Scientific Forecaster Discussion
fxus66 kpqr 110516
Area forecast discussion
National Weather Service Portland or
857 PM PST sun Dec 10 2017
Synopsis...high pressure will continue to bring cool temperatures
and areas of morning fog and frost before a transition to a more
typical wet December pattern returns towards the weekend.
Short term...tonight through Wednesday...an upper ridge of high
pressure will bring few changes to the weather through midweek with
cool temperatures and areas of morning frost. A shortwave trough
passing by to our west and north Monday night will drag a
weak front into the waters off the Pacific northwest coast and
perhaps squeeze out a few drops across our northern coastal zones.
Otherwise, surface high pressure should strengthen a bit over
western Oregon in the wake of this system, which should reduce the
winds a bit across the Columbia River gorge. However, it should
still be windy in locations like Troutdale and Corbett Tuesday and
Wednesday...just not as windy as the past 24-72 hours. /Neuman
Long term...Wednesday night through Sunday...ridging will keep US
dry into Thursday. Models have pushed back the onset of
precipitation to 10pm Thursday night at the earliest on the coast
and Friday morning in the Willamette Valley. This may help lower
the chance for freezing rain in the Hood River Valley. The later the
precip arrives in the morning, the less chance of freezing rain at
onset. This is still a time period to watch over the next few days.
Once rain starts, we get into more of a zonal flow pattern as the
ridge of high pressure gets squashed down by a longwave trough
digging into the Gulf of Alaska over the weekend. This means our
winter rains return. We may get a break in the precipitation on
Saturday before another system swings through on Sunday, but the
timing of this break is uncertain at this time.
Snow levels will be coming down to 3000 to 4000 ft behind this first
front Friday night, bringing snow to the resorts passes Friday night
into early Saturday morning. Snow levels lift back up to 6000-8000
ft for the onset of the next system on Sunday. -McCoy
Aviation...no changes, as strong high pres remains east of the
Cascades. This will maintain dry offshore flow, with gusty
easterly winds in and near the western Columbia River gorge
tonight and Mon. Air mass remains dry, so will see just VFR under
mostly clear skies tonight into Mon. Stratus in the Columbia
Basin will remain jammed against the east slopes of the Cascades,
and into the Columbia Gorge/Hood River Valley. Also, like past
several nights, will see areas of IFR fog/stratus form from
ks12/kcvo area southward through keug and krbg.
Pdx and approaches...VFR tonight and Mon, with continue easterly
winds. Gradients big stronger tonight, so east winds may not
slacken until after 08z, with 5 to 10 kt from 10z to 16z, then
east winds pick up again. Rockey.
Marine...high pres over the inland pac NW, with lower pres well
offshore. This will maintain east to southeast winds on the
waters, with stronger winds over the far outer waters. Will also
have occasional gusts around 20 kt in the coastal terrain gaps.
Generally, seas running 5 to 7 ft tonight and Mon, but a mix of
swell trains will keep periods bouncing between 12 and 16 sec.
Seas will build a bit more Mon night and Tue, but look to stay in
the 8 to 10 ft range at this time.
A weakening front will produce bit more in way of souther winds
Mon night. Could get gusts up to 25 kt at that time, but hard to
say as the front is weakening as it arrives. Strong front will
arrive later in the week, with higher potential of 25 to 30 kt
or...air stagnation advisory until 2 PM PST Thursday for central
Willamette Valley-lower Columbia-south Willamette Valley.
Washington...air stagnation advisory until 2 PM PST Thursday for Greater
Vancouver area-I-5 corridor in Cowlitz County.
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