Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 210540

Area Forecast Discussion
National Weather Service Portland OR
940 PM PST Tue Feb 20 2018

Updated Aviation section.

.SYNOPSIS...Cool weather stays over the region for this week under
a north flow aloft. A low pressure system is dropping south along the
Oregon coast today, with a secondary low up near the mouth of the
Columbia River. This will bring several inches of low elevation late
this afternoon and tonight. The next shortwave trough will provide
another chance for low-elevation snow Wednesday night into early Thu.
Additional troughs will continue a threat for rain and low-elevation
snow this weekend into early next week.


.UPDATE...Confirmed 2 inch per hour rates here at the office in NE
Portland as of 7:30 PM. Expect these extreme rates to continue for
the metro area for the next hour followed by light snow through 11 PM
or so. Total snow accumulations still appear on track despite the
later start on the west side where warmer temperatures delayed the
start of accumulating snow. Expect this band to continue to drop
heavy snow down the valley with heavy snow reaching Salem between
8:30 and 10 PM and Eugene between 10 PM and 11 PM.

Will start to drop the warnings and advisories from north to south as
the precipitation comes to an end starting with the WA coast and
Willapa hills in the next hour. /Bentley

.SHORT TERM...Tonight through Friday...
As is typically the case for low-elevation snow events west of the
Cascades, today is presenting some unique forecasting challenges. An
upper level shortwave and associated surface low dove south along the
Oregon coast through the day and appears to be almost due west of the
OR/CA border at 3 PM. A weak warm front associated with this low
brought persistent light precipitation across much of the region
through the day, especially over the northern half of the fcst area.
Precipitation has fallen mainly as snow for the northern interior
lowlands, where temps have remained in the low to mid 30s due to
light offshore flow. However, road and subsurface temperatures are
warm enough that no significant accumulations have occurred at the
lowest elevations. There have been several reports of a few inches of
snow for elevations at 500-1000 ft. South of the the PDX metro area
and along the coast have seen southerly flow develop during the
afternoon, which has allowed temps to warm into the upper 30s to low
40s this afternoon. These areas saw precipitation change over to rain
or a rain/snow mix.

Things become more complicated later this afternoon and this evening.
A secondary low just off the north Oregon coast has been developing
and will continue to strengthen through early this evening. KLGX
radar shows a semi-linear feature stretching from around the mouth of
the Columbia River up along the WA coastal waters. This is appears to
be low-level frontogenesis associated with the low development. At
the same time, water vapor satellite shows a line of cooling cloud
tops  in central WA along the leading edge of a shortwave trough
dropping south of British Columbia, indicating upper level lift from
vorticity advection. The fcst models indicate that these upper level
and lower level forcing features will phase together over the next
few hours, which should drive a several hour period of heavier
precipitation. This is expected to begin over the northern portions
of the fcst area between 4-6 PM. The boundary will sag south through
the rest of the CWA through this evening and early in the overnight
period. The 18Z NAM and GFS runs also indicate a second band of
precipitation will develop over the southern half of the CWA between
00Z-03Z, which may be associated with a deformation band between the
primary surface low to the south and the secondary low, which will be
moving onshore by this time.

Dynamic cooling with the heavy precip rates, combined with diurnal
surface cooling this evening, should allow accumulating snow to fall
down to the lowest elevations as the line moves through. We have
lowered overall storm total snowfall accumulations due to the warm
ground conditions to start, but confidence is still high that most
locations will see at least an inch of snow before the night is over.
The biggest question marks are at the coast and the southern
Willamette Valley. These locations have warmed into the 40s today,
and are modeled to hang onto southerly winds until the frontal band
arrives. Still think that they may be able to eek out an inch of snow
or so on the backside of the front, though. We decided to keep all of
the warning and advisory areas unchanged from the morning package for
messaging continuity, but think that totals will be more on the
advisory end for most of the zones. There is still potential to see
warning criteria hit in spots over the northern half of the CWA, with
the best chances in areas of higher terrain.

Precipitation wraps up late this evening for the further north
portions of the CWA, then overnight further south. By daybreak Wed,
there should be nothing left but maybe some light snow showers in the
Lane County Cascades. The northern half of the CWA may see skies
clear out overnight, which combined with the likelihood of some snow
on the ground, should lead to some overnight lows down into the 20s.
Expect a mainly dry day Wed. There will be increasing clouds through
the day, but temps should be able to warm into the mid 30s to low
40s, helping to melt some of the snow.

Yet another low elevation snow event is then on tap for Wed night
into Thu morning. A fast-moving upper level shortwave and surface low
are modeled to drop along the B.C. coast, then move just enough
offshore to pick up some moisture before moving onshore somewhere
over our CWA overnight Wed. This fcst is highly dependent on the
exact track of this low, and there is still some uncertainty.
Precipitation that does fall has a chance to fall as snow close to
the lowest elevations. Southerly flow ahead of the system may keep
snow levels a little higher at the onset of precipitation, but flow
turns offshore behind the low as it passes. There appears to be a
pretty good chance that some areas see more low-elevation snow
accumulations. The best chances at this point appear to be at the
coast and Coast Range, and southern portions of the Willamette Valley
at this point.

