Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 062217
AFDPQR

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

.SYNOPSIS...Clouds will be decreasing from the north this afternoon
and evening as drier northerly flow spreads in. An upper ridge will
build in tonight and Wednesday for drying, with increasing offshore
winds on Wednesday. A strong winter storm will begin to spread in
from the south and impact the area Thursday with 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. Strong east winds
through the Gorge and south to southeast winds along the coast are
expected. 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...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...Showers have mostly dissipated this afternoon, but
some MVFR and IFR clouds remain. Expect to see clearing through
the evening hours from north to south. Clear skies tonight may
allow IFR freezing fog to form into Wed AM but there is also a
possibility that IFR ceilings will form instead of fog. Either
way, expect deteriorating conditions by Wednesday morning.
However, developing offshore flow should prevent fog from forming
in areas adjacent to the gorge and at the coast, where skies
should remain clear.

KPDX AND APPROACHES...Clearing conditions this afternoon. Expect
light offshore flow to prevent any fog formation overnight into
Wed. Bowen
&&

.MARINE...Seas are on their way down this afternoon with local
buoys showing 9 to 11 ft. Expect the downward trend to continue
through tonight, with seas dropping to around 6 ft by Wednesday
morning. Winds will remain light through tonight and then begin to
pick up Wednesday morning as offshore flow strengthens. The
strongest winds Wednesday and Wednesday night will likely be near
coastal gaps.

A strong frontal system will then approach Thu. There is some
disagreement on exactly what track this system will take, but it
appears likely that we will see a period of gale force southerly
winds at some point Thu. Expect winds to remain gusty out of the
west Fri on the back side of the low, but confidence is low on
the details right now. Peak seas will be delayed from the winds
Thursday, approaching 20 ft late Thursday night and into Friday
but then quickly decreasing into the day Saturday. Right now winds
and seas look fairly benign for much of the weekend. Bowen
&&

.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.

     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.

PZ...Small Craft Advisory for winds from 7 AM Wednesday to 10 AM PST
     Thursday for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 60 nm.

     Small Craft Advisory for hazardous seas until 4 PM PST this
     afternoon for Coastal Waters from Cape Shoalwater WA 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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