Arch Cape, Oregon Weather Forecast Discussion

918
FXUS66 KPQR 212125
AFDPQR
Area Forecast Discussion
National Weather Service Portland OR
225 PM PDT Sun Sep 21 2025
.SYNOPSIS...Drying and warming expected through midweek with
unseasonably warm temperatures Tuesday and Wednesday as high
pressure builds nearby. Rainfall chances increase late into the
weekend as troughing moves over the northeastern Pacific, but
the long-term forecast remains low confidence at this time.
&&
.SHORT TERM...Now through Tuesday...A fast moving cold front
brought decent rain to portions of southwest Washington and
northwest Oregon earlier this morning. Rainfall has varied
widely with the highest amounts around 0.5-0.75 inches around
the Long Beach peninsula and around Astoria. Inland locations
generally saw 0.2-0.4 inches, closer to 0.10 inches for the
Southern Willamette Valley and the central Oregon Cascades.
Cloud cover and the return of onshore flow will keep
temperatures in the low 70s inland and in the 60s along the
coast and Cascades.
The trough that brought the cold front through the area will
quickly exit the region this afternoon with 500 mb heights
rebounding late this evening. Highs Monday afternoon will be
around average with inland locations reaching the mid to upper
70s while the coast and Cascades remain in the 60s. A weak
thermal trough is expected to develop in the morning, leading to
a period of light offshore flow across the western slope of the
Cascades.
The center of the ridge at 500 mb will shift inland on Tuesday,
centering over Oregon with lows positioned over the Rockies,
another off the southern California coast and a third over the
northeast Pacific. This will set the stage for temperatures to
jump into the mid to upper 80s inland along with strengthening
the thermal trough. Easterly downslope winds west of the Cascade
crest are expected to be strongest early Tuesday morning with
sustained winds to 15 kt and gusts to 25 kt. Through the
Columbia River Gorge, winds will be stronger as the pressure
gradient between The Dalles and Troutdale increase to -4 to -5
mb, leading to sustained winds to 25 kt and gusts to 40 kt.
Models continue to trend slightly upward with the winds on
Tuesday. Winds are expected to weaken during the afternoon while
relative humidities are at their minimum. RH values are
expected to drop below 25% for some areas of the Cascades and
Cascade Foothills. Luckily, the strongest wind gusts are not
expected to coincide with the low RHs through the NBM does
indicate a 20-30% chance for overlap Tuesday evening with RH
values below 25% and sustained winds of 15 kt. This will
continue to be monitored and evaluated. -Batz
.LONG TERM...Tuesday Night through Saturday...Ensembles are in
fairly good agreement in the ridge remaining within the general
area through Thursday morning then becoming more zonal. As the
end of this week approaches, a broad, nearly flat trough is
expected to move into the Pac NW. The low associated with this
trough looks to be the next synoptic system and depending on how
it tracks will dictate is we get some precipitation or remain
dry. Models in general are split with around half showing a dry
solution, while others are leaning into a moderate PoP/low QPF
solution. Also, the Climate Prediction Center (CPC) is leaning
into a warmer and wetter solution for days 6-10. Overall, the
forecast confidence is very low given the wide solutions shown
within the models, their families and other forecast tools. /42
&&
.AVIATION...A few lingering showers persist across portions of the
Cascades early this afternoon, with skies otherwise clearing as
behind a departing cold front. Flying conditions have improved to
VFR at terminals across the region, with additional clearing
expected through this evening and tonight. Southwest to northwest
winds around 5-10 kt will ease to less than 5 kt by 06z Mon. With
clearing skies, light winds, and a moist surface following recent
rain, there remains a 20-30% chance in low stratus or patchy fog
yielding restricted cigs or vis, most likely from 12-15z Mon.
KPDX AND APPROACHES...Rain showers have exited eastward while
shallow post-frontal instability continues to support sct-bkn cu
at 3-4 kft. Building high pressure will see skies trend clearer
through this afternoon. Diurnal northwest winds around 5 kt will
ease overnight. There are low chances, around 20%, of areas of fog
or low stratus yielding MVFR conditions late tonight, most likely
from 12-15z Mon. -Picard
&&
.MARINE...Surface high pressure has begun to rapidly build over
the Northeastern Pacific behind the departing cold front, bringing
generally northerly winds across the waters. Thermal troughing
along the coast will yield a tightening pressure gradient and high
confidence in wind gusts reaching 20-25 kt from Tuesday through
Wednesday; conditions hazardous to small craft are very likely.
Seas of 6-8 ft at 10-12 seconds will build to 7-9 ft midweek as
winds increase, before easing into the weekend as high pressure
and associated winds weaken. -Picard
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&
$$
www.weather.gov/portland
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