Precipitation likely lingers into Thu morning for the southern
portions of the CWA. But skies should be clearing from north to south
late in the day and Thu night. Dry conditions then continue through
much of Fri as a shortwave ridge moves over the region. However, yet
another low pressure system arrives late Fri and Fri night. Temps
appear to moderate just a bit before this system. Snow levels will
still likely be quite low, but probably not down to the valley floor.

.LONG TERM...Friday night through Tuesday...Stuck in a colder north
to northwesterly flow pattern that will continue to maintain pretty
low snow levels through the period.

The models show the system starting to drop down from the north on
Friday will move through Friday night, followed by trailing short
waves through Saturday into Saturday night. This series of short
waves, while quite cool, has a decent onshore flow with them with an
increasing southerly gradient across our forecast area. The net
result would be that this should begin to lift snow levels a bit off
the valley floors, probably closer to 1000 feet or so by later
Friday night or Saturday morning. The Cascades, foothills and
coastal mountains could see pretty decent snow accumulations through
this time frame.

The next system drops in late Sunday and Sunday night, continuing
Monday. The ECMWF has a surface low that would induce offshore flow
and probably drop snow levels back down near the valley floors, the
other models less certain of that. Nevertheless, the snow levels
will continue to be quite low. Precipitation is likely to ease by
Tuesday but with snow levels continuing quite low. pt


.AVIATION...Still expecting precipitation to switch to snow all
the way south through the Willamette Valley tonight. Then snow will
come to an end north to south, ending south by 4 AM. As snow comes
to an end, clouds should be breaking up which will allow
temperatures to drop down into the 20s at inland terminals, and to
around 30 along the coast. With clouds breaking up, expect VFR
conditions starting after the snow ends. VFR conditions to persist
through Wednesday morning. Still low confidence in Wednesday
evening/overnight forecast, but chance for more snow in the
interior, which would bring IFR conditions.

PDX AND APPROACHES...Snow coming to an end then expect improvement
to VFR overnight. Could see more snow and IFR conditions sometime
after 00Z Thursday, but confidence in precipitation type is low. If
it falls as rain, conditions would likely stay MVFR. Bowen


.MARINE...Weak low pressure system moving down the coast this
afternoon. This has generated southerly winds within 20 nm of the
coast gusting to 25 kt generally south of Cascade Head. As the
low slides south, winds will turn northeasterly tonight, and
should generate gusts up to 25 kt, mostly beyond 10 nm of the
coast. The next system will slide down the coast, ramping up
winds from the northwest on the back side of this low after 7pm
Wednesday evening. Expect solid small craft advisory winds with
this system as well. Seas with this system will build back up
around 7 to 8 feet. Winds drop back below 20 kt late on Thursday,
and conditions remain fairly benign until the next system drops
down out of the Gulf of Alaska later on Friday into the weekend.

The weekend system has a decent fetch of winds with it, so seas
will build back up into at least the middle teens by Sunday.
Right now winds should peak out at around 30 kt on Saturday, but
we have a chance for some gales as another boundary drops down
into the waters on Sunday. -McCoy


OR...Winter Storm Warning until 7 AM PST Wednesday for Central
     Columbia River Gorge-Upper Hood River Valley-Western
     Columbia River Gorge.

     Winter Storm Warning until 4 AM PST Wednesday for Coast Range
     of Northwest Oregon-Greater Portland Metro Area-Lower
     Columbia-Northern Oregon Cascade Foothills.

     Winter Weather Advisory until 7 AM PST Wednesday for Central
     Coast Range of Western Oregon-Northern Oregon Cascades.

     Winter Weather Advisory until 7 AM PST Wednesday for Cascade
     Foothills in Lane County-South Willamette Valley.

     Winter Weather Advisory from 6 PM this evening to 7 AM PST
     Wednesday for Central Oregon Coast.

     Winter Weather Advisory until 2 AM PST Wednesday for North
     Oregon Coast.

     Winter Weather Advisory until 4 AM PST Wednesday for Central
     Willamette Valley.

WA...Winter Storm Warning until 7 AM PST Wednesday for Central
     Columbia River Gorge-Western Columbia River Gorge.

     Winter Storm Warning until 4 AM PST Wednesday for Greater
     Vancouver Area-I-5 Corridor in Cowlitz County-South
     Washington Cascade Foothills-Willapa Hills.

     Winter Weather Advisory until 9 PM PST this evening for South
     Washington Coast.

     Winter Weather Advisory until 1 AM PST Wednesday for South
     Washington Cascades.

PZ...Small Craft Advisory for winds from 7 PM this evening to 7 AM
     PST Wednesday for Waters from Cape Shoalwater WA to
     Cascade Head OR from 10 to 60 NM.

     Small Craft Advisory for winds until noon PST Wednesday for
     Waters from Cascade Head to Florence OR from 10 to 60 NM.

     Small Craft Advisory for winds until 7 PM PST this evening for
     Coastal waters from Cascade Head 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.