Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 221030

Area Forecast Discussion
National Weather Service Portland OR
330 AM PDT Thu Mar 22 2018

.SYNOPSIS...A strong cold front will move through southwest
Washington and northwest Oregon this morning, with snow levels
lowering into the coastal mountains and Cascade foothills later today
and tonight, and possibly as low as around 1000 feet Friday and
Saturday. Look for accumulating snow in the mountains especially
tonight and Friday in the onshore flow behind the front. Look for
some breezy winds today. There will be a chance of thunder at times
especially near the coast through Saturday, and possibly inland on
Saturday. The pattern should dry out quite a bit next week as an
upper ridge builds, though there are some hints that the upper ridge
may be a bit "dirty" at times.

.SHORT TERM...Today through Saturday...A strong cold front is just
off the coast, pulling up ample moisture ahead of it from a system
off the California coast. The models say the front will come onshore
along the coast around 5 AM this morning, then move through the
inland areas by mid to late morning. Breezy conditions will accompany
the front, but the winds should stay below warning or advisory

Snow levels will drop dramatically behind the front, falling into the
coastal mountains and the Cascade foothills this afternoon and
evening. Onshore flow is forecast to pick up tonight as a lot of the
could air cumulus seen out between 130W and 140W spreads in under
southwesterly low level flow. The 850 mb winds reach 40 to 45 kt
tonight which should aid the snowfall in the coastal mountains,
foothills, and higher Cascades. Have issued several winter weather
advisories to deal with this. The 850 mb winds feeding all of this
onshore don`t let up until later Friday and Friday evening, which is
when the snowfall will ease. Road surfaces will start out quite warm,
so it will take some time for snow to start to accumulate on roadways
through the coastal mountains and the Cascades, but eventually it
will. But this will cause the heaviest snow accumulations to be at
the elevations of the ski resorts such as Timberline and Mount Hood

Snow levels will fall to around 1000 feet Friday into Saturday, but
accumulations will be light. There is always a risk of a few flakes
below this level early Saturday, but this is not certain. It is
getting a bit late in the season to get snow down to the valley
floor, and the offshore low has taken a circuitous and lengthy route
over the water which will further modify how cold the air is. All in
all, don`t think we will see any snow to the valley floor.

The parent cold upper low will dumbbell around and come onshore late
Saturday and Saturday night, keeping showers around. Model lifted
indices suggest thunder is possible near the coast the next few days,
possibly clipping the Kelso area Friday afternoon, and a greater part
of the inland valleys on Saturday as the low moves onshore and
through the area. Tolleson

.LONG TERM...No Changes. Previous discussion follows...
Saturday night through Wednesday. Forecast models in slightly better
agreement today concerning the weekend and into early next week. The
upper level trough is expected to advance inland into far southern
Oregon or northern California late Saturday night before reaching the
Great Basin on Sunday. This will enable a cold air mass to move into
the Pacific Northwest and maintain lower snow levels (1,500 to 2,000
feet) through Sunday afternoon or evening. Meanwhile, a building
upper level ridge over the Pacific will eventually bring rising
heights late Sunday through the start of next week. While showers
will continue in the northwest flow aloft on Sunday, the flow will
turn to a more north or perhaps northeast direction early next week.
Thus, restricted mention of precipitation to a slight chance on
Monday and removed all mention of PoPs starting Monday night.
Depending on the strength and position of the ridge, it is possible
some light precipitation may be possible across the northern portions
of the area at times given some weak impulses embedded in the flow.
Given this pattern, have trended temperatures gradually warmer for
the start of next, with temperatures again approaching 60 in the
interior lowlands for the middle of next week.Cullen

.AVIATION...12z TAF expectations: MVFR cigs at 012-025 under a modest
rain band will spread north over the next few hours as a cold front
over the waters moves east toward the coast. MVFR vsbys will occur at
times with an outside possibility of IFR vsbys under the heavier rain
bands. Behind the front, expect predominantly VFR conditions to
return after 17z. However, an unstable airmass and breezy winds will
result in wind gusts 25 to 30 kt this afternoon and evening. Also
cannot rule out potential for CB and thunderstorms. Main threat will
be at the coast with 20% probability and 10% or less inland. Would
like to have higher confidence before specifically adding VC or TEMPO
wording just yet. Coastal thunderstorm threat will peak this evening
and diminish overnight. COnditions should remain unstable enough with
sufficient wind to block development of fog and/or a low stratus

KPDX AND APPROACHES...MVFR cigs likely to prevail through 17z until
the cold front passes east of the terminal area. May see periodic
MVFR cigs under showers and from a very outside chance of a
thunderstorm through the evening. /JBonk

.MARINE...A cold front is crossing the waters this morning. West
winds will increase behind this front with seas beginning to build.
The winds and seas will intensify more this afternoon as the next
stronger front approaches. Expect Small craft conditions for the
waters in earnest by this afternoon and continuing through parts of
Saturday. Models have lessened the threat of gales this afternoon and
evening ahead of the second front but still cannot rule out an
occasional gust to around 35 kt. Just do not have the confidence to
justify the warning at this time.

Seas will build above 10 feet this afternoon, peaking at 12 to 14
feet late tonight, then subsiding below 10 feet Friday night across
the north and Saturday morning south. The seas will be a mix of wind
wave, fresh SW swell and distant source NW swell.

Models continue to flirt with the idea of a closed low dropping south
along the western edge of the larger parent upper low for Saturday
which could bring another threat of gales Saturday afternoon and also
prolong the elevated seas into Saturday night. Have continued to take
a blend of the models which currently clips the far southwest corner
of the waters with brief gusts around 35 kt and does extend 10 ft+
seas into saturday night. Will take a wait and see attitude for the
models to develop a more consistent approach of that system. JBonk/TJ

OR...Winter Weather Advisory from 10 PM this evening to 6 PM PDT
     Friday for Cascade Foothills in Lane County-Northern
     Oregon Cascade Foothills.

     Winter Weather Advisory from 6 PM this evening to 6 PM PDT
     Friday for Northern Oregon Cascades.

     Winter Weather Advisory from 11 PM this evening to 11 PM PDT
     Friday for Cascades in Lane County.

     Winter Weather Advisory from 6 PM this evening to 5 PM PDT
     Friday for Central Coast Range of Western Oregon-Coast
     Range of Northwest Oregon.

WA...Winter Weather Advisory from 6 PM this evening to 1 PM PDT
     Friday for South Washington Cascades.

     Winter Weather Advisory from 6 PM this evening to 5 PM PDT
     Friday for Willapa Hills.

     Winter Weather Advisory from 6 PM this evening to noon PDT
     Friday for South Washington Cascade Foothills.

PZ...Small Craft Advisory for winds until 2 PM PDT Friday for Waters
     from Cape Shoalwater WA to Florence OR from 10 to 60 NM.

     Small Craft Advisory for hazardous seas from 5 PM this
     afternoon to 11 PM PDT Friday for Coastal Waters from Cape
     Shoalwater WA to Cascade Head OR out 60 NM.

     Small Craft Advisory for hazardous seas from 5 PM this
     afternoon to 5 PM PDT Saturday for Coastal Waters from
     Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory for winds from 8 AM this morning to 2 PM
     PDT Friday for Coastal Waters from Cape Shoalwater WA to
     Florence OR out 10 NM.


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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA or forecast area. is the U.S. government's official web portal to all federal, state and local government web resources and services.