Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 250415

Area Forecast Discussion
National Weather Service Portland OR
857 PM PDT Tue May 24 2016

.SYNOPSIS...Increasing moist northwest flow should bring increased
cloud cover, increasing chances of light showers and/or drizzle and
cooler temperatures through at least Friday. Another upper level
storm system looks to move across the northern half of the Pacific
Northwest over Memorial Day Weekend and keep temperatures cool and
bring a threat of wet weather each day west of the Cascade Crest.


.SHORT TERM...(Tonight through Friday)...A few light showers
developed over the coast range and Cascades this afternoon and early
this evening. A few sprinkles were reported in the Willamette
Valley, but most populated lowland locations remained dry. A couple
showers producing mainly sprinkles are hanging on across the
Clackamas and Marion Counties, but even these should come to an end
over the next couple of hours.

Onshore pressure gradients have increased a couple millibars across
northwest Oregon and southwest Washington relative to 24 hours ago.
As a result, stratus off the Washington Coast should push into our
coastal zones later tonight and in some shape or form spread inland.
KLGX radar is already picking up on a few areas of light
rain/drizzle associated with this stratus off the Washington Coast.
Model soundings depict this shallow moist and somewhat unstable
layer producing these areas of light rain and drizzle pushing into
the coast later tonight and Wednesday morning. Low level winds will
be most westerly Tillamook northward so PoPs appear highest across
our northern coastal zones and lowest towards Florence and Newport
where low level winds remain more north than west and thus
orographic enhancement will likely be minimized. Models suggest
there is a slight chance enough low level moisture will sneak inland
late tonight and Wednesday morning that drizzle or light rain could
fall, but at this point, it appears most interior locations should
start the day dry. As daytime heating ensues across the interior
later Wednesday morning, a few light showers may develop as this
moisture turns over, but like today, any rain will likely be short
lived and light.

As a shortwave trough currently entering the northern portion of the
Gulf of Alaska drops southward closer to the region late Wednesday
and Thursday, onshore pressure gradients will likely intensify. This
should result in a near repeat of Wednesday on Thursday, but perhaps
with a slight decrease in high temperatures and a slight upward
trend in morning rain/drizzle chances. This will likely be a result
of a slightly deeper marine layer and a bit more low level lift,
particularly near the Cascade foothills.

As the aforementioned shortwave trough settles into southern British
Columbia and Washington late Thursday and Friday, mid level cooling
will allow instability to increase modestly across the area. This
should increase shower coverage and intensity Friday, particularly
across our northern zones. It should be noted that instability still
appears it will remain shallow enough that thunder chances appear
minimal. /Neuman

.LONG TERM...Friday night through Tuesday....Previous Discussion
Below...By late Friday night, the upper low over southern B.C.
continues to advance south into eastern Washington. Latest models
suggest a slightly more inland (easterly) track, but with a slowly
evolving upper trough remaining over the Pacific Northwest through
the holiday weekend. Plenty of spread in ensemble members as well as
a lack of run-to-run consistency over the past 24 hours has
maintained lower than typical confidence in the particular details of
the forecast. However, the general trend does support the building of
a weak ridge over the eastern Pacific and into the region on Sunday
and Monday. So while a shower cannot be ruled out through Monday for
most locations, confidence is increased in the trend of tapering
moisture in the increasingly northwest flow aloft. As such, edited
the PoPs to reflect a chance of showers Saturday transitioning to
slight chance south/chance north Sunday, then only a slight chance
north for Monday. Also expect temperatures to gradually trend warmer
through the extended period - from a few degrees below normal
Saturday to near or a few degrees above normal
on Monday.  Cullen


.AVIATION...Widespread VFR conditions across SW Washington and NW
Oregon this evening. Developing high pres offshore will result in
a better onshore marine push tonight. Expect MVFR cigs to spread
into the coast between 06Z-09Z, then move into much of the
interior by around 12Z. Cigs should lift and improve to VFR for
the interior by around 18Z. The central Oregon coast, including
KONP, also appears likely to improve to VFR by late Wed morning.
However, the south Washington and north Oregon coast, including
KAST, may hang on MVFR cigs well into the afternoon.

PDX AND APPROACHES...VFR conditions this afternoon through this
evening. Good chance of cigs lowering to MVFR with bases around
2000-2500 ft by around 12Z Wed morning. Cigs should lift to VFR
by around 18Z. Pyle/McCoy


.MARINE...Conditions are fairly benign over the waters this
evening as higher pres continues to build offshore. Expect the
high pressure to remain in place over the Northeast Pacific
through Friday. This means northerly winds over the waters,
increasing to 15 to 20 knots during the afternoons and evenings
through Friday. Out west 20 nm offshore may see a few gusts to 25
knots. Fresh swell from a decent fetch of northwesterly winds will
allow seas to build to near 7 feet tomorrow afternoon. This will
also shorten dominant wave periods to around 8 seconds. This is
fairly steep and approaching criteria for a small craft advisory
for hazardous seas. There is a chance seas in the northern waters
out west of 10 nm sees seas get up around 8 ft, especially farther
out closer to Buoy 89. This would meet the square seas criteria
for an advisory, but holding off on the advisory for now due to
some uncertainty. Will see a chance for square seas in the
northern outer waters again Thursday afternoon and evening. Fetch
starts to break down on Friday so may not see seas get quite as

Over the weekend the forecast is more uncertain. There is a system
moving through which, depending on the track, will either disrupt
this pattern of northerly winds or just keep northerly winds a bit
weaker over the weekend. In regards to seas, combined seas will
probably stay below 10 ft through the weekend. Pyle/McCoy





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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.