Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 240522
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
915 PM PST Thu Feb 23 2017

.SYNOPSIS...A series of cool storm systems will bring cooler than
average temperatures, off and on showers and occasional snow to the
Coast Range and Cascade foothills into early next week. The best
chance for snow to dip to sea level and the valley floors will be on
Friday morning, but any accumulations should remain rather short
lived as temperatures warm Friday afternoon. Saturday looks to be
the driest day in the near future.

&&

.SHORT TERM...Tonight through Sunday night...Today was a near repeat
of Wednesday with morning fog giving way to hit or miss afternoon
showers with the strongest storms containing a mix of snow and small
hail down to sea level. Surface temperatures have cooled into the mid
to upper 30s and the low level atmosphere should continue to allow
snow to fall all the way down to sea level through Friday. Showers
are currently weakening as they push inland and should continue to do
so over the next few hours with areas away from the coast and west
slopes of the Coast Range likely drying out overnight.

Our focus has been on a shortwave trough diving southward out of
British Columbia that is just beginning to enter the northeast
Pacific off Vancouver Island. A surface low pressure will develop
just off Vancouver Island over the next 12 hours before it drops
southward parallel to the coast on Friday, but likely closer to 130W
than the actual coastline. This will keep the bulk of the shower
activity offshore over the next 24-36 hours. Nonetheless, a closed
500mb circulation will pinch off from the main shortwave trough and
place northwest Oregon and southwest Washington in an area of upper
level diffluence and cooling temperatures aloft Friday morning. This
should help showers increase across northwest Oregon and southwest
Washington late tonight and Friday morning. The last several runs of
the NAM and GFS suggest a weak trough and/or 925mb low pressure will
pivot onto the coast and hang up over the north Oregon Coast Range
and provide a focus for enhanced shower activity Friday morning.
While these models may be a bit overdone with this feature, do not
feel they can be ignored entirely so suspect if any interior location
receives accumulating snow western Yamhill, Washington,  and Columbia
Counties stand the best chance of seeing an inch or so of snow
especially across any of the higher hills and tried to trend the snow
forecast towards this idea. Even if this feature doesn`t materialize,
the west slopes of the north Oregon coast range and Willapa Hills
will likely see enough snow showers stream onshore that several
inches of snow over a 12 hour period appear probable with winter-like
travel conditions along Highway 6 and 26. As a result, these zones
were included in the Winter Wx Advisory for now. However, did not
have the confidence that enough of the lower Columbia River and
Portland metro zones will see accumulating snow especially on
pavement to warrant an advisory at this time. Same goes for the
central Oregon coast so will let the mid shift watch trends before
including any additional zones in the advisory.

Temperatures should rise during the late morning and afternoon hours
on Friday into the upper 30s and 40s for lowland locations so any
impacts from snow accumulations below 1000` should be relatively
short lived. Nonetheless, snow will likely continue to mix down to
near sea level through the day, but additional accumulations will
likely be hard to come by and very short lived...similar to the past
couple of days.

Shortwave ridging still looks on track to bring drier weather to the
area Saturday with another similar storm system to tomorrows on track
for late Saturday night and Sunday. /Neuman


.LONG TERM...Sunday night through Thursday...Showers will linger
across the region Sun night and Mon behind a departing shortwave
trough. The Cascades should pick up several inches of snowfall
accumulation during this time. Showers will become increasingly
confined to the Cascades later Mon and Tue. The trend will be toward
drier weather for the middle of next week as a ridge begins to
strengthen over the NE Pac. The ridge will not be all that warm, and
temps still likely remain several degrees below average through the
extended fcst period. Pyle


&&


.AVIATION...Cool air mass remains over region, with unstable
northwesterly flow aloft. Showers decreasing, and may see patchy
fog tonight especially in Willamette Valley where cloud breaks
will be linger longer. But overall, most areas will remain VFR.
Upper level disturbance over Vancouver Island will spread showers
back into region later tonight and through Fri. As showers
increase, so will areas of MVFR CIGS, along with VIS briefly
dropping to 2 to 4 miles under heavier showers. Snow levels
remain low, generally 700 to 1000 feet through Fri.

KPDX AND APPROACHES...VFR tonight, with some patchy fog possible.
Do not think will be extensive as clouds will be increasing later
tonight as another disturbance arrives from the northwest. Will
see mix of VFR and MVFR cigs after 12Z Fri, with showers
increasing. Snow levels remain low, and will again see a mix of
small hail, graupel, and wet snow may mix with the rain...but no
runway accumulations expected. Weaglerock


&&


.MARINE...The coastal waters remain relatively calm for late
February, as a series of weak low pressure systems drop down from
BC through the Pac NW coastal waters. One such low will move
across the waters Fri, but winds should generally remain 20 kt or
less. A similar low will take a similar track Sun/Sun night. Seas
will likely remain below 10 ft through Sat. ENP suggests there
may be just enough fetch from the Gulf of Alaska late Sun/Mon to
push seas up to 10 ft.Weagle


&&


.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 3 AM to noon PST Friday for Coast
     Range of Northwest Oregon-North Oregon Coast.

WA...Winter Weather Advisory from 3 AM to noon PST Friday for South
     Washington Coast-Willapa Hills.

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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