West Linn, Oregon 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

564
FXUS66 KPQR 261507 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
806 AM PDT Tue Aug 26 2025

...Updated public discussion...

.SYNOPSIS...Warm today with monsoonal moisture in the area. A
weak area of low pressure inland and a low offshore will be the
driving force for weather over the next several days. 10-25%
chance of thunderstorms today over the Cascades. Cannot rule out
storms inland though too. Slight warm up on Thur/Fri with active
weather returning over the weekend.

&&

.MORNING UPDATE...Radar imagery as of 8 AM Tuesday depicts showers
and isolated thunderstorm activity across the Willamette Valley and
Coast Range, driven by an upper level shortwave with positive
vorticity advection. Precipitation and thunderstorm chances were
increased to slight chance (15-20%) across most of the Willamette
Valley, Coast Range, and coast as these showers and thunderstorms
are moving westward due to easterly 700 mb flow. Main threats from
passing thunderstorms include lightning, gusty outflow winds, and
heavy downpours. Heavy rain could lead to urban and small stream
flooding, especially in low-lying and poor drainage areas. When
lightning roars, go indoors!      -Alviz

&&

.SHORT TERM...Today...Synoptically, the main pattern is
comprised of a weak mesoscale low over eastern Oregon and a
broad low over the northeast Pacific. The combination of these
two systems is wrapping monsoonal moisture over the Pacific
Northwest. Radar is detecting weak showers passing over the area
from Sweet Home northward though minimal precipitation has been
observed. This is due to the dry layer in the near surface
atmosphere. Precipitation is tending to evaporate before
reaching the ground. Through the day, humidity will lower as the
850-250 mb layer shows an average humidity of around 60% along
the Cascades. This is enough to feed storms though.

In the upper levels, a vorticity maxima will move directly over
the region this afternoon. As it does so, this upper level
support will enhance surface level mixing and instability.
Forecast soundings show lapse rates of around 8.5-10C/1000 ft.
These steep lapse rates combined with moisture and the overall
instability will promote more thunderstorm development. The area
most at risk for thunderstorms will be the Willamette Natl
Forest and associated foothills. Here, chances are around
20-25% with lower chances over the foothills.

With all of these features in mind we can transition to our
temperature forecast. This amount of moisture will increase
cloud cover which will throw a wrench into the forecast. Within
the Willamette Valley, Columbia River Gorge and the Upper Hood
River Valley, models are suggesting highs in the mid-90s.
However, the cloud cover and even smoke aloft from nearby fires,
will inhibit this type of heating. Let`s use Monday for example.
The NBM suggested a high temperature at Portland Intl Airport of
95F. However, due to the cloud cover the highest temperature was
88 degF. This trend continued through the central Willamette
Valley. However, clearer skies to the south allowed areas around
Eugene to heat up to near 96F. Today with the added moisture,
cloud cover will be more and therefore, temperatures will be
cooler once again than models are suggesting. Have trended
towards the 5th percentile of the NBM incorporating a bit of the
persistence forecast.


.LONG TERM...Wednesday through Monday...The mid-range and long
term forecast is complicated as there is uncertainty in the
overall pattern shift. The previously mentioned low in the
northeast Pacific will slowly drop down to western Washington on
Wednesday. This shift will create a more persistent onshore
flow through the day with wrap around drier air aloft. AT that
point there will be less of a monsoonal moisture influence so
chances for thunderstorms have decreased. Cannot rule them out
completely though as there will still be instability through all
layers of the atmosphere. On Thursday through Friday the low
will advect northward over Vancouver Island and a second low
aloft will form over the inner mountain West. The combination of
these two features will promote weak ridging over the area.
Will see a brief drying and warming shift on Thu/Fri though not
expected to be impactful.

Over the weekend another round of instability is expected as a
strengthening low aloft moves over the Pacific Northwest. At
this point it is unclear of exactly the impact, but there will
likely be wrap around monsoonal moisture once again on Sunday
and a vorticity maxima. The jet stream is projecting a jet
streak moving directly overhead on Saturday afternoon with
stronger southwesterly flow. This type of scenario could
produce another round of thunderstorms. Based on the ensemble
models there is around a 30-50% chance of accumulating rain from
Corvallis north, and around a 15-25% chance within the southern
Willamette Valley and central Oregon Cascades. This trend is
fairly consistent between the GFS and the ECMWF AIFS.

