Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 251007

Area Forecast Discussion
National Weather Service Portland OR
307 AM PDT Wed May 25 2016

.SYNOPSIS...A deep marine layer and onshore flow will result in
plenty of clouds across southwest Washington and northwest Oregon
today and Thursday. Some areas of drizzle will be possible each
morning, with a few showers each afternoon. Due to the deep marine
layer and cloud cover, temperatures will probably be a few degrees
below late May normals today and Thursday. Low pressure dropping down
the west coast of Canada will likely bring a few more showers
Thursday afternoon through Friday, along with a reinforcing shot of
cool air and onshore flow. Cool and showery conditions look to stick
around for the first half of Memorial Day Weekend, but milder and
drier weather appears to start Monday as a ridge of high pressure
builds in for the first part of next week.


.SHORT TERM...(Tonight through Friday)...Satellite 11-3.9 um fog
product shows stratus filling in across SW Washington and NW Oregon
this morning. The clouds are somewhat trapped below a strengthening
subsidence inversion as shortwave upper ridging slowly transits the
region. 00z NAM/GFS soundings continue to show a sharpening inversion
near 800 mb today, with abundant moisture trapped below. The result
will likely be a good bit of cloud cover today. There are some very
weak echoes on KLGX radar, seeming to suggest some -DZ over the
coastal waters. As onshore flow pushes this moisture up the west
slopes of the Coast Range and Cascades, there will probably be some
areas of drizzle wrung out of out of the marine layer... possibly
enough to count as measurable rainfall. Some patchy drizzle is
possible in the lowlands as well this morning, but less widespread
than in the higher terrain.

There remain a few breaks in the clouds, and that will probably be
the case throughout the day. However, any prolonged sunshine will be
"self-destructive" in the sense that it will create mixing in the
low-level moist layer which in turn will cause stratocumulus clouds
midday and afternoon. A few of these will produce showers, but QPF in
general will probably be negligible. Tonight and Thursday appear to
be mostly a re-run of today, though afternoon showers may be more
widespread Thu due to an approaching upper trough and cold front from
the BC Coast. The incoming upper trough will relax the subsidence
inversion Thu night and Friday, resulting in somewhat deeper and more
widespread showers...particularly Salem northward. Even with the
deeper mixed layer, thunder does not appear likely as the bulk of the
cold air aloft stays north over WA and the Idaho Panhandle.

All the cloud cover will probably keep high temps 5-10 degrees below
seasonal averages through Friday. Despite many locations reaching the
lower 70s yesterday in a somewhat similar pattern, it appears the
deep low-level moisture and resulting cloud cover will hold most
lowland locations in the lower to mid 60s today...and possibly even
cooler than that Thursday and Friday as the cold front and upper
trough usher in a cooler air mass in general. One caveat is if cloud
cover ends up less than expected today or Thu, temps could end up
about 5 degrees warmer than forecast due to the strong late May
sunshine.  Weagle

.LONG TERM...Friday night through Tuesday....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.

With confidence building in the upper ridge for next week, decided to
pull any precip mention from the forecast Mon Night through Wed. Per
the 00z runs of the GFS, ECMWF, and GEM - along with most of their
ensembles, it appears reasonable to expect we will once again
approach 80 degrees in the Willamette Valley by the middle of next
week.  Cullen/Weagle


.AVIATION...VFR conditions prevailing inland early this morning
with a cloud layer around 4 to 6 kft. Coastal areas predominantly
MVFR with cigs around 2 to 3 kft. Model soundings and statistical
guidance generally agree in MVFR cigs between 2 and 3 kft
developing at most inland TAF locations this morning between 12Z
and 18Z. Onshore low level flow is weakening and BKN to OVC layer
around 6 kft inhibiting radiational cooling. A weak disturbance
will move across the area this morning to enhance the onshore
moisture and may provide the additional lift to develop the MVFR
layer. The central Oregon coast, including KONP, also appears
likely to improve to VFR by 18Z. However, the south Washington and
north Oregon coast, including KAST, may hang on MVFR cigs well
into the afternoon. MVFR returns to the coastal areas this evening
then inland overnight into Thu morning.

PDX AND APPROACHES...VFR conditions prevailing today and tonight.
However there is a chance of cigs lowering to MVFR with bases
around 2000-2500 ft around 14Z through 18Z.


.MARINE...Conditions are fairly benign over the waters this
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.
West of 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 later today into Thu. This will also shorten dominant
wave periods to around 8 seconds to bring fairly steep waves.  There
is a chance seas in the northern waters out west of 10 nm sees seas
get up around 8 ft on Thu, especially farther out closer to Buoy 89.
This would meet the square seas criteria for an advisory, so an
advisory for hazardous seas may be need then. 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 high.

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.