Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 222136

Area Forecast Discussion
National Weather Service Portland OR
236 PM PDT Tue Aug 22 2017

.SYNOPSIS...An upper ridge that spread into the Pacific Northwest
Monday will slowly move east tonight and Wednesday as a system slowly
approaches the B.C and Pacific Northwest coast. Southwest flow aloft
will increase through Wednesday with increasing onshore flow, with
the marine layer being entrenched along the coast and extending
farther inland by Wednesday. This will result in some cooling
Wednesday. There is a slight chance of a shower or a thunderstorm in
the Oregon Cascades tonight and early Wednesday from instability
moving north from a weak low over California. The offshore system
will move through the Pacific Northwest Wednesday night and Thursday,
bringing significant cooling to the area with more clouds and a few
showers. Friday will see drying and warming for a typical summer day,
then look for significant warming this weekend into early next week.

.SHORT TERM...Tonight through Friday...Skies were mostly clear across
southwest Washington and northwest Oregon this afternoon except that
low clouds and fog were hanging in a the coast and smoke continued
over much of the inland areas. The low level flow was turning onshore
this afternoon and the upper flow was turning southwesterly as well
as evidenced by the smoke plumes from the multiple fires in the area.
However, it will take some time for the winds especially aloft to
increase and the smoke to be evacuated from the western valleys, but
expect improvement tonight and Wednesday.

The other issue is that the models indicate some higher level
moisture and elevated instability may lift north from Douglas County
into our southern Cascade zones later tonight and early on Wednesday,
resulting in some nocturnal convection. This moisture and instability
is originating from a weak low over California. For now. we will keep
the convection south of Detroit for now. This should not spread into
the valleys due to the southwest flow aloft.

Wednesday will see greater coverage of low clouds inland in the
morning when compared to today, coming inland along the Columbia
River as well as the southern coastal gap towards and past Eugene.
Even though it will burn off inland in the afternoon, this will cool
temps back down closer to seasonal normals on Wednesday. Increasing
southwest flow aloft will ensure that any convection will be east of
our Cascades in the afternoon.

Wednesday night and Thursday will see the approaching offshore system
move through the Pacific Northwest. This will bring cloudy conditions
to the area with a few showers, as well as cooling of afternoon
temps. Thursday should see 60s at the coast and 70s inland.

Westerly flow spreads in Thursday night and Friday, with morning
clouds especially north Friday giving way to sunshine in the
afternoon. Temps will be closer to seasonal normals as Friday is a
very typical summer day. Tolleson

.LONG TERM...Friday night through Tuesday...Shortwave ridging will
build over the Pacific Northwest and bring warming temperatures
through the weekend. Several days of 90F+ temperatures appear
increasingly likely for the Willamette Valley with Sunday and Monday
appearing the hottest. While some models suggest even hotter
temperatures are possible on these days, most generally keep
temperatures closer to reasonable levels so the forecast was trended
towards this idea being the most likely to occur.

This upcoming hot stretch will likely produce a multi-day period of
critical fire weather conditions as well. East to northeasterly winds
should allow smoke to filter back into the region Sunday and Monday.
The flow may turn a little more southerly aloft as the ridge begins
to break down late in the weekend or more likely early next week so
will need to monitor for the possibility of thunderstorms even though
models at this time do not really show it despite the synoptic
pattern appearing somewhat favorable. /Neuman/Tolleson

.AVIATION...Conditions along the coast are generally MVFR this
afternoon, with patchy IFR stratus. Expect coastal stratus to lower
this evening, with a mix of IFR/LIFR conditions and local fog likely
after 03Z. Coastal IFR/LIFR conditions look to persist through at
least late Wed morning.

Conditions in the interior will remain predominately VFR through late
tonight, with occasional cigs due to smoke. A decent marine push will
likely bring a period of MVFR cigs to most of the interior between
10-18Z Wed. Conditions in the interior look to become VFR by 18Z Wed.

KPDX AND APPROACHES...VFR through late tonight with occasional cigs
due to smoke. A period of MVFR stratus around 2000 ft is likely
between 11-18Z Wed. /64

.MARINE...Conditions remain relatively benign across the coastal
waters, with winds less than 15 kt and seas around 5 ft. A weak front
will move across the waters Wed, but impacts are minimal. High pres
rebuilds behind the front Wed night, with thermal low pres
strengthening later this week. This will likely bring SCA winds to
the central waters by Fri, and to the northern waters over the
weekend. Seas generally remain around 4 to 6 ft through the forecast
period, but may build to around 7 ft and become steep at times over
the weekend as the winds increase. /64




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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 forecast area. is the U.S. government's official web portal to all federal, state and local government web resources and services.