After Saturday the pattern is incredibly messy and chaotic so
confidence remains extremely low in the overall sensible
weather. -Muessle


&&

.AVIATION...Inland terminals will see VFR conditions and high
clouds through the TAF period. Along the coast, LIFR/IFR
conditions continue as the coastal waters remain under dense fog
this morning. There is low confidence that coastal terminals will
improve to MVFR/VFR by the afternoon. Probabilistic models suggest
a 20-30% chance for MVFR conditions at KAST and a 50-70% chance
for LIFR/IFR conditions at KONP between 19-22Z Tuesday (best time
frame for conditions to improve). As a result, expect LIFR/IFR
conditions to persist through most of the TAF period for coastal
terminals, with potential improvement of conditions around 19-22Z
Tuesday. Otherwise, expect northwesterly to westerly diurnally-
driven winds around 5 to 10 kt through the period for all
terminals, with gusts up to 20 kt possible for coastal terminals.

While the bulk of potential isolated thunderstorms will stay in
the Cascades and Cascade foothills, there is low confidence that
easterly flow aloft could cause a thunderstorm to stray towards
the I-5 corridor after 11-13Z Tuesday.

PDX AND APPROACHES...VFR conditions and high clouds through the
period. Low confidence for a stray shower/thunderstorm from the
Cascades to wander into the vicinity of the terminal after 11Z
Tuesday and through the rest of the TAF period.-Hall

&&

.MARINE...A continued typical summertime pattern is expected
through midweek as surface high pressure remains offshore over
the northeastern Pacific. This feature will help to maintain
northerly winds through late Wednesday, with gusts increasing
above 25 kt, especially beyond 30 NM in the outer waters south of
Cape Falcon. A mid-period northwest swell at around 5 ft at 8
seconds will contribute to total seas of 6-8 ft, which, at such
short periods, will be steep enough to pose a hazard to small
craft, also especially beyond 30 NM. As such, a Small Craft
Advisory remains in effect through 11 AM today across the outer
waters south of Cape Falcon. In addition, stratus atop the marine
boundary layer will remain very close to the surface, and periods
of dense fog are expected to reduce visibilities to 1 NM or less
across the waters. A Dense Fog Advisory remains in effect through
11 AM Tuesday, when diurnal mixing will deepen the boundary layer
sufficiently to improve surface visibility.

Late today through Friday, seas look to continue diminishing as
both winds and the northwest swell ease. An upper-level and weak
surface low moving over the northeastern Pacific by the end of the
workweek will see low-level flow turn out of the west on Thursday
and the south to southwest by Friday. -Picard

&&

.FIRE WEATHER...Red Flag Warnings for thunderstorms and high
instability remain in effect 9AM through 9 PM Tuesday across the
Willamette National Forest and Linn/Lane County Cascade
foothills. Increased monsoonal moisture and instability is
contributing to a 15-25% chance for thunderstorms across fire
weather zones 689, and 690, mainly in the afternoon. Morning
showers have been forming throughout nearly all central district
fire weather zones with around a 50% chance of lightning
projected within the storms. The afternoon thunderstorms that do
develop could produce outflow wind gusts up to 40 mph, which
may travel outward up to 25 miles from thunderstorm cores. High
instability may result in fire growth and pyrocumulus (PyroCB)
forming after the overnight inversion break around 0900-1000.
The overall environment will see minimal change to that of
Monday so similar chances for rapid fire growth on any new fires
that start. Unstable conditions continue into the middle of the
week. -Schuldt/Alviz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Red Flag Warning until 9 PM PDT this evening for ORZ689-690.

WA...None.
PZ...Dense Fog Advisory until 11 AM PDT this morning for PZZ251>253-
     271>273.

     Small Craft Advisory until 11 AM PDT this morning for PZZ272-
     273.
&&

$$

www.weather.gov/portland

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