Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 212312
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
312 PM PST Tue Feb 21 2017

.SYNOPSIS...Steady precipitation will gradually change to showers
from northwest to southeast tonight. Mountain snows will taper off
from north to south with a few more inches of new accumulation
mainly this evening. Colder air associated with a large Pacific
upper trough will sag into the forecast area starting later tonight.
Expect snow levels generally below 1500 feet Wednesday through the
weekend. Overall precipitation amounts will remain relatively light
but the heaviest showers could mix in snowflakes to sea level into
early next week.
&&

.SHORT TERM...Tonight through Friday...The region is under a fairly
complex upper pattern with this morning`s low having already moved
across eastern Oregon earlier today with a compact and filling
secondary low currently crossing the central Oregon Cascades. A
deformation zone currently stretches from about Corvallis northeast
to Mt. Hood. The band is becoming more diffuse although a trailing
burst of rain in the last hour has brought a new month of February
record to the Portland Airport. The monthly total is at 10.04 inches
as of about 215 pm which breaks the record of 10.03 inches from
February, 1996.

Accumulating snow levels are currently around 4000 feet to the south
and about 3000 feet to the north they will lower back to around 3500
feet to the south and 2500 feet to the north as a pseudo front
begins to push southeast over the region behind the primary
deformation band. Will leave the snow advisories  in place as
planned with the North Oregon Cascades ending at 6 pm and the Lane
County Cascades ending at 10 pm tonight.

A large scale upper trough will continue to shift southeast to cover
the entire Pacific Northwest by this time tomorrow. The colder and
drier air mass is becoming apparent at the coastal sites with dew
points lowering into the mid 30s. As the trough digs south, another
reinforcing and smaller scale closed low will slide down the western
periphery of the parent trough tomorrow. This will bring a brief
period of particularly unstable air over the region and possible
thunderstorms to the Coast Range westward. This pattern could be the
first shot of low elevation snow mixing in with the rain showers.
The upper trough will linger over the broad region through Friday
and models do show a fairly vigorous closed low dropping offshore to
the west on Friday. So far, it seems far enough west the southerly
flow will bring slightly warmer air along the coast where the best
threat of precipitation could clip our area.

Temperatures will be noticeably cooler the next several days,
however, an overall lack of modest easterly flow will keep the best
chances for accumulating snow above 1000 feet. That said, all of the
showers over the next few days will not likely bring more than a
couple inches of snow over any given 12-hour period. That would
remain below advisory amounts of snow to the foothills. It`s even
more unlikely to get more than 6 inches for the Cascades. The lowest
elevations, including the Willamette Valley and the Coast, will
likely see periods where snow mixes into the showers...particularly
during the late overnight and early morning hours. Any accumulating
snow at the lowest terrain will occur under a particularly heavy
shower but would be unlikely to stay frozen on the ground for more
than 1-2 hours. /JBonk

.LONG TERM...Friday night through Tuesday...Models and ensembles
show good agreement that below normal temperatures will persist
through the extended forecast period. The mean longwave upper trough
axis moves little through Monday, allowing additional reinforcing
shots of cold air to push down from Alaska. The associated
shortwaves will keep our weather unsettled through early next week,
but none of these disturbances look particularly strong. The 12z
ECMWF holds 850 mb temps generally in the -4 to -6 deg C range,
which in onshore flow would support some snow showers in the hills
above around 1000 feet in elevation. With snow levels this low,
cannot rule out a few wet snowflakes making it down to the valley
floors in heavier showers. However, accumulating snow appears
unlikely this weekend or early next week for elevations much below
1000 feet.

The longwave pattern appears to open up a bit toward Tuesday, with
most guidance ejecting our upper trough eastward into the Rockies.
However there are signs of yet another cool upper trough digging
down from the Gulf of Alaska mid- to late next week, which is likely
the primary reason CPC is showing high probabilities of below normal
temperatures across the Pac NW for the first week of March.  Weagle

&&

.AVIATION...-RA turning to -SHRA from northwest to southeast this
afternoon, although there doesn`t appear to be much activity in
the shower area. Also seeing improvement to VFR from NW to SE as
-RA clears out. Expect these trends to continue with VFR across
the area this evening and through the overnight hours for most
places, but isolated MVFR CIGs may develop for a few hours
overnight. Winds will be mainly light and variable.

KPDX AND APPROACHES...VFR conditions with southerly flow starting
out this afternoon then turning light and variable overnight.

&&

.MARINE...Winds have dropped below 20 kt, so have cancelled the
Small Craft Advisory for Winds slightly early. Winds and seas now
look like they will remain benign through the rest of the
workweek and into the beginning of the weekend. Seas will
generally remain between 7 and 9 ft. Weak low pressure may slide
south along the WA/OR coasts Thu this week, but at this point it
doesn`t look like it will produce any significant winds or seas
in the local waters. Bowen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 6 PM PST this evening for
     Northern Oregon Cascades.

     Winter Weather Advisory until 10 PM PST this evening for
     Cascades in Lane County.

WA...None.
PZ...None.
&&

$$

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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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