Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 232242

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

.SYNOPSIS...A cold upper level trough will remain over the Pac NW
through the weekend. A shortwave will drop south along the WA and OR
coast late tonight and Fri, bringing a period of more intense shower
activity. This will bring accumulating snow below 1000 ft, and
locally down to the lowest elevations. The best chance for low
elevation accumulations looks to be along the south WA and north OR
coast Fri morning, although light accumulations are possible almost
anywhere during the morning under heavier showers. The snow level
will rise during the afternoon as surface temps rise. A brief ridge
will result in drier weather Friday night and Saturday. Then another
cold system arrives from the north on Sun. It does not appear to be
quite as cold as the one Fri, and expect accumulating snow to remain
in the Coast Range and Cascades, with snow flakes mixing in at times
to the lowlands. Showers continue into early next week, but become
increasingly confined to the Cascades by Tue. The trend is then
toward drier weather for the middle of next week.


.SHORT TERM...Tonight through Sunday night...Things are playing out
pretty much as expected today. After some reports of light snow
accumulations in showers along the coast this morning, surface temps
have now warmed well into the 40s for most of the lowlands. Showers
are now mostly rain, with some graupel or small hail mixed in at
times. The showers have also expanded from the coast into the rest of
the interior in response to increased low-level instability from
diurnal heating. The showers will continue across the region through
early this evening. Then as surface temps begin to cool again,
showers will become confined to the coastal waters and coast.

A shortwave trough currently apparent on water vapor satellite over
central B.C. will dive south and west tonight, then move south just
off the WA and OR coasts on Fri. Diffluent flow aloft will enhance
the shower activity over the coastal waters and coast going into
early Fri morning. An attendant surface low will also bring moist
southwest flow into the coast Fri as it also moves over the waters,
which should allow the showers to bring a little better QPF totals
compared to those we have seen over today and yesterday. Model 850 mb
temps will remain around -7C to -8C, which is around where we
typically start to see snow mixing down to the lowest elevations in a
light onshore shower pattern. Overnight low temps will again drop
down into the low to mid 30s, so think there is a pretty good chance
that many low elevation locations will experience a mix of rain and
snow or all snow with any showers occurring tomorrow morning. The
best shower chances in the morning will be over the south WA and
north OR coast and Coast Range due to the relatively warmer ocean
temps and the proximity to the low to the north. Expect there will be
spots in these zones that pick up a quick inch or two of snow in
heavier showers. Showers will weaken as they come over the east side
of the Coast Range into the interior lowlands. However, they may hold
together enough to produce some light valley accumulations to the
lowest elevations as well. Think most locations will see a dusting to
1/2 at most, with accumulations mainly on grass and non-road
surfaces, as roads will likely be well above freezing. However,
elevations above 500 ft may see localized higher amounts. Decided to
issues a Winter Weather Advisory for the south Washington and north
Oregon coastal zones. Will keep an SPS covering the rest of the
lowlands. Bottom line is that there is potential for light snow
accumulations pretty much anywhere Fri morning, but it will be
localized and dependent on the locations of heavier showers. The low
will be pushing southward through the day, shifting the main shower
focus over the southern tier of the CWA in the afternoon and evening.
There will still be potential for snow mixing down to the lowest
elevations, but accumulations will be much harder to come by as
surface temps again warm into the 40s.

Showers will taper off from north to south Fri evening as the low
moves south toward the Calif coast. Then expect a dry period
overnight Fri and Sat as a shortwave ridge builds over the Pac NW.
Depending on how fast the cloud cover clears, there will be potential
for fog Sat morning, particularly over the northern half of the CWA.
However, fog and low clouds should clear during the day, providing a
nice, albeit slightly cooler than average afternoon with some sun.

Another upper level shortwave and surface low will approach from the
north on Sun. This system actually looks pretty similar to the Fri
system, with QPF totals a little more impressive. However, it appears
that there is a slight warm front that the surface low brings
onshore. Think that the snow levels will not be quite as low with
this event. There should be some decent accumulations, potentially
advisory worthy, down into the Coast Range and Cascade foothills.
However, do not think it will be cold enough to produce any
accumulations in the lowlands. Would not be surprised to see flakes
mixing in to the lowest elevations again, though. Pyle

.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...Similar to Wednesday, a moist and unstable
environment led to rapid development of showers once morning
valley fog burned off across SW WA/NW OR. Heavier showers may
produce small hail, graupel, or even a burst of wet snow. Any
accumulations should only last a few minutes and should not
affect runways. A stray lightning strike or two cannot be ruled
out from the heavier showers, either. Showers will likely end
inland with the loss of solar heating this evening; returning the
focus of shower activity to the coast overnight and Fri morning.
The air mass may be just cool enough for some accumulating snow
along the coast Fri morning. Conditions will generally be VFR
outside of the above-mentioned valley fog or heavier showers.
Depending on how fast mid-level clouds develop later tonight,
there may be some valley fog after 08z if it stays clear long
enough...particularly KUAO/KSLE southward.

KPDX AND APPROACHES...Generally VFR through this evening, though
some scattered afternoon showers may briefly reduce cigs/vsbys
into the MVFR category. If showers become heavy enough, some
small hail, graupel, or wet snow may mix with the rain...but
runway accumulations are not expected.Weagle


.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


OR...Winter Weather Advisory from 3 AM to noon PST Friday for North
     Oregon Coast.

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



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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. is the U.S. government's official web portal to all federal, state and local government web resources and services.