most of the coast will remain VFR, though will
see some patchy fog in sheltered bays and rivers.

Kpdx and approaches...generally, broken VFR cigs of 3500 to 4500
ft this evening, but will see clouds break up further later this
evening. With light winds and cool air mass, should see areas of
of fog and low stratus reform between 07z and 09z, with vis
likely under 1/2 mile at times. Improvement to VFR will be slow
on Thu, and likely sometime between 19z and 20z. /Rockey

&&

Marine...quiet marine weather through the period with 6 to 8
foot seas for the next 7 days and winds likely to remain below
Small Craft Advisory criteria. Instability over the waters will
cause some squally showers at times...with gusty winds and some
potential for lightning. There will be a chance for Small Craft
Advisory-level winds on Friday night/Saturday as a surface low
tracks south along the Washington coast, but there is still some
uncertainty how closely this surface low will track to the or
coast which will impact the winds on the waters. A similar low
pressure system is expected to drop down the pac northwest coast sun/Sun
night.Weagley

&&

Pqr watches/warnings/advisories...
or...none.
Washington...none.
Pz...none.



&&



$$

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