Cornelius, Oregon Weather Forecast Discussion

148
FXUS66 KPQR 232158
AFDPQR
Area Forecast Discussion
National Weather Service Portland OR
258 PM PDT Tue Sep 23 2025
.SYNOPSIS...
High pressure will maintain warm and dry conditions across the
Pacific Northwest through midweek. A thermal trough remains in
place, supporting offshore flow and continues low relative
humidity, particularly across the Cascades and interior valleys.
Conditions moderate Thursday as onshore flow returns, bringing
slightly cooler temperatures and higher humidity. Another
warming trend is expected over the weekend ahead of an
approaching frontal system, which looks increasingly likely to
bring rain to the region Sunday into Monday.
&&
.SHORT TERM...Now through Wednesday Night...Tranquil weather
continues across northwest Oregon and southwest Washington as a
strong upper-level ridge dominates the region. The ridge has
been amplified in part by surrounding upper-level troughs over
the Gulf of Alaska, the Rockies, and southern California. This
pattern has reinforced the warm, dry airmass in place across the
Pacific Northwest.
Offshore flow persists, with the thermal trough extending across
western Oregon. While pressure gradients supported stronger
winds earlier this morning and afternoon, they have largely
diminished as of 3 PM. The key feature through Wednesday will
instead be the very dry air. Afternoon relative humidity values
will remain low, around 30% or less in many areas east of the
Coast Range, mostly notably the southern Willamette Valley and
the Cascades. These dry conditions, combined with above- normal
temperatures, will continue through Wednesday.
Highs both today and Wednesday will generally reach the low to
mid 80s across interior valleys, while the coast holds in the
upper 60s to low 70s. Nights remain seasonable, with lows in the
upper 40s along the coast and low to mid 50s inland. Early
Thursday, the ridge begins to flatten and the thermal rough
weakens, allowing onshore flow to gradually reestablish itself.
While conditions remain dry, the introduction of cooler marine
air will lower temperatures a few degrees compared to midweek.
.LONG TERM...Friday through Monday...Zonal flow aloft will be
the dominant feature Friday into early Saturday, leaving
conditions relatively unchanged to close out the work week
Saturday will act as a transitions period as a longwave trough
approaches the coast, beginning to shift temperature slightly
cooler. Ensemble spread highlights the uncertainty in exact
values, but generally supports highs in the upper 60s to upper
70s across higher terrain and valleys, with interior valleys
holding in the low to mid 80s. Attention then turns to a frontal
system associated with the deepening trough. Recent model runs
have trended toward slower timing, with each successive cycle
pushing precipitation onset later. The latest guidance suggests
light rain could begin at Astoria as early as Saturday, though
for most of the area, rain is not expected until Sunday, with
the bulk of precipitation arriving Monday. Probabilities for
measurable rain range from 40-50% across much of northwest
Oregon and southwest Washington, with chances somewhat lower
farther south. Associated chances of wetting rain (0.25 in or
greater) run around 20-40%, with the lowest likelihood in the
southern Willamette Valley and Lane/Linn County foothills and
Cascades. Confidence in the timing has increased somewhat but
is still best described as low to moderate, with ensemble
clusters converging on the Sunday-Monday timeframe as we move
further into the week.
Overall, this system looks to bring a noticeable cool-down, with
highs returning to near or slightly below seasonal averages by
early next week. Moisture associated with the front will also
provide a welcome break from the extended stretch of dry
weather.
~Hall
&&
.AVIATION...High pressure will support widespread VFR conditions
into tonight. Easterly flow through the Columbia Gorge and east-
west oriented Cascade gaps/valleys will see gusty winds at KTTD,
while northerly to northeasterly flow is expected elsewhere. Along
the coast, winds out of the north-northwest at 10-15 kt may gust
to 20-25 kt through 03-04z Wed before easing this evening, and
restricted vis/cigs are possible (40-60% chance of MVFR, 25-35%
chance of IFR) as low stratus or mist develops after 12z Wed
before improving around 18z Wed. Diurnal winds increase Wednesday
afternoon to 8-12 kt inland and 15-20 kt along the coast.
KPDX AND APPROACHES...VFR conditions with gusty easterly flow
extending westward from the Columbia Gorge to KTTD. At KPDX,
surface winds have turned easterly at 8-12 kt beneath flow of
30-35 kt at 1500 ft elevation, which could result in residual low
turbulence until winds off the deck weaken through this evening.
Light northwesterly winds below 5 kt returns tonight. -Picard
&&
.MARINE...Surface high pressure offshore will continue to yield
northerly flow into the weekend. A tightening coastal pressure
gradient will see winds increase to 15-25 kt across the outer
waters tonight, and across the inner waters by Wednesday
afternoon, while seas of 6-9 ft build to 7-10 ft as winds
strengthen. Small Craft Advisories therefore remain in effect
beyond 10 NM, and will go into effect at 11 AM PDT Wednesday
closer to shore, continuing through early Thursday morning.
Conditions will ease Thursday and Friday before building again
ahead of an approaching frontal system this weekend. -Picard
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory from 11 AM Wednesday to 5 AM PDT Thursday
     for PZZ251>253.
Small Craft Advisory until 5 AM PDT Thursday for PZZ271>273.
&&
$$
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NWS PQR Office Area Forecast Discussion