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Area forecast discussion 
National Weather Service Portland or
230 PM PST Sat Jan 31 2015

Synopsis...an upper ridge will remain over the region tonight before
shifting inland on Sunday. High clouds ahead of an approaching front
may limit fog development in some areas late tonight...but still
expect some valley fog...especially for the southern Willamette
Valley. The first of a series of fronts will move across northwest Oregon
and SW Washington Sunday...followed by a slightly stronger front on
Monday. There is uncertainty on whether Tuesday and Wednesday will
be dry or rainy...but there is greater confidence that the end of the
weak will be wet. Snow levels generally stay high...but there will be
chances for new snow at the higher Cascades throughout the week.
&&

Short term...tonight through Monday night...water vapor imagery
shows an upper ridge over northwest Oregon sandwiched between two large
upper lows...one over the NE Pacific and the other over
Arizona/Southern California. At the surface...visible imagery shows a
frontal system offshore with a warm front just offshore the British
Columbia coast...and a cold front around 135w. This is resulting in
an easterly surface pressure gradient with the Troutdale to dalles
gradient around 6 mb early this afternoon.

Gusty east winds blowing through the Columbia River gorge have helped
erode the fog and low clouds from the northern Willamette Valley
early this afternoon. The fog has improved for the southern
Willamette Valley...but areas of low clouds will likely hang around
through tonight and possibly into Sunday morning. The upper ridge
will remain over the area tonight and expect a return of at least
stratus to the inland areas that have cleared today. The presence of
high clouds ahead of the front should inhibit fog development
somewhat and do not expect as much dense fog as has been observed the
past few nights. The easterly gradient will continue through tonight
limiting the chances for dense fog to the north. However...think the
southern valleys still have a chance for some at least spotty dense
fog.

The cold front should begin to spread light rain to the region early
Sunday morning...with rain continuing through late Sunday afternoon.
This front should generate 0.25 to 0.5 inch of rain for the higher
terrain of the Coast Range and the Cascades with less than 0.25 inch
for the low lands. Snow levels should be between 6kft and 7kft Sunday
and possibly lowering to below 6kft with the Post-frontal showers
Sunday night.

Another frontal system will approach Sunday night with a warm front
bringing rain mostly to the southern areas Monday morning before the
cold front pushes in more widespread rain Monday afternoon and
evening. This front is slightly stronger than the Sunday system and
will produce slightly gustier winds for the coast and slightly higher
rainfall totals. Expect 0.5 to 0.75 inch of rain for the higher
terrain Monday morning through Monday afternoon with between 0.2 and
0.4 inch for the low lands.

Snow levels will be between 5500 and 6500 feet Monday with possibly 6
to 8 inches of new snow for the very high elevations of the South
Washington and northern Oregon Cascades. Snow levels will lower to
near the Cascade passes Monday night into Tuesday morning. Weakening
Post-frontal showers may bring a dusting of snow to the Cascade
passes overnight Monday and Tuesday morning with spotty 1 to 3 inches
of new snow above 5000 feet. Tj

Long term...Tuesday through Friday night...models begin to diverge
Tuesday with the GFS forecasting an upper ridge to return over the
Pacific northwest and the European model (ecmwf) forecasting a weaker ridge further
offshore...and a shortwave trough impacting the region. There is a
possibility that both of these models are overdoing their
forecasts...with the GFS ridge and the European model (ecmwf) trough being too strong.
The GFS solution would result in dry weather for northwest Oregon Tuesday
and Wednesday whereas the European model (ecmwf) would result in wet weather. Think
there will be a bit of both and expect a warm front will bring very
light rain to the region Tuesday with drying for Wednesday. This pans
out as a low confidence chance of rain forecast.

The models are then in surprising agreement that a period of wet
weather will impact the region Thursday and Friday...and possibly
into next weekend as a swath of subtropical moisture combined with a
series of atmospheric waves sets up over the NE Pacific. This weather
pattern will likely bring a substantial amount of rain to parts of
the Pacific northwest...but would like to see the first weather
systems this weekend and early next week pan out before even trying
to determine where the heaviest rain will be. Tj
&&

Aviation...stubborn LIFR fog remains over the south Willamette
Valley this afternoon...while elsewhere fog has scattered out.
High clouds are starting to increase over the region ahead of an
approaching front...with increasing middle clouds tonight. Patchy
shallow fog should impact locations from 04z-12z along the
Columbia River like kkls and the south Willamette Valley before
middle clouds reach them. East winds through The Gorge will keep the
pdx/vuo metropolitan areas clear of fog tonight. Coastal sites should
remain VFR tonight. Light rain should impact the SW Washington/far northwest or
coasts early Sun morning.

Kpdx and approaches...VFR conditions will prevail tonight with
gusty east winds. Strongest gusts will be nearest The Gorge. Middle
and high clouds increase overnight with ceilings between 5k-10k feet
after 14z sun. Kpdx may see some brief and shallow IFR fog
develop if east winds hold off until after 06z sun. /27
&&

Marine...the first in a series of progressively stronger frontal
systems can be seen approaching the region this afternoon...and
will move through Sunday morning. This first front on Sunday
brings the possibility of some Small Craft Advisory gusts above 20
kts while bringing seas up to around 8 feet...but confidence is
rather low on solid gusts reaching small craft criteria this
afternoon. The second frontal system moves through on
Monday...bringing solid Small Craft Advisory winds with gusts 30
to 35 kts likely. Strong winds will ramp up early Monday morning
and persist through the day until winds ease behind the front late
Monday night or early Tuesday. This system will also bring seas up
to around 12 to 13 feet on Monday. Confidence is increasing for an
even stronger system Thursday or Friday which could generate
southerly gales and bring seas up to near 20 feet. /27

&&

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

$$

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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.

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