Area Forecast Discussion
Issued by NWS Portland, OR

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542
FXUS66 KPQR 251630
AFDPQR

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

.SYNOPSIS...A deep marine layer and onshore flow will result in
plenty of clouds today and Thursday. Some areas of drizzle will be
possible each morning, with a few showers each afternoon, and
temperatures will be a few degrees below late May normals. 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.

&&

.UPDATE...Significant cloud cover continues to blanket the local area
this morning with additional clouds moving in from the north and
likely helping to further fill in areas which are seeing any breaks
this morning. It will likely be a day when diurnal heating will
refill any attempts at cloud breaks this afternoon. May see some
drizzle this morning along the coast, possibly spreading to the rest
of the area during the afternoon hours. Since POPs are based on
measurable precip, continued with slight chance at best. Mostly it
will be a cloudy day but not particularly rainy. Because of all of
the clouds, temperatures will remain in the low to mid 60s, which is
around 5 degrees below normal for inland areas for this time of year.
Inland normal high temperatures have just within the past week or so
generally hit 70 degrees.  /Bowen

.SHORT TERM (previous)...Today 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 (previous)...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...Mix of VFR and MVFR cigs across the area this morning.
Already starting to see improvement inland with cigs starting to
lift to around 3000 ft at impacted terminals, with the exception
of KEUG which will take a bit longer to improve to VFR. At coastal
terminals, seeing improvement as well, with VFR conditions at KONP
and cigs up around 2800 feet at KAST. Will probably see VFR
conditions at most sites by 18Z this morning as cigs continue to
lift. The far north Oregon and south Washington coast may hold on
to MVFR cigs a bit longer, but should improve to VFR by early
this afternoon. VFR conditions continue across the area this
afternoon into this evening. MVFR cigs return to the coast this
evening, with scattered MVFR cigs impacting some terminals inland
Thursday morning similar to this morning.

PDX AND APPROACHES...MVFR cigs improving to VFR by 17Z. VFR
conditions continue through tonight. Chance of MVFR cigs returning
Thursday morning around 13-16Z.   -McCoy

&&

.MARINE...High pressure over the waters this morning will remain
in place through early Friday. Added a small craft advisory for
winds in the northern outer waters. Feeling more confident that we
will see gusts to 20 to 25 kt this afternoon due to current winds
being observed just outside our waters at Buoy 89. Could see a
few gusts to 23 kt in the southern waters west of 20 nm offshore,
but right now not thinking they will be frequent enough to warrant
a small craft advisory. Winds should decrease a bit Thursday
morning for a period of winds below 20 kt in the northern outer
waters before winds increase to near 20 kt with another chance
for gusts to 20 to 25 kt Thursday afternoon and evening, with the
best chance in the northern waters out west of 20 nm offshore.
Friday should see winds decreasing and evening shifting to
westerly Friday night as high pressure breaks down with an
approaching low pressure system which moves through on Saturday.
High pressure rebuilds over the waters early next week, bringing
back northerly winds over the waters.

Seas currently around 5 to 6 feet. Should see seas build up near 7
feet this afternoon, particularly west of 10 nm offshore where
winds will be stronger. Dominant wave periods will drop to around
8 seconds this afternoon creating fairly steep sea conditions in
the outer waters, but doesn`t quite meet advisory criteria. Better
chance on Thursday that we will need an advisory with seas getting
up closer to 8 feet. Wave heights start to decrease on Friday with
the approaching trough, with fairly benign sea conditions through
the weekend. -McCoy


&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory for winds from 2 PM this afternoon to 5 AM
     PDT Thursday for Waters from Cape Shoalwater WA to Cascade
     Head OR from 10 to 60 nm.

&&

$$

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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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