Area Forecast Discussion
Issued by NWS Portland, OR

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901
FXUS66 KPQR 070602
AFDPQR

Area Forecast Discussion
National Weather Service Portland Oregon
1002 PM PST Tue Dec  6 2016

.SYNOPSIS...Skies have been slow to clear this evening between Salem
and Roseburg, but most of western Oregon is expected to clear out by
Wednesday morning as cold and dry offshore flow increases across the
region. The east winds will become fairly strong through the Columbia
Gorge and into the Portland Metro beginning late Wednesday afternoon,
continuing into Thursday as a strong winter storm approaches from the
southwest. This storm will bring a mix of snow and sleet changing to
freezing rain and sleet and eventually to rain from south to north
through the entire day of Thursday. Most places will be rain Thursday
night and rain showers Friday except mixed precipitation will
continue in the Gorge into Friday. More typical unsettled weather
will continue through the weekend and into early next week.

&&

.SHORT TERM...Tonight through Thursday...Overall gist of the forecast
remains on track as models continue to have some variation of a low
pressure system...currently in the general vicinity of
40N/140W...approaching our forecast area from the southwest Wed night
and Thursday. The new 00z GFS seems to slow down the onset of
precipitation by a couple more hours, with precip beginning around
4am Thu near Newport and Eugene, then spreading north to Astoria and
Portland by around 10am Thu. Will be interesting to see if the 00z
ECMWF follows this trend. While the 00z GFS continues to develop a
strong low outside of 130W, the secondary low appears a little weaker
which would tend to allow cold air to hang in a little longer,
especially near the Columbia Gorge. This type of solution would tend
to favor more snow and less ice for the PDX metro, with frozen or
freezing precip also holding on longer along the North Coast and
Willamette Valley. The 00z NAM keeps the parent low dominant inside
130W and below 990 mb, which would be a warmer solution and would
result in a faster changeover to freezing rain and eventually rain
outside of the Columbia Gorge and Hood River Valley. Given that
models are still struggling with the details of this system, decided
to make no major changes to the forecast other than some tweaks to
the winds.

Strong easterly pressure gradients are expected to develop across the
Columbia Gorge later Wednesday and Wednesday night, causing east
winds to become rather strong through the Gorge and across the
Portland and Vancouver metro areas. Decided to issue a Wind Advisory
for the Portland/Vancouver metro late Wed afternoon through Thu
afternoon. With KTTD-KDLS gradients expected to be up around -10 to
-11 mb and KGEG-KOTH gradients up around +20 to +25 mb, we should
easily meet advisory criteria, especially near the Gorge and in the
West Hills of Portland. If the wind were completely constricted
through the Gorge, this would be a high-end event with gusts up to
50-60 mph across the east PDX metro and 100-110 mph gusts at Crown
Point...and we would possibly need warnings for the west end of the
Gorge and nearby portions of the PDX metro. However, it appears the
eastside cold air should be deep enough to spill over the Cascade
crest, which would take a little bit of the edge off the winds.
Still, it will be windy and cold later Wed through Thu, with wind
chills in the teens to mid 20s across much of the PDX Metro Wed
night/Thu. The winds will likely cause very hazardous, blizzard-like
conditions at times in the Gorge as snow develops early Thu.  Weagle

Previous discussion from 219 pm Tue Dec 6 2016 below...

Weather continues to slowly improve from north to south this
afternoon with decreasing clouds. Low clouds in the valley may take a
while to break up late today especially south of Portland.

An upper ridge will build in tonight and Wednesday for dry weather.
Expect areas of valley freezing fog and low clouds to redevelop
tonight and Wednesday morning. A light easterly gradient will develop
in the Gorge later tonight, then increase on Wednesday with most
places seeing some wind Wednesday. Tonight will be cold in the wake
of the Monday system, and Wednesday will continue to be cold and
somewhat bitter feeling from the effects of the wind.

Attention then turns to the very significant winter storm for later
Wednesday night and Thursday, the strongest winter storm of this sort
we have had in quite a while. Not much change from the ideas
presented this morning.

Look for mixed precipitation to slowly lift north through the area
Wednesday night, probably just approaching the Portland area around 4
am, and may not start before decision makers make calls about school
closures. The mixed precipitation will then spread north Thursday
morning. Look for a brief period of an inch or two of snow or sleet
in the inland areas from about Salem south late Wednesday night
before changing to freezing rain and sleet Thursday morning and rain
late Thursday morning and afternoon. Farther north in the inland
areas around Portland northward look for 1 to 3 or 2 to 4 inches of
snow in the morning before changing over to sleet then freezing rain
in the afternoon with up to about a quarter inch of ice possible. The
Portland area north should change to rain in the evening. The Gorge
and Upper Hood River Valley will see several inches of snow Thursday,
with freezing rain and sleet moving in Thursday night, possibly
lingering into Friday. The Cascades will mainly see some snow with
some rain below the rising snow levels.

The coast may start as mixed precipitation as well, with just a brief
period Wednesday night on the central Coast but a longer period
lasting well into Thursday morning on the north coast.

The models are showing a decent low moving up the coast Thursday into
Thursday night that will bring strong east winds to the Columbia
River area, and southeast to south winds along the coast. Details
differ from model run to model run but will need to watch for high
winds along the coast late Thursday and Thursday night.

