Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 201129
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
329 AM PST Mon Feb 20 2017

.SYNOPSIS...A strong low pressure center is expected to make landfall
this morning near or just south of Florence and move northeastward
through northwest Oregon the remainder of this morning and early this
afternoon, producing windy conditions south of its path. A second
strong low is forecast to move onshore and move through southwest
Oregon Tuesday, but with less wind in northwest Oregon. Both these
lows will produce ample rain and mountain snow. Colder air associated
with the parent upper trough will sag into the forecast area during
the middle part of the week and lower snow levels as a series of
disturbances move through, for more snow in the mountains possibly
even mixing down to low elevations in the valleys that will continue
into next weekend.
&&

.SHORT TERM...Today through Wednesday...A tricky forecast this
morning is highly dependent on the track of a strong low off the
southern Oregon coast as it makes landfall and moves through
northwest Oregon today. Our models have been struggling to reach a
decent consensus of that track and point of landfall. Model solutions
are now suggesting landfall will be close to Florence, then track
northeastward through the central Oregon coast range and the mid
Willamette Valley before moving into the Cascades mainly south of
Portland. As a result, have opted to not issue any wind headlines for
the central Oregon coast, but did issue a high wind warning for the
central Oregon coast range and the Lane County and north Oregon
Cascade foothills, and a wind advisory in the central and south
Willamette Valley for this morning and early afternoon. The models
indicate that the low will be east of the Cascades by 00z.

Based on this track, the models suggest that the wraparound band or
bent back occlusion with this low will extend across the north Oregon
Coast range and part of the Portland and Vancouver areas and the
Kelso/Longview area into the south Washington Cascades for steady
rain and mountain snow through much of today, but with showers
spreading inland behind the low center farther south in the
afternoon.

The models suggest snow advisory amounts of snow in the south
Washington Cascades and the north Oregon Cascades, with 6 to 11
inches in Washington and 4 to 7 inches in Oregon. Have issued a snow
advisory for this. The precipitation will decrease to some showers by
this evening.

Right on the heels of this first low, a second strong low will follow
late tonight and Tuesday. The models are indicating the track of this
low will be on a slightly more southerly track than the first low,
and if so, will keep any stronger winds mainly south of our forecast
area. But the deformation zone on the north and northwest side of the
low will track across our forecast area for more rain and mountain
snow, especially in the Oregon part of our forecast area.

Cooler air with the parent offshore long wave trough will then sag
into the forecast area Tuesday night and Wednesday, dropping snow
levels down to around 1000 feet or so and bringing showers onshore
with it. Expect some snow accumulations in the coastal mountains and
passes and in the Cascade foothills. Snow could get close to mixing
down to the valley floors but accumulations will probably be spotty
at most if any snow does reach the surface. Tolleson

.LONG TERM...No Changes. Previous discussion follows...
Wednesday night through Sunday...An upper level ridge over the NE
Pacific will start to move the broad upper level trough over the
western conus to the east late next week. However, before the trough
moves inland, an upper level shortwave will drop south from the Gulf
of Alaska towards the Pacific NW Wednesday night and Thursday. Models
are in fair agreement that this low will move away from the coast as
it slides south which may limit moisture over Oregon and Washington.
Another shortwave trough will drop south from the north this weekend.
Colder air will result in low snow levels and unseasonal cool
temperatures Thursday through Sunday. Snow levels will likely be
below 1000 feet with a chance for showers. With snow levels this low,
can not rule out a few flurries making it close to the valley floor,
but limited moisture and warm road surface temperatures will minimize
the threat for accumulating snow and major impacts. ~TJ
&&

.AVIATION...Rain increasing across the area this morning as a warm
front lifts north over the area. VFR conditions prevailing as of 10Z
with local MVFR CIGS. As the warm front moves into the area expect
flight conditions to lower to MVFR and possibly high end IFR. The
front may slow or stall north of KSLE this afternoon and delay
improving conditions north of KSLE until this evening. Strong gusty
winds are possible at TAF sites later this morning and afternoon.
Gusts at inland TAF sites could peak around 35 kt and close to 40 kt
at KONP. But strength of the winds will be highly dependent on the
track & strength of the low, which at this time various models are
having difficulty agreeing on a solution.

KPDX AND APPROACHES...VFR to start the 12Z TAF package with CIGS
becoming 1500 to 2500 feet after 15Z and continuing through 21Z-00Z.
Then a mix of VFR and MVFR expected. Will probably also see a couple
hour period of southerly winds gusting 25 to 30 kt between 19Z and
23Z today.
&&

.MARINE...A surface low 41N/129W is tracking toward the Oregon coast.
Models are not in great agreement on the track. Based on current
satellite trends and short term model guidance it appears it will
make landfall near Florence. If the low makes landfall south of
Florence winds will be substantially weaker over the waters. A
prudent move is to clip the central Oregon waters with a Gale Warning
for a few hours this morning. Expect gales with gusts 35 to 40 kt
south of Newport.

Seas are currently in the low teens and may rise another foot or two
overnight. The seas fcst for Mon is tough, given the uncertainty in
the surface low track, but the likeliest scenario still appears to be
seas peaking in the mid teens Monday afternoon, highest along the
central coast south of Newport. The northern waters may not be much
above 10 ft through much of tomorrow. Another low will move onto the
southern OR/northern CA coast later tonight or Tuesday, potentially
bringing wind gusts up to about 30 kt to the central and northern OR
waters.

Seas in the low teens drop below 10 ft Tuesday, but may also stay
above 10 ft until Tuesday night, depending on the second low. After
that, the pattern calms down for a few days as a trough settles over
the WA/OR coastal waters with more benign winds and seas through the
remainder of the workweek. /mh
&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 7 AM this morning to 3 PM PST this afternoon
     for Central Willamette Valley-South Willamette Valley.

     Winter Weather Advisory until 4 PM PST this afternoon for
     Northern Oregon Cascades.

     High Wind Warning from 7 AM this morning to 3 PM PST this
     afternoon for Cascade Foothills in Lane County-Northern
     Oregon Cascade Foothills.

     High Wind Warning from 7 AM this morning to 2 PM PST this
     afternoon for Central Coast Range of Western Oregon.

WA...Winter Weather Advisory until 4 PM PST this afternoon for South
     Washington Cascades.

PZ...Small Craft Advisory for hazardous seas until 4 PM PST this
     afternoon for Coastal Waters from Cape Shoalwater WA to
     Cascade Head OR out 60 nm.

     Gale Warning from 6 AM to 11 AM PST this morning for Coastal
     Waters from Cascade Head OR to Florence OR out 60 nm.

     Small Craft Advisory for hazardous seas until 6 AM PST early
     this morning for Coastal Waters from Cascade Head OR to
     Florence OR out 60 nm.

     Small Craft Advisory for Rough Columbia River Bar until 2 PM
     PST this afternoon.

&&

$$

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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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