Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 030516
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
916 PM PST Fri Dec  2 2016

.SYNOPSIS...Very light precipitation will continue overnight behind a
weak cold front. Another front will move south into the Pacific NW
Sat night and Sun, ushering in the coldest air of the season so far
for early next week. Expect heavy snow with this front on Sunday in
the Cascades, and snow levels falling through the day down to the
Cascade Foothills and Coast Range by Sunday evening. Will see the
potential for low elevation snow down to the valley floor on Monday
morning, turning to all rain below around 500 feet by midday and
tapering off in the evening. However, expect roads will be too warm
for significant impacts at the valley floor. Another system is
expected to move through later next week, which bears watching with
the potential for significant impacts.

&&

.SHORT TERM...Tonight through Monday...Radar shows light rain across
much of the area this evening along and behind a weakening cold front
which has mostly moved through at this point. Expect only very small
accumulations tonight and mostly just a nuisance drizzle for those
out and about. Made only very minor changes to the forecast this
evening as it appears generally to be on track. Bowen

The rest of the short term discussion remains unchanged...
Behind the front, snow levels will fall to around 3500 to 4500 feet
Saturday morning behind the front, but by the time snow levels drop
down below the passes, precipitation should be coming to an end.
Saturday should be mostly dry during the day, but can`t rule out a
stray shower, mainly in our SW Washington zones. Saturday night, the
front associated with the upper-level trough sags south into SW
Washington bringing precipitation and rapidly lowering snow levels.
Moisture associated with this front suggests we`ll get a moderate
amount of rain across the area through the day on Sunday. With cold
air coming down out of western Canada with the upper-level trough,
snow levels will be falling through the day. Due to 24-hour QPF
totals in the Cascades around 1 to 1.5 inches through the day on
Sunday with this front, we could see a foot or more of snow up above
the Passes at the ski resorts, and depending on how quickly the snow
levels fall below the Passes, the Passes could see anywhere from 5 to
10 inches of snow on Sunday. Sunday evening, the snow levels should
fall down to around 1500 feet which will bring snow down into the
Coast Range, Willapa Hills, and Cascade Foothills during the evening
hours and overnight. QPF will be lighter during this period as it
will be between systems, so right now thinking there won`t be a lot
of accumulation until the next system comes in Monday morning.

A shortwave moves across the area Monday morning, bringing
precipitation just as snow levels are already down around 800 to 1200
feet. If we can get a decent break in the precipitation late Sunday
night into early Monday morning, we could see enough drying for
temperatures to wet-bulb down for at least a rain-snow mix down to
the Valley floor. Areas Salem northward, including Portland and
Vancouver, may see heavy enough precipitation for a period of all
snow Monday morning, with accumulations up to around an inch. These
low-elevation snow events are very tricky and we really won`t feel
confident until it is on our doorstep, so keep an eye on the forecast
this weekend for the Monday morning commute. During the day on
Monday, right now it looks like temperatures should come up enough
for a switchover to all rain on the Valley floor below around 500
feet, but it is looking likely that snow will continue to accumulate
above 500-1000 feet through the day on Monday. Travel on Monday could
be difficult through both the Cascades and Coast Range.

A high surf advisory is in effect for the Coast for Saturday morning
as large swell out of the northwest is expected to build seas to
around 20 to 24 feet by early Saturday morning.       -McCoy


.LONG TERM...No Changes. Previous discussion follows...Monday night
through Friday...Unsettled winter weather will be the rule to start
the period. It looks as though a few showers will continue Monday
night into Tuesday in the wake of the system on Monday. Breaks in the
clouds and a generally cool air mass will lead to lows near freezing
many areas Tuesday morning, meaning some of the showers will be of
snow even at low elevations but with light spotty accumulations if
any. The models show brisk north to northwest flow aloft taking hold
later Tuesday, with offshore flow taking effect Tuesday night and
Wednesday leading to brisk cold and rather widespread east winds over
the forecast area. Gradients through the gorge could be on the order
of 8 to 10 mb, so these winds are not likely to be garden variety
east winds. Wednesday looks generally dry as a result. The faster GFS
and the slower ECMWF are coming together slowly and will go with an
increasing likelihood of precipitation Wednesday night and Thursday.
The moisture will override the colder air in place so expect some
winter weather with this system as well. Thermal profiles and
thickness values suggest that any low elevation snow might brief and
the main impact will be a period of freezing rain. The actual threat
of freezing rain will be determined by how cold the surfaces can get
Wednesday night and Thursday morning, which will also be influenced
by events earlier in the week. At this point, model differences are
too large to pinpoint the transition back to rain with any certainty,
but probably by Friday. An interesting week is ahead of us. Tolleson

&&

.AVIATION...A front moving across the region will bring MVFR and
high end IFR conditions for the remainder of this evening.
However, instability behind this front will bring a mix of MVFR
and VFR flight conditions between 09z and 18z Saturday before
transitioning to mainly VFR. Conditions should begin to
deteriorate across the region as we approach 06z Sunday.

KPDX AND APPROACHES...A period of MVFR and high end IFR conditions
associated with an area of light rain accompanying a front this
evening will likely give way to a mix of MVFR and VFR conditions
after 09z Saturday as the atmosphere becomes slightly more unstable.
/Neuman

&&

.MARINE...A weak front pushed across the waters this evening.
Winds have since subsided and should remain below small craft
advisory thresholds of 21 kt through Saturday evening. A front and
possibly a low pressure will push into the waters late Saturday
night and possibly bring a brief bout of southerly small craft
advisory level winds before northerly winds kick up behind frontal
passage Sunday. Another weak storm system Sunday night should
bring another round of northwesterly winds Monday. In the
meantime, a large northwesterly swell is approaching the region
with seas likely to jump quickly to around 20 ft between midnight
and 6 am across the waters. There is a good chance seas will
likely peak out in the 20 to 24 ft range around 7 am Saturday
before gradually subsiding into the mid teens towards Saturday
evening. Seas appear likely to stay in the low to mid teens
through Monday before subsiding below 10 ft Tuesday. /Neuman


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 2 AM to noon PST Saturday for Central
     Oregon Coast-North Oregon Coast.

WA...High Surf Advisory from 2 AM to noon PST Saturday for South
     Washington Coast.

PZ...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 4 AM
     PST Sunday.

&&

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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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