Friday will see a lot of onshore flow and showers with snow in the
Cascades as showers rotate inland along the southern periphery of the
low to our north. Looks like a good accumulation of snow in the
Cascades on Friday.

Bottom line is that Thursday for much of the area weather is going to
cause a lot of problems. Tolleson

.LONG TERM...Friday night through Tuesday...Continuing onshore
orographic flow will continue wet conditions through the weekend.
Snow levels will hover between 2 and 3kft resulting in continuing
Cascades snow and even some snow on the higher peaks of the Coast
Range.

By the beginning of next week, model uncertainty increases with
several different pieces of energy moving through the Pacific NW,
but differing model solutions on the strength and timing. It is
worth noting that the arctic high which has brought our cold air
this week is continuing to recharge in Canada. Most models keep the
coldest air further east, however, the GFS locks the cold air into
the Columbia Basin once again. Around half of the GFS ensembles are
showing a high greater than 1035mb surface high settling in to the
Columbia Basin at some point in the first half of next week. Once we
get through the storm system this Thursday, models should come into
better agreement in this timeframe.

Despite the medium range model variance, the GFS, GEM, and ECMWF all
show a fairly strong low pressure system moving inland along the
Oregon coast next Thursday. Typically would wait to mention a system
almost 9 days out, especially with the model uncertainty in the
medium range, but this system has been showing up on the GFS now for
a few days and now that the other models are on board, it is worth
watching as it could be our next impactful weather system.

/Bentley
&&

.AVIATION...Building high pressure over the Inland Northwest will
bring increasing offshore flow through Wednesday, which will
bring predominantly vfr conditions to most taf sites. The main
exception to this will be at KSLE and KEUG where mvfr stratus
appears likely to continue through the night. If these clouds
dissipate, ifr/lifr conditions appear likely to develop shortly
thereafter. Farther north, KHIO and KPDX may see some brief stints
with mvfr and ifr conditions with areas of frost between 10-16z
Wednesday, but any flight restrictions should come to an abrupt
end as east winds increase towards 15z Wednesday.

KPDX AND APPROACHES...Light winds tonight may allow some mvfr and
ifr flight restrictions to develop with frost development likely.
East winds should kick in around 15-18z and increase through 00z
Thursday. Vfr conditions should be dominant during this period.
/Neuman

&&

.MARINE...Seas are currently subsiding across our waters this
evening. High pressure building over the Inland Northwest will
bring increasing offshore flow and easterly winds to the waters
Wednesday. Expect small craft advisory level wind gusts of 25 to
30 kt to build across the waters Wednesday afternoon and evening.
Models suggest there is possibility easterly winds could increase
to low end gale force with gusts of 35 to 40 kt later Wednesday
night into Thursday morning. This case is not that dissimilar to a
case in Nov 2014 where this actually occurred so went ahead and
issued a Gale Watch.

A low pressure system is then forecast to move towards the Pacific
Northwest Thursday, which will turn winds southerly and likely
bring a more substantial round of Gale force winds to the waters.
Held off on a Gale Watch for the central Oregon waters for now
though. The strong south to southwesterly winds will likely result
in seas climbing towards 20 ft late Thursday night and Friday.
Additional storm systems will likely move through the waters over
the weekend and into early next week, but these appear weaker and
should allow seas to subside during this period. /Neuman


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Watch from late Wednesday night through late
     Thursday night for Central Columbia River Gorge-Upper Hood
     River Valley-Western Columbia River Gorge.

     Winter Storm Watch from late Wednesday night through Thursday
     afternoon for Coast Range of Northwest Oregon-Greater
     Portland Metro Area-Lower Columbia-Northern Oregon Cascade
     Foothills.

     Winter Storm Watch from Wednesday evening through Thursday
     morning for Cascade Foothills in Lane County-Central Coast
     Range of Western Oregon-South Willamette Valley.

     Wind Advisory from 3 PM Wednesday to 4 PM PST Thursday for
     Greater Portland Metro Area.

     Winter Storm Watch from late Wednesday night through Thursday
     morning for Central Willamette Valley.

WA...Winter Storm Watch from late Wednesday night through late
     Thursday night for Central Columbia River Gorge-Western
     Columbia River Gorge.

     Winter Storm Watch from late Wednesday night through Thursday
     afternoon for Greater Vancouver Area-I-5 Corridor in
     Cowlitz County-South Washington Cascade Foothills-Willapa
     Hills.

     Wind Advisory from 3 PM Wednesday to 4 PM PST Thursday for
     Greater Vancouver Area.

PZ...Gale Watch from Wednesday evening through late Thursday night
     for Coastal Waters from Cape Shoalwater WA to Cascade Head
     OR out 60 nm.

     Small Craft Advisory for winds from 10 AM to 10 PM PST
     Wednesday for Coastal Waters from Cape Shoalwater WA to
     Cascade Head OR out 60 nm.

     Small Craft Advisory for winds from 10 AM Wednesday to 10 AM
     PST Thursday for Coastal Waters from Cascade Head OR to
     Florence OR out 60 nm.

&&

$$

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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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