Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 240355

Area Forecast Discussion
National Weather Service Portland OR
854 PM PDT Wed May 23 2018

.SYNOPSIS...South to Southwest flow aloft will be over the forecast
area Thursday. The threat of thunderstorms Thursday afternoon and
evening will be primarily over the Lane County Cascades. Increasing
southwest flow aloft and a strengthening marine layer help to
stabilize the north part of the area. The whole trough pattern moves
onshore Friday, maintaining the deep marine layer and pushing the
threat of thunder east of the Cascades. Onshore flow will continue
Saturday, then a return to dry and slightly above normal daytime
temperatures can be expected Sunday into early next week.

.SHORT TERM...Tonight through Saturday...Evening water vapor imagery
depicted a complex upper level trough with multiple low pres centers.
One such circulation center was noted over East-Central Washington.
Convective activity has been diminishing since 01Z. However,
scattered thunderstorms developed over the Cascades during the
afternoon. These were slow-movers due to weak steering flow. One such
storm dumped around an inch of rain in a couple hours at a RAWS site
in the South Washington Cascades. Infra-red satellite imagery at 03Z
showed some lingering cells in eastern Douglas County. These could
drift into the Lane County Cascades and have left a mention of
thunderstorms through 06Z.

The marine layer this evening is fairly extensive along the central
coast and coastal valleys. The KOTH-KSEA gradient was nearly 6.0 mb
at 03Z, with a 3.6 mb KAST-KPDX gradient. Also of note is that SW to
W wind has pushed into the west side of the central Willamette
Valley. KMMV was SW at 13 kts and had cooled to 60 deg by 03Z. KUAO
was SW 8g16, dropping the temp to 66. All in all, expect a rather
deep marine layer Thu morning, likely extending to near the Cascade

The offshore trough gets closer to the coast Thursday, with a low
developing on the southern end that will be heading into California.
This should help the marine air to move into all of the western
valleys tonight and Thursday. The models suggest some drizzle along
the coast late tonight and Thursday morning from the good marine
layer. There is some southeast flow aloft north of the developing
upper low that is moving toward California, but this is mostly into
southwest Oregon. The flow aloft turns more southwesterly in
northwest Oregon and the bulk of the precipitation and convection
will likely be in the Cascades on Thursday.

On Friday, the southern low makes its move into California, with the
northern piece of the offshore upper trough starting to move into the
Pacific Northwest. This will maintain the onshore flow and marine
layer over the area. The unstable lifted indices are east of the
Cascades, so the threat of any thunder looks pretty minimal Friday,
though some showers are still possible over the Cascades in the flow
northward out of the California low. The models show some significant
rains moving northward but most of this looks to be in central

Good onshore flow continue Friday night and Saturday as the whole
pattern and the California low move inland and eastward. The models
show some drizzle or light showers along the coast spreading inland
along the Columbia River late Friday night and into Saturday and have
some low PoPs for that. Weishaar/Tolleson

.LONG TERM...Saturday night through Wednesday...The models are in
good agreement with a positive tilt upper ridge over the Pacific
Northwest Sunday in the wake of the system moving eastward from
Saturday, with the pattern continuing into Monday. The net result is
afternoon sunshine after areas of night and morning low clouds. Temps
will likely trend back toward or perhaps slightly above normal. The
models then show a weak trough moving through after that, the ECMWF
possibly getting it here late on Memorial Day and Tuesday with the
GFS focusing on Tuesday and Wednesday. This will increase the onshore
flow and marine layer again, with some spotty precipitation possible.

.AVIATION...Increasing onshore flow this evening will result in
marine clouds and MVFR cigs along the coast pushing inland
overnight. Most of the Willamette Valley should have MVFR cigs
spread overhead between 06-12z Thursday, which should then
persist through ~18-20z Thursday. There is a chance light drizzle
could develop along the coast and result in cigs lowering into
IFR thresholds at KAST and KONP, but confidence is low in this
scenario as they typically stay or even lift slightly in this
type of pattern.

KPDX AND APPROACHES...Increasing onshore flow will result in VFR
conditions giving way to MVFR cigs around 09-12z Thursday. These
lower flight restrictions should persist through ~18-20z
Thursday. /Neuman


.MARINE...High pressure over the northeast Pacific and lower
pressure over northern California will maintain northerly winds
across the waters for much of the next week. As a result, seas
should generally remain in the 4 to 7 ft range. Winds may
increase a bit early to mid next week, which should result in
seas climbing into the 7 to 10 ft range, and possibly becoming
steep and hazardous, but confidence is low given how far out it
is. /Neuman





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