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Area Forecast Discussion
National Weather Service Portland Oregon 
1025 AM PST Sat Dec  3 2016

.SYNOPSIS...A modest westerly flow aloft will keep a few showers over
southwest Washington and northwest Oregon today with snow in the
Cascades. A strong cold front will push into southwest Washington and
northwest Oregon tonight then move south through the area on Sunday
for more rain and mountain snow. Expect heavy snow in the Cascades
affecting travel through the passes. Snow levels will drop into the
coastal mountains and the Cascade foothills mid day Sunday but
precipitation will be dwindling, limiting any accumulations. The next
system will be approaching from the northwest later Sunday night and
move through the area on Monday. Expect more accumulating snow in the
mountains. Snow levels will drop close to the valley floors Monday
morning with some light accumulations possible but not certain. Some
precipitation will linger into early Tuesday then start to dry out.
Rather strong east winds are expected to develop Tuesday night and
Wednesday, setting the stage for more possible winter weather
Wednesday night and Thursday.

.SHORT TERM...Today through Monday...A westerly flow aloft combined
with some modest onshore flow is keeping a few showers over the area
today. The snow level will flirt with the Cascade passes today.

The next system is starting to take form offshore as a mass of clouds
and moisture approaches the coast. This front will spread in tonight
with increasing rain and mountain snow. The cold front will be quite
strong with strong cold advection behind it. The front will move
south through the forecast area Sunday. Look for heavy snow in the
Cascades tonight and Sunday, starting first in the south Washington
Cascades then spreading southward through Sunday. There will be
decent valley rains with it as well. The snow level on Sunday will
lower into the coastal mountains and Cascade foothills Sunday
afternoon as the front passes through, but precipitation will be
dwindling so any accumulations will be limited.

The next system will be dropping in from the northwest later Sunday
night and Monday. Look for more significant snows in the Cascades
with this system, perhaps 6 inches or a bit more. Drying overnight
Sunday night behind the cold frontal passage of Sunday will lead to a
wet bulb cooling situation over our forecast area that could drop
snow levels close to the valley floor by early Monday. NAM and GFS
model soundings still support this possibility. At this point expect
a rain or snow mix to develop after midnight Sunday night that could
change to mainly snow through a good part of the morning Monday.
Roads will be warm so the snow may take a while to stick and
accumulate, but close to an inch is within the range of
possibilities. The best chance of accumulations will be from around
Salem northward as the precipitation will be starting later in the
south and mix over to rain a bit faster. A light low level southerly
flow should eventually lift the snow level above the valley floors in
the afternoon but it may still hover in that 1000 foot range. These
exact details are still not certain, it just looks as though this is
the most likely scenario at this point. Tolleson

.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...Wide mix of conditions across the region with the bulk
of the TAF locations remaining VFR today. Exceptions are KONP
where light winds are allowing sea mist to limit Vsbys to 4 sm for
the next couple hours and KHIO currently has LIFR stratus and no
wind. Feel it may be another couple hours for conditions to
improve at KHIO when the low clouds will quickly mix out to bring

A cold front will drop from the north later this evening bringing
MVFR cigs to the terminals 03-06z along with the chance for IFR
Vsbys to the coast. Later the front arrives and expect IFR Cigs
along with IFR Vsbys at the coast and MVFR inland.

KPDX AND APPROACHES...Expect VFR Conds with local Cigs above 040
to prevail until after 05z when Cigs will lower from the
approaching cold front. However, There will continue to be Cascade
terrain obscuration through roughly 20z today. Cigs eventually
drop to 015 later overnight.

.MARINE...Minimal changes for the morning update aside from minor
adjustments upward for the afternoon waves to cover current swell
trends. Also pushed the Rough Bar Advisory out through 4 pm
Sunday. Remaining previous discussion follows. /JBonk

Winds should remain below small craft advisory criteria through
tonight. Then a surface low will approach from the west late
tonight into Sun AM. There is some disagreement on how strong this
system will be, be was confident enough in the overall pattern to
issue a small craft advisory for winds starting at 3 AM Sun. There
is a slight chance of low-end gales over the central OR waters. A
cold front dropping down from the north will bring another round
of small craft winds Sun night into Mon morning. There is
potential for a much stronger system toward the middle of the next

Seas are beginning increase over the past few hours as a long
period NW swell begins to build into the waters. Based on the
decay of the peak seas observed at the offshore buoys yesterday,
think that we will see seas peak over the coastal waters at 21-24 ft
between 6-9 AM. Decided to go several ft above the model guidance
through this morning. A high surf advisory is in effect through
this evening. Seas remain in the upper teens through this
afternoon and the mid teens through tonight. Seas appear likely to
stay in the low to mid teens through Monday before subsiding below
10 ft Wed. Pyle

OR...High Surf Advisory until noon PST today for Central Oregon 
     Coast-North Oregon Coast.

     Winter Weather Advisory from 1 AM to 4 PM PST Sunday for 
     Northern Oregon Cascades.

     Winter Weather Advisory from 4 AM to 4 PM PST Sunday for 
     Cascades in Lane County.

WA...High Surf Advisory until noon PST today for South Washington 

     Winter Weather Advisory from 6 PM this evening to 10 AM PST 
     Sunday for South Washington Cascades.

PZ...Small Craft Advisory for winds from 3 AM to 10 PM PST Sunday 
     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 4 PM 
     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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