Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Portland or
220 am PST Wednesday Nov 25 2015
Synopsis...high pressure will settle in today over the interior
Pacific northwest northwest...and remain through the weekend. This
will bring an extended period of dry cool conditions.
Short term...water vaspor pictures showed an upper low sinking
slowly southeast through northern California and northern Nevada early this morning.
Radar and surface observations indicated precipitation had mostly
come to an end...but will retain a slight chance pop for snow
showers in the Lane County Cascades this morning in the event of any
more wrap around type precipitation brushing the area. Clouds
remained over roughly the southeast half of the forecast area this morning.
With a dry offshore flow increasing today expect the clouds to
break up during the day today.
Models agree on a Rex type blocking pattern evolving in the next few
days...with a ridge of high pressure aloft building in over southern
British Columbia...and the upper low lingering in the vicinity of Nevada. Offshore
flow brought on by a surface high slipping down east of the Cascades
will keep the region with a dry but cool offshore flow during the
next few days. There is general agreement on the offshore gradients
through The Gorge increasing late Thursday into Friday for windy conditions
in and near The Gorge. Wind protected valleys will see temperatures in the
coming nights dipping down to near 20...which would be the coldest
temperatures seen since the very beginning of January.
Long term...no changes. Previous discussion follows. Friday night
through Tuesday. Dry and cold weather continues through the weekend
and likely continues into early next week. Low level east winds will
continue to keep temperatures below normal...with highs struggling
to reach 40...except locations along the coast which will get up
into the middle 40s. Widespread nighttime lows below freezing will also
continue through the weekend...with wind chills dropping down into
the teens mainly around the Columbia River gorge and eastern
portions of the Portland metropolitan. Lowered probability of precipitation on Monday and
Tuesday...as current model runs show the Rex block not breaking down
until late Tuesday/early Wednesday. Left some probability of precipitation on Tuesday
because GFS ensembles keep a slight chance of precipitation starting Monday
night into Tuesday...but feel more likely that dry weather continues
until the blocking pattern breaks down late Tuesday or Wednesday.
Models have a tendency to try to break these blocking systems down
too early...so could very well see the next system to bring US rain
continue to be pushed back to later next week. -McCoy
Aviation...surface low over the Oregon/California/Nevada triple
point continuing to sag south tonight. Generally VFR across SW Washington
except for local IFR along the coast and interior valleys. VFR north
and west of a kpdx-konp line as well...except with local LIFR/IFR
along the coast. A mix of VFR/MVFR S and east of the aforementioned
line. 10z inland metar dew point spreads generally 4-8 degree.
Thus...fog/stratus formation may not be all that extensive. Expect
coastal areas to improve to VFR shortly after sunrise. Inland
areas will have a mix of conditions through the morning...then
will improve to VFR by 20z. Offshore gradient will strengthen
later today and Wednesday evening...with gusts 30-40 knots at the west end
of the Columbia Gorge by 06z Thursday.
Kpdx and approaches...VFR clear at 10z with dew point spread of 6
degree. With north-NE surface gradient dew Point May drop a little more
during the next few hours...which would not be favorable for fog
formation. IFR ceilings over the eastern approaches at 10z which will
likely persist until sunrise. There is the chance some of this IFR
stratus drifts west to impact the terminal. Otherwise...VFR to
prevail after 18z. Weishaar
Marine...NE wind continues over the waters early this morning
with gusts around 25 knots over the outer waters. Do not expect much
change in the overall pattern for the next 36-48 hours. Wind
gradually veers to the east Wednesday evening. Surface thermal trough along
the north cal and S Oregon coast this morning strengthens and expands
north this afternoon and tonight. Strongest wind through Friday will be
over the outer southern waters. May have to extend the Small Craft
Advisory for wind in that area but will let the next couple shifts
make the decision. Wind speeds gradually diminish late Friday through
the weekend. Latest model runs show varying solutions sun through
early next week. Agree with the opc forecaster assessment
regarding pretty low confidence level beyond day 5. For now have
leaned toward GFS and dgex...indicating potential Small Craft
Advisory wind Monday night then a chance for gales late next Tuesday or
Seas trending down nicely...as shown by enp and spectral bulletin
data. Will hold on to the Small Craft Advisory for seas through
18z Wednesday...although seas will drop below 10 feet in the northern waters a
little sooner. Offshore pattern Wednesday night through Sat will keep
seas around 5 feet. Boosted seas to 10 feet Sun night and Monday but not
overly confident that will occur. Weishaar
Pz...Small Craft Advisory for winds until noon PST today for waters
from Cape Shoalwater Washington to Cascade Head or from 10 to 60 nm.
Small Craft Advisory for hazardous seas until 10 am PST this
morning for coastal waters from Cape Shoalwater Washington to Florence or
out 60 nm.
Small Craft Advisory for winds until 4 am PST Thursday for
waters from Cascade Head to Florence or from 10 to 60 nm.
Small Craft Advisory for rough Columbia River bar until 5 am
PST early this morning.
Small Craft Advisory for rough Columbia River bar from 2 PM
this afternoon to 6 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. The area is
commonly referred to as the forecast area.