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Area forecast discussion 
National Weather Service Portland or
930 PM PST Sat Nov 22 2014

Synopsis...showers will gradually decrease this evening and late
tonight. The next front will bring rain and mountain snow to
southwest Washington Sunday morning...moving south throughout the
day. Snow levels will remain near the Cascade passes...but
accumulations are expected to be minor for the northern Oregon
passes. A flat upper ridge will build into California and Oregon
Monday and Tuesday...but should remain weak enough for systems to
clip our northern zones with occasional wet weather. Valley
inversions will likely build in the central and south Willamette
Valley...with fog possibly becoming more persistent next week.
Another organized frontal system may bring more rain to the region
for Thanksgiving weekend.

&&

Evening update...with the loss of daytime heating...shower activity
has decreased significantly over the past few hours. Some scattered
showers remain...but overall the air mass is becoming more stable.
Coverage of snow showers in the Cascades has also decreased...so we
cancelled tonights Winter Weather Advisory a little early. Scattered
snow showers may still result in localized accumulations of an inch
or two above 3500-4000 feet.

There have also been some patches of clearing this evening...
allowing for some radiational cooling. Therefore we cooled low temperatures
by a few degrees tonight in the valleys...and added a little more fog
to the forecast.

Looking further north...a well-defined and compact low pressure
system is pushing into haida gwaii...with a secondary low developing
off Vancouver Island. This secondary system will be the one
responsible for pushing another front southward across the forecast
area...with rain developing along the Washington coast by sunrise and
spreading south throughout the day. The snow level will remain around
4000 feet much of the day. Combine this with decent orographics and
strong jet dynamics...and it appears our Snow Advisory for the South
Washington Cascades looks good. For now it appears the best dynamics will be
in the left-exit region of the jet over Washington. Model guidance
has been fairly consistent in showing a sharp quantitative precipitation forecast gradient south of
the Columbia. Therefore the Oregon Cascades remain out of the Snow
Advisory for Sunday...but Mount Hood may receive close to the
necessary 6 inches for an advisory. Areas further south will see much
less. Weagle

/prev disc issued 256 PM PST Sat Nov 22 2014/

Short term...tonight through Tuesday...a few upper short wave
troughs moving across the far NE Pacific will maintain convective
showers across the Pacific northwest this evening. Freezing levels are are
currently around 4500 feet. Web cams shows snow showers at the
Willamette...santiam...and Tombstone passes which are at around
5100...4800...and 4200 feet respectively. This suggests that the snow
level is currently between 4000 and 4000 feet. The snow level will
change very little tonight and Sunday.

The showers are decreasing this afternoon...but satellite imagery
shows more showers offshore approaching the area. Lightning has been
detected with these offshore showers and have continued the slight
chance for thunderstorms through 7pm. Since these showers may not
make it to the Cascades until around 9 PM am hesitant to end the
Winter Weather Advisory earlier than the current midnight end
time...even though there may be a considerate break in showers
between 4 and 9 PM.

The showers will decrease after midnight tonight as an upper level
ridge tries to build over the area. Cloud breaks may lead to
radiation cooling and fog development late tonight into Sunday
morning...especially for the interior valleys south of Salem. The
north is less likely to have fog due to slightly stronger winds and
possibly cloudier skies. The cloudier skies will be due to the next
approaching front that will be approaching from the northwest.

Despite the building upper ridge...another surface front will sneak
into the area Sunday. This front does not look well defined on
satellite imagery yet...but the models are in great agreement that
the front will begin rain at the SW Washington shortly after
sunrise...sliding southward through the day.

The highest quantitative precipitation forecast will be for areas north of the Columbia River where
the SW Washington coast and the Willapa Hills can expect around a
half an inch of rain Sunday and Sunday night. The SW Washington
Cascades will see higher rainfall totals...around 0.75 to 1 inch for
elevations below 4000 feet...and possibly 4 to 9 inches of additional
additional snow above 4000 feet. Have therefore issued another Winter
Weather Advisory for the SW Washington Cascades for elevations above
4000 feet.

Precipitation will be less for northwest Oregon and expect amounts to
become gradually less from north to south...with totals from 0.40
inch just near the Columbia River to 0.10 inches across Lane County.

Showers will dramatically decrease Sunday night and Monday and expect
more valley fog Monday morning. Monday will be mostly dry...but northwest
Oregon will be between two weather systems...one to the SW and one to
the northwest. Models are leaning towards keeping these storms offshore
Monday and have reduced probability of precipitation quite a bit. However the presence of
these two storms warrant at least a slight chance for the coast and
Cascades.

The two weak storms may merge offshore Monday evening and push
onshore Monday night into Tuesday. The models are not in great
agreement on this but have increased probability of precipitation. The southern system should
bring warmer air to the region and raise snow levels well above the
passes Monday and Tuesday. Tj

Long term...Tuesday night through Saturday...the models are
currently showing the upper level ridge flattening out on Tuesday.
They tend to keep most of the moisture to our north which justified
lowering the probability of precipitation through Wednesday afternoon...but keeping a slight
chance of rain for the northern portions of the forecast area and
along the coast. The ridge builds slightly on Thursday...but a weak
shortwave could bring another round of rain through Friday. A much
stronger and wetter system looks to move in over the weekend which
could pose a threat to travelers returning after the Thanksgiving
Holiday. Details at this point are still unknown and will be refined
with future updates. The good news is that temperatures in the
extended will be at or above seasonal normals. /64

&&

Aviation...expect some patchy MVFR/IFR fog or low stratus will
form overnight in the valley particularly. Think it will stay
well mixed enough to keep VFR in control at the coast. Another
front will work in from north to south Sun morning...bringing
increasing rain and likely MVFR ceilings to the northernmost taf sites
starting between 15z and 18z. The front will sag slowly south
through the day...but the main impacts will continue to be for
locations further north.

Kpdx and approaches...a Post frontal showery air mass will lead
to predominantly VFR conditions into Sun morning. Patchy IFR fog
is possible after 12z if skies clear out. A front moving in from
the north will spread light rain and possibly MVFR ceilings in by
around 18z sun. Bowen/pyle

&&

Marine...a general lull in west/northwest winds should last through this
evening...but another front will move into the waters from north
to south overnight....bringing another round of solid Small Craft
Advisory winds...mainly over the northern waters. Winds will
subside again Sun afternoon and evening before yet another front
brings more small craft winds on Monday.

Seas remain around 12 to 14 feet and should change very little
through tonight. With the approach of the front Sunday...seas may
increase into the middle teens. With another front on Monday and
several systems moving through to our north early next
week...expect seas to remain above 10 feet until midweek. The latest
enp wave guidance then suggests they fall below 10 feet Tuesday or Wednesday.
Bowen/pyle

&&

Pqr watches/warnings/advisories...
or...none.
Washington...Winter Weather Advisory from 7 am to 10 PM PST Sunday for South
Washington Cascades.

Pz...Small Craft Advisory for winds from 1 am to 2 PM PST Sunday for
coastal waters from Cape Shoalwater Washington to Cascade Head or out
60 nm.

Small Craft Advisory for hazardous seas until 4 am PST Monday
for coastal waters from Cape Shoalwater Washington to Florence or out
60 nm.

Small Craft Advisory for rough Columbia River bar until 4 am
PST Monday.

&&

$$

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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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