Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 151623
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
922 AM PDT Wed Aug 15 2018

Updated aviation section.

.SYNOPSIS...Warm and hazy conditions continue today. Increasing
onshore flow will improve air quality for the interior valleys on
Thursday. A slow moving upper level trough will result in deeper
marine pushes tonight and Thursday night for cooler afternoon
temperatures Thursday and Friday. This feature also results in a
slight chance for thunderstorms along the Oregon Cascade crest this
afternoon and Thursday afternoon. An upper level ridge returns
Saturday for slight warming.

&&

.SHORT TERM...Today through Saturday...Haze and smoke from regional
wildfires will continue to impact NW Oregon and SW Washington today.
Haze higher in the atmosphere are from the wildfires across south
Oregon and north California, and are being transported to the Pacific
NW via S-SW 700mb winds. Smoke and haze at the lower levels are from
wildfires north and east, and were transported into the interior
valleys yesterday (Tuesday) from light low-level offshore winds. The
winds are no longer offshore so do not expect additional low-level
smoke to filter into the area today. However, calm winds this morning
is creating stagnant conditions trapping the already present smoke in
the interior valleys and continuing unhealthy air quality. The Oregon
DEQ and SW Washington Clean Air Agency air quality monitors show poor
air quality in the interior valleys from Salem to Chehalis this
morning. The HRRR model which has been forecasting the smoke very
well this year forecasts that the smoke and air quality will improve
tonight and Thursday for the Willamette Valley and the valleys of
Cowlitz County, but may linger in the Columbia River Gorge and along
the Cascade passes through Thursday evening. This forecast aligns
well with modeled winds near the surface and at 700 mb.

A thermal trough is just offshore the Oregon coast early this morning
generating light S-SW winds along the coast. Satellite imagery shows
expansive low clouds over the coastal waters. The S-SW coastal winds
overnight and this morning combined with a fairly deep marine layer
(courtesy of an approaching upper trough) has allowed the low clouds
to fill in along the coast, the central Oregon coast range, and up
the lower Columbia River Valley. These clouds will continue to
slowly move inland this morning and there is a good possibility that
the clouds will impact much of the south Willamette Valley this
morning. Can not rule out some clouds making it into the north
Willamette Valley, but think it is more likely that the clouds
moving up the lower Columbia River Valley will not make it much
further than Kelso. These clouds will provide cooler afternoon
temperatures along the coast and the coastal mountains. Even though
low clouds will not fill in the entire Willamette Valley, cooler
marine air will move in resulting in slightly cooler afternoon
temperatures.  There is still a good chance though that many areas
in the interior valleys will reach 90 degrees today.

The thermal trough shifts to the Cascades later today for increasing
onshore winds this afternoon and evening. Instability will increase
over the Cascades this afternoon as an upper level trough nears.
Models show a slight increase in mid-level moisture near the Cascades
that may result in cumulus development or possibly a thunderstorm.
Steering flow suggests that the best possibility for any
thunderstorms to form will be along the Cascade crest south of Mt
Jefferson. A deeper marine push is expected Wednesday night into
Thursday for cooler inland temperatures Thursday afternoon. There
will be another slight chance for thunderstorms along the Oregon
Cascades Thursday afternoon as the upper trough moves a little
further inland.

The upper level trough finally moves over the area on Friday with
Friday afternoon temperatures likely peaking in the upper 70s to low
80s. This will be the coolest day in the 7-day forecast. An upper
ridge moves in behind the departing trough Friday night and Saturday
limiting the inland intrusion of marine air, and warming the interior
temperatures back to the mid to upper 80s Saturday afternoon. ~TJ

.LONG TERM...Saturday night through Wednesday...The mid-range models
are in good agreement on a strong upper ridge building over the B.C.
coast Saturday night and Sunday as a large upper low deepens near the
Aleutian Islands. A weaker upper low will undercut the ridge and move
over the Pacific NW Sunday and Monday. This will continue marine
clouds along the coast in the nights and mornings with clearing in
the afternoons. The interior will remain dry with afternoon
temperatures warming a little each day. Light onshore flow should
prevent the temperatures from getting excessively hot. The models
vary on the departure of the upper trough, but all agree that an
upper ridge will dominate Tuesday or Wednesday. Increased subsidence
from this ridge will limit the inland impacts of the marine air, and
the interior afternoon temperatures will likely warm into the low 90s
Tuesday and Wednesday. ~TJ

&&

.AVIATION...Low level smoke continues to impact the interior
lowlands north of KSLE today. Expect a mix of VFR and MVFR
visibilities in these areas through the afternoon, with more
significant improvement likely tonight. Meanwhile, IFR marine
stratus and fog remains at the coast this morning, and will
likely persist through most of, if not for the entire day. The
best chance of seeing any clearing later this afternoon with be
along the south Washington and far NW Oregon coast. The marine
layer was actually deep enough last night for some 1000 ft cigs
to reach KEUG this morning. However, this deck should dissipate
over the next hour or two, with VFR expected for the south valley
through the rest of the day. A better marine stratus surge is
expected tonight into Thursday morning. MVFR cigs should return
to KEUG, and may impact some of the northern TAF sites as well.

KPDX AND APPROACHES...Low level smoke to bring a continued mix of
VFR and MVFR visibilities into early this evening. Conditions
should improve tonight. MVFR stratus will likely impact the
terminal after 12Z Thu. Pyle

&&

.MARINE...There will be little change to the overall weather
pattern through the upcoming weekend. Surface high pressure will
remain in place over the northeastern Pacific, while a thermally-
induced surface trough will sit over northern California and
southern Oregon. This will result in persistent northerly winds
across the waters. These winds may become a bit gustier on
Thursday and Friday during the afternoon and evening hours, with
the latest models showing gusts of 25 kt over the southern zones
(south of Cascade Head). These low end small craft advisory
strength winds likely continue at times through the weekend.

Seas will remain mostly 5 to 7 ft, perhaps touching 8 ft at times
late in the week. The dominant swell will be out of the NW at 8
seconds through tomorrow. Then strengthening winds off of
Vancouver Island will bring a bit longer period NW fresh swell
into the waters by early Thursday.   Cullen

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning from 2 PM this afternoon to 11 PM PDT this
     evening for Willamette National Forest.

WA...None.
PZ...None.
&&


$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA or forecast area.


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