Area Forecast Discussion
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FXUS66 KPQR 250501 CCA

Area Forecast Discussion
National Weather Service Portland OR
839 PM PST Fri Feb 24 2017

.SYNOPSIS...An upper level shortwave and surface low are moving south
along the Oregon offshore waters. The snow level is around 1500 feet
though showers are slowly coming to an end this evening. Clearing
skies overnight will likely allow for patchy fog Sat morning.
Otherwise expect a dry and somewhat chilly day Sat with some
sunshine. Then another low will approach from the north Saturday
evening for another round of rain and snow showers Saturday night and
Sunday. Moist onshore flow will maintain showers Sunday night through
at least Wednesday.


.SHORT TERM...Tonight through Monday...A vertically stacked low
about 420 miles southwest of Astorial continues to drift southward
this evening. Another weaker low about 90 miles southwest of Astoria
is seen by NWS network radar as well as satellite. The combination
of the two is bringing scattered showers to the coast and coastal
mountains.  The overall trend will be for the showers to decrease
tonight as the low shifts further away from the forecast area. As
this happens clouds will thin which will open the door for lowland
fog and freezing fog, with lows dropping below freezing for most
locations. Any lowland fog and stratus should clear late Sat morning
to early Sat afternoon. Expect a dry and slightly chilly day as a
shortwave ridge moves over the Pac NW. But expect most locations
will see quite a bit of afternoon sun especially in the mountains.

Another upper level shortwave and associated surface low will drop
down from the B.C. coast late Sat night and Sun. This will bring
another round of mountain snow, with a rain/snow mix over the
interior lowlands. This system looks very similar to today`s system,
but there are a couple of subtle differences. First, the models are
suggesting a weak warm front associated with the initial rain band
will moderate temp aloft a bit. 850mb model temps increase to around
-4C to -5C. Also, the surface low is modeled to move right along the
coastline, which would allow for a decent southerly gradient to
develop over the region Sun morning. Southerly winds are generally
not favorable for low elevation snow. Forecast lows Sat night only
drop into the 35 to 37 deg range for most of the lowlands. Increasing
cloud cover overnight should limit radiational cooing, and the
increasing southerly winds will limit cooling as well. So expect
accumulating snow will be limited to the Cascades and the higher
elevations of the Coast Range and Cascade foothills. We may need to
consider advisories for these zones. Expect generally rain at 1000 ft
and below, with some flakes mixing in at times during the morning

Light shower activity will linger into Mon in the cyclonic flow on
the back side of the departing trough. Expect temps remaining a few
degrees below normal, with snow levels well below the Cascade passes.
Pyle /mh

.LONG TERM...Monday night through Thursday night...A series of weak
disturbances embedded within an upper trough near the Gulf of Alaska
will move over the Pacific NW early next week with moist onshore flow
continuing showers for SW Washington and NW Oregon through Wednesday
night. Snow levels will remain relatively low (2000 to 3000 feet
range). An upper ridge over the NE Pacific and California will begin
to intensify Thursday and Friday for drier conditions for Oregon.
However, showers will likely continue across SW Washington through
Friday as the upper low to the north challenges the building ridge.


.AVIATION...Cool air mass in place, with lingering low level
moisture and light winds. Upper low off the south Oregon coast
will continue drifting southward, with showers south of a KONP to
KEUG line ending by midnight. Otherwise, will see gradual
clearing overnight. With cool air mass, will see areas of fog and
low stratus form over many inland areas, and patchy fog/stratus
on the coast. With little to no wind and poor mixing, any fog
will persist well into Sat am...possibly as long as 20z.

KPDX AND APPROACHES...VFR clouds over region at moment, but will
see this deck gradually break up through 10Z. But air mass
remains cool and moist, and will see IFR fog/low stratus forming
tonight. Likely not to see much improvement later tonight
through at least 18Z Sat, as the IFR will persist. Rockey.


.MARINE...Weak low pressure about 200 NM west of Florence will
continue southward tonight, then into California later Sat/Sat
night. Winds of 10 to 15 kt will ease overnight, with seas
running 5 to 7 ft.

Next low pressure system from the Gulf of Alaska will arrive
Sat night into Sun as a surface low tracks south down the
Pacific NW shoreline. There will undoubtedly be small craft winds
west of this surface low, but there is a bit more uncertainty
regarding the strength of winds south of this surface low and
east of the surface low. A small craft advisory may be needed,
especially if the surface low tracks a bit further east. Beyond
Sunday, a lack of organized surface low pressure centers will
limit chances for small craft strength winds until later in the
week when a front approaches the area. Seas will follow this same
pattern, approaching near 10 feet on Sunday/Sunday night and
again later in the week ahead of the next front. Weagle/Bentley





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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.