283
FXUS66 KPQR 220530
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
1030 PM PDT Fri Aug 21 2026

Updated aviation discussion.

&&

.SYNOPSIS...Low pressure brings chances for showers tonight and
into Saturday across northwest Oregon and southwest Washington,
with a 15-20% chance for thunderstorms over the Cascades and
5-10% chance elsewhere. Southerly winds tonight will become more
west/southwest on Saturday and return to a west/northwest
pattern Sunday. Seasonable weather returns Sunday through the
middle of next week.

&&

.DISCUSSION...Tonight through Thursday...An upper level trough
over the northeast Pacific will continue to move eastward
towards the PacNW through the overnight hours. This will support
upper level cloud cover, a decrease in temps and a low chance
for precipitation through Saturday.

Hi-res models seem to be struggling with the development and
progression of precipitation across the area as the low shifts
over the area. The most up to date CAMs continue to show the
potential for light precipitation starting this evening and
continuing into the overnight hours. As winds shift more
southwesterly, PWAT values are expected to increase to between
1.0-1.30 inches across NW OR and SW WA. The biggest question
will be CAPE over the region. CAMs and high resolution models
continue to show surface based CAPE (SBCAPE) values 400-800 j/kg
is possible over the Cascades but a strong capping inversion of
75-175 J/kg of CIN could stifle any development. Most unstable
CAPE (MUCAPE) values are a bit higher in the 700-1100 J/kg range
over the Cascades which supports elevated instability in the
mid levels of the atmosphere, this lends credence to the
Cascades maintaining the highest probability of thunderstorms
through the overnight hours. It should also be noted that CIN
values over the Cascades are around 100-200 J/kg and that could
be enough to stifle any development for the Cascades as well.
Therefore, will continue to maintain the slight chance PoPs and
thunder (both 10-25%) over the area with the Cascades remaining
most likely for any precipitation initially then shifting north
of the OR/WA border as a weak cold front moves onshore late
tonight into Saturday morning. Any showers or storms will be
isolated with most locations likely remaining dry.

The upper level trough will continue to progress northeast through
the weekend. The weak cold front moving through the area early
Saturday will support daytime highs in the upper 70s to low 80s
for inland valleys as well as highs in the 60s along the coast
and 70s for the Cascades. Southerly winds which developed today
will become more west/southwest tonight and into Saturday. Winds
through west- east aligned mountain gaps could be gusty,
especially through the western Columbia River Gorge. Gusts
around 30 mph are expected with a 30-50% chance for isolated
gusts approaching 40 mph Saturday afternoon.

Zonal flow aloft develops Sunday as the upper level trough continues
to trek northeast away from the region. Seasonable highs in the
upper 70s to low 80s are expected across the interior lowlands.
Looking ahead into early next week, ensemble clusters favor
ridging over the central CONUS and the PacNW. Model spread for
high temperatures on Monday increases with the 10th-90th
percentiles ranging from the upper 70s to low 90s for the
interior lowlands. Confidence increases Tuesday and Wednesday
with highs likely in the mid 70s to mid 80s. Clusters indicate
another trough influencing the region by the middle of the week
but exact timing and upper level pattern are uncertain but
onshore flow and more seasonable temperatures would be favored.
-42/19

&&

.AVIATION...Through 06z Sunday...An upper-level low tracking to
the north offshore will bring periods of vis/cig restrictions to
area terminals throughout the period. Along the coast, widespread
low stratus coverage will bring persistent IFR/low-end MVFR cigs
at 500-1500 ft. Highest confidence in IFR cigs to the south at
KONP (70-80% chance) while MVFR conditions are more likely at
KAST. Periods of MVFR vis are likely overnight at coastal
terminals. Cigs will tend to lift to 1.5-2.5 kft after 15-18z Sat
while south-southwesterly winds increase to 12-16 kt with gusts of
20-25 kt through the afternoon. Cigs look to trend downward again
late in the period as winds ease Saturday evening.

VFR conditions are favored to continue ahead of the inland
penetration of low stratus for I-5 terminals. Chances for MVFR
cigs are 40-60%, with the highest confidence in the southern
Willamette Valley (KEUG) as early as 09z Sat, and less so in the
central Willamette Valley and Portland area. If coverage of low
clouds is high enough, restricted cigs are most likely from 13-17z
Sat before mixing out and returning to VFR conditions. South to
southwest winds will build to 6-12 kt by Saturday afternoon.

KPDX AND APPROACHES...VFR conditions favored through at least 12z
Sat. Low cloud coverage will increase after 08z Sat, with the
chances for MVFR cigs reaching 50-60% from 13-18z Sat. As low
clouds mix out, VFR conditions are again highly favored after 18z
Sat. South to southwest winds of 3-5 kt overnight will build to
8-10 kt by Saturday afternoon. -36

&&

.MARINE...South to southwest winds continue today as a low
pressure system moves over the northeast Pacific. A cold front
moves through Saturday, bringing a 50-80% chance for maximum
wind gusts to exceed 21 kt for the inner waters, including the
Columbia River Bar. As a result, a Small Craft Advisory has been
issued for the inner waters from 5 AM to 5 PM Saturday and for
the Columbia River Bar from 11 AM to 5 PM Saturday. For the
outer waters, chances are 20-50% with the highest values of that
range between 10 and 30 NM.

High pressure and northerly winds return by Sunday, with a 40-70%
chance for northerly wind gusts to exceed 21 kts in the evening
and continuing into Monday. Seas of 4-6 ft persist through the
period.~12

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.

WA...None.

PZ...Small Craft Advisory from 11 AM to 5 PM PDT Saturday for
     PZZ210.

     Small Craft Advisory from 5 AM to 5 PM PDT Saturday for
     PZZ251>253.

&&

$$

www.weather.gov/portland

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