Area Forecast Discussion
National Weather Service Portland OR
1007 AM PDT Thu Mar 22 2018

...Updated aviation and marine sections.

.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...A rain band associated with a weak cold front is
moving through the interior this morning. Based on radar trends,
it should be fully through the lowlands and into the Cascades by
18Z. Post-frontal showers will develop this afternoon, along with
some breezy southerly winds. There will be potential for a few
thunderstorms as well, but confidence in timing and location not
high enough to include in the TAFs at this time. Expect a mix of
VFR/MVFR through 18Z, then predominantly VFR conditions this
afternoon. However, brief periods of MVFR conditions are likely
during as heavier showers pass. A secondary front will bring more
organized shower activity to the coast this evening, then into
the interior lowlands overnight. Expect conditions to deteriorate
to predominantly MVFR as the steadier showers move in.

PORTLAND AND APPROACHES...Expect mainly VFR conditions during the
late morning as rain tapers to showers. However, an occasional
heavy shower may bring a brief period of MVFR There is also
potential for an isolated thunderstorm at some point this
afternoon. Pyle


.MARINE...A cold front moved through the waters earlier this
morning. Southwesterly winds behind the front are now gusting in
the 20 to 25 kt range, and will continue through the afternoon. A
stronger secondary front will bring southerly winds of 25 to 30
kt this evening and early in the overnight hours. The 12Z NAM and
GFS runs suggest that there may be a brief period of low-end
gales in PZZ255/PZZ275 in the evening. Decided not to issue a
gale warning at this time, as the duration appears to be very
short and the area affected rather small. Winds will remain above
small craft advisory criteria into Fri morning, but should
decrease through the day. An offshore low dropping south towards
the CA waters may bring another round of small craft winds Sat.
Then more benign conditions are expected late in the weekend and
into early next week as higher pres builds toward the waters.

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.


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
     Coastal Waters from Cape Shoalwater WA to Florence OR out
     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 Rough Columbia River Bar from 8 AM to
     noon PDT Friday.


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.