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Area forecast discussion 
National Weather Service Portland or
857 PM PST Monday Mar 2 2015

Synopsis...drier north flow aloft will settle over SW Washington and
northwest Oregon later today and tonight. However...another weak disturbance
slides south along the eastern Cascade slopes tonight. High pressure
will then dominate the weather pattern middle-week through the weekend.
There is the possibility of a pattern change early next week.


Update...inherited forecast appears on track so few changes were
made to the forecast this evening. Fog coverage was reduced a
bit...with river valleys of the Coast Range and the southern
Willamette Valley most likely to see some patchy fog Tuesday morning
based on current dewpoints. Low temperatures will bottom out near
freezing for many locations tonight...and look to be even a couple
degrees cooler Tuesday night. Models continue to suggest a shortwave
trough will slide over the top of the ridge of high pressure into
southwest British Columbia late Thursday into Friday. Rain chances look to remain
north of our forecast area...but this system may allow marine clouds
to increase across the area...particularly if a southwest marine
push materializes as some models suggest. Otherwise...expect daytime
temperatures to gradually warm well above normal into the upcoming
weekend as high pressure looks to dominate our weather. /Neuman

Previous discussion below...

Short term...tonight through Thursday...water vapor loop shows a
small and compact upper level low diving south along the central
California coast early this afternoon. Other large-scale features
include a high-amplitude ridge centered near 140w and an upper trough
over the northern rockies. Visible shows an extensive stratocumulus cloud
field from the west slopes of the Coast Range to the coast and also over
the Cascade crest and into the S Washington Cascade foothills.

Brisk northerly flow aloft will be over the forecast area tonight and Tuesday.
The latest NAM and European model (ecmwf) hint at a weak back-door system sliding S
along the eastern Cascade slopes tonight with some very spotty quantitative precipitation forecast
associated with it. Will keep the minimal probability of precipitation for the higher
Cascades tonight...but not overly confident anything will occur. The
bigger story for tonight will be widespread freezing temperatures. May also
see a little east wind develop in The Gorge late tonight as the NAM
shows a -3 to -4 kttd-kdls gradient by 12z Tuesday.

An upper ridge will then be the dominant feature for the remainder of
the short it has been for most the winter. Some low-level
offshore flow persists through Wednesday so night and morning fog will be
rather localized. Overnight lows will drop to near or below freezing
Tuesday night...with the latest guidance showing lows in the middle to upper
20s for many inland valleys. Latest GFS and NAM show a weak
short-wave moving across the area 00z-12z Tuesday. Would expect just high
clouds with this feature. The GFS and European model (ecmwf) runs also show the upper
ridge flattening a bit Wednesday night and Thursday. NAM cross-section from near
kast through the Columbia Gorge hints at some high level
moisture...above 450 mb...Wednesday night and Thursday so would suspect another
surge of cirrus clouds. Maximum temperatures Wednesday will be near seasonal
normals...but warm to slightly above normal Thursday. Some far southern areas
could get to near 60 degree as early as Thursday afternoon. Weishaar

Long term...and the ridge continues. European model (ecmwf) actually shows the ridge
amplifying Friday as a short-wave drops through the northern rockies. 850 mb
temperatures warm a bit more Friday...which should be enough to produce maximum
temperatures in the lower 60s for a good portion of the southern two-thirds of
the forecast area. Upper ridge axis pushes a bit closer to the coast
Sat for a little more warming. European model (ecmwf) would suggest a little low-level
offshore flow Friday night and Sat. Little change sun... although the
GFS starts moving the ridge east...with the ridge axis along a line
from kuil-kreo. The gefs and naefs mean charts valid 00z Monday suggest
the ridge axis will shift east of the Cascades. Bigger model
differences crop up the GFS tries to bring an upper low
closer to the coast and the European model (ecmwf) maintains the ridge. Would tend to
lean closer to the European model (ecmwf)...since the ridge has been the dominant
feature this winter. Even if the GFS turns out more
tends to break down the ridge a little too fast. Introduced some
slight chance probability of precipitation for the the South Coast and Coast Range sections
Monday night for now. Weishaar

Aviation...widespread VFR conditions in control this evening as
high pressure builds over the region. Drier northerly flow cleared
skies from north to south during the afternoon and early evening.
Some stratocumulus continue to obscure the Cascades...but these
clouds should dissipate overnight. Clearing skies and fairly
light winds will lead to good radiational cooling overnight...with
many valley locations falling near or just below freezing.
However...given the relatively low dew points to start it is
unlikely that fog will be an issue. The exception may be in the
southern Willamette Valley...where dew points are starting out a
bit higher. Any fog that does form should burn off by middle to late
morning...with widespread VFR through the rest of Tuesday.

Kpdx and approaches...VFR through Tuesday. Light-moderate north-northwest
flow surface and aloft...diminishing overnight. Pyle


Marine...weak high pressure moving down the coast earlier today is
now dissipating off the California/or border. This has allowed winds over
the central or waters to drop below 20 let the Small
Craft Advisory for winds expire at 8 PM. Winds will continue to
decrease overnight and Tuesday morning as high pressure builds over
the region...turning light offshore. A thermal trough building
north from California may cause a modest increase in north winds
midweek...mainly for our southern waters. Otherwise a strong upper
level ridge will keep conditions benign for the rest of the the Pacific storm track stays well to our north.


Pqr watches/warnings/advisories...


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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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