Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 041104
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
304 AM PST Sun Dec 4 2016

.SYNOPSIS...A strong cold front will push se through southwest
WA and northwest OR today today bringing a round of rain and
mountain snow. Cold air behind the front will lower snow levels and
support freezing temperatures Sunday night into Monday morning as
the next front arrives. Snow levels will drop close to the valley
floors Monday morning complicating the morning commute.
Precipitation will linger into early Tuesday. East winds Tuesday
night and Wednesday will set the stage for more winter weather
Wednesday night and Thursday.
&&

.SHORT TERM...Today through Tuesday...Rain has spread in across much
of the forecast area early this morning ahead of a cold front that
was moving through nw WA early Sun morning. Rainfall rates through
09z over western WA have been on the order of 0.3 to 0.5 inches per
6 hour, which agrees well with most model qpf estimates. These rates
expected to continue today as the main area of rain moves through nw
OR and cold front pushes south into Lane county by the end of the
day. Winter weather advisories for snow in the Cascades look on
track today, ending this morning in the south WA Cascades as the
cold front pushes through, but lasting through the afternoon in the
Oregon Cascades where another 6 to 12 inches are still possible. The
front will usher in colder air today and tonight, with snow levels
lowering into the Coast Range and Cascade Foothills by evening.

Moist onshore flow brings increasing pops tonight across the
forecast area for light showers, but more organzied precipitation
precedes a cold front arriving early Mon, originating today along
the se Alaska coast. 1000-500 mb thickness values on the order of
522 dm and h8 temps around -6 deg C continue to suggest snow levels
dipping by Mon morning to around 1000 feet. Cold front still appears
in models to be fairly dynamic, and moving south down the lower
Columbia Valley into the north Willamette Valley during the morning.
With the threat of dynamic cooling accompanying the front, there
remains a possibility for snow to temporarily lower to near the
valley floors during the morning Mon. As the front moves south
through the rest of nw Oregon during the afternoon hours Mon, the
likelihood of snow levels dipping below 1000 ft in the interior
valleys becomes less. For the Willapa Hills, North Oregon Coast
Range, and South WA Cascade Foothills, where snow begins late Sun
night and continues into Mon, will go ahead and issue winter weather
advisories for snow this morning. Further south the potential for
accumulating snow does not kick in in earnest until Mon morning, so
will hold off on any more advisories at this time.

Behind the cold front later Mon and Mon night, precipitation will
decrease as the low level flow veers to the n. Air mass becomes more
continental looking in nature with the change to more of a light
offshore flow. With drier air moving in, chances for showers will
diminish, and largely come to an end on Tue except over the Oregon
Cascades. The colder air will likely keep high temps limited to the
30s over the interior Tue.

.LONG TERM...No Changes. Previous discussion follows...Tuesday night
through Saturday...An upper level ridge will move across the Pac NW
late Tuesday/early Wednesday ushering in a brief period of dry
weather. With cloud cover decreasing slightly overnight we could see
the coldest temps of the season thus far, with temps at the coast
near 30. Temps in the interior valley will generally be in the upper
20s, with temps in the Cascades in the low teens. In addition to the
cold temps, residual surface moisture combined with clearing skies
could provide ideal conditions for radiational fog so decided to add
the mention of freezing fog for much of the interior valley. However,
brisk east winds near the Gorge should prevent any fog development.

Unsettled winter weather looks to return by late Wednesday. The GFS
and ECMWF are starting to slowly converge on one another, but the
GFS remains the more aggressive solution. Both models continues to
suggest that moisture will initially overrun the cold air in place.
Model soundings show a brief period of valley snow will be possible
at the beginning. However, as the warm front lifts north Thursday
morning expect valley snow to transition to a mix of snow, freezing
rain, or rain before turning to all rain later in the day. However,
there is still a lot of uncertainty with this system, and the exact
threats and impacts will be influenced by how cold the surface gets
over the coming days.

Active weather looks to continue on Friday and Saturday as a series
of shortwaves move across the region. Both models show a warmer air
mass over the area, with snow levels generally around 3500 ft. As
such, the threat of valley snow will be minimal late next week. /64
&&

.AVIATION...A cold front is slowly moving through SW Washington
and NW Oregon from the northwest this morning. An associated band
of steady rain and a mix of VFR/MVFR cigs is draped across most of
the region as of 3 AM. Rain will taper to showers from north to
south through the morning hours. Expect conditions to transition
to mainly VFR behind the front, with occasional showers through
the rest of the day and this evening. The next cold front will
begin to spread in from the north late tonight...likely bringing
MVFR cigs into KAST between 09-12Z Mon.

KPDX AND APPROACHES...Expect a mix of VFR/MFR conditions in steady
rain through about 15Z-17Z. A cold front will move through around
14Z, with winds switching to the NW behind the front. Rain taper
to showers later this morning and this afternoon, with mainly VFR
expected. Pyle

&&

.MARINE...A cold front moving through the coastal waters this
morning is bringing low-end small craft advisory winds, with gusts
to around 25 kt. NW winds behind the front will continue to gust
20-25 kt at times through this afternoon. Another cold front will
move down tonight into Mon morning, likely bringing another round
of small craft winds. Winds subside below 20 kt late Mon through
early Wed. However, a strong storm system appears to be on tap for
the Wed through Fri timeframe. There is some considerable
differences in model timing at this point, but it appears likely
that gales will affect the waters sometime late Wed or early Thu,
with another round of gales possible toward the end of the week.

A longer period westerly swell train will keep seas around 12 to
14 ft for the next several days. Expect seas to finally drop below
10 ft by Wed. However, the next approaching strong system is
expected to quickly push seas back into the teens later Wed or
early Thu. Seas may push above 20 ft again on Thu into Fri. Pyle

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 4 PM PST this afternoon for
     Northern Oregon Cascades.

     Winter Weather Advisory until 4 PM PST this afternoon for
     Cascades in Lane County.

     Winter Weather Advisory from 1 AM to noon PST Monday for Coast
     Range of Northwest Oregon-Lower Columbia.

WA...Winter Weather Advisory until 10 AM PST this morning for South
     Washington Cascades.

     Winter Weather Advisory from 1 AM to noon PST Monday for I-5
     Corridor in Cowlitz County-South Washington Cascade
     Foothills-Willapa Hills.

PZ...Small Craft Advisory for winds until 10 PM PST this evening for
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     60 nm.

     Small Craft Advisory for hazardous seas until 4 PM PST Tuesday
     for Coastal Waters from Cape Shoalwater WA to Florence OR
     out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 10 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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