Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 201010

Area Forecast Discussion
National Weather Service Portland OR
309 AM PDT Sun May 20 2018

.SYNOPSIS...Very slow changes in the overall pattern, with a broad
upper trough over the western CONUS. High pressure building
offshore, will return a more seasonal weather pattern, with late
night/AM clouds and mix of clouds and sunshine in the afternoons.
Less morning clouds are expected Monday and Tuesday for warmer
afternoons. An upper low will bring a threat for showers mid week
that will likely persist into next weekend.

.SHORT TERM...Today through Wednesday...An upper level short wave
approaching the Oregon coast is supporting light precipitating
showers over the Oregon Cascades early this morning. The main upper
trough moves inland over the area today continuing the possibility
for showers. Do not expect much if any measurable rain from these
showers, but there could be a hundredth or two here and there. The
threat for showers ends tonight as the upper trough splits, and an
upper ridge noses into the Pacific NW.

Dry weather is expected Monday and Tuesday as high pressure settles
over the area. Increasing N-NW flow at the lower and mid levels will
result in nighttime marine clouds along the coast moving inland up
the Columbia River into parts of the WIllamette Valley in the
mornings. Clearing in the afternoon will lead to sunshine and warming
with temperatures above the seasonal normals. Less clouds are
expected Monday night into Tuesday morning for temperatures likely
warming into the 80s Tuesday afternoon. Showers associated with a
large upper low south of the area may near the Lane County Cascades
late Tuesday afternoon.

An upper shortwave along the coast early Wednesday morning may result
in a weak SW marine push which may bring marine clouds from the coast
up into the south Willamette Valley. AN upper low offshore of
California will result in diffluence over NW Oregon Wednesday
afternoon favoring showers and possibly thunderstorms over the
Cascades. ~TJ

.LONG TERM...Wednesday night through Sunday...Convective showers are
possible over the Cascades Wednesday night and Thursday as an upper
low approaches northern California. Models are in decent agreement
that the upper low will move inland Friday and Saturday, but vary on
how far north the low will track. The GFS forecasts the low to move
over NW Oregon and Washington, whereas the ECMWF forecasts the low will
move over north California and east Oregon. Both of these solutions
will support showers over NW Oregon and SW Washington Friday nigh,
Saturday, and possibly into Sunday. The upper low will moderate
temperatures over the weekend with afternoon temperatures likely
peaking near or just above 70 degrees. ~TJ

.AVIATION...Cigs have lowered to predominantly MVFR and should
remain there through much of this morning. Marine stratus will be
slow to break up this afternoon inland, breaking up between
19Z-21Z. MVFR stratus will likely hold on to the coast through
the rest of today. Could see a few light showers across the
region today.

.KPDX AND APPROACHES...MVFR cigs to hold through much of this
morning, lifting and starting to break up around 20Z. Expect VFR
conditions after 20-21Z through the rest of this afternoon and
evening. -McCoy


.MARINE...Benign conditions through the rest of today with light
NW winds. Seas are hovering around 3 to 4 ft with a mixed swell,
mostly coming from the west. The pressure gradient increases over
the waters tomorrow, with a chance for wind gusts above 20 kt
Monday evening through Tuesday. Will also see increasing westerly
swell with seas building to 6 to 8 ft midweek.

Stronger NW winds retreat farther offshore later this week,
bringing back benign conditions to our inner water zones Thursday
and Friday. Pattern change going into next weekend as a split-
flow pattern sets up over the waters. This should keep fairly
benign conditions with more westerly winds over the waters. This
will also allow seas to fall back down to around 5 to 6 ft going
into next weekend. -McCoy



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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.