Forest Grove, Oregon Weather Forecast Discussion

515
FXUS66 KPQR 220408 AAC
AFDPQR
Area Forecast Discussion...UPDATED
National Weather Service Portland OR
908 PM PDT Sun Sep 21 2025
Updated Aviation, Marine and Watches, Warnings and Advisories
.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...As the post frontal air-mass settles over the area,
the current VFR as of 04Z Monday, are expected to settle as high
pressure slowly develops over the region along with weak easterly
flow. This will result in development of MVFR to LIFR conditions
along the coast. MVFR/IFR conditions along the coast are expected
to develop through 08Z Monday. IFR/LIFR conditions likely to
develop around KAST around 10Z-14Z Monday with KONP maintaining
the current LIFR conditions. For inland locations, generally VFR
is expected. However, there is a 30-50% probability of MVFR
conditions and 10-20% probability of IFR or lower conditions
developing around 12Z-14Z Monday. Conditions expected to improve
towards VFR across the airspace, starting around 16Z-18Z Monday
with west to north winds returning. Gusts up to 20 kt also
expected along the coast from 18Z Mon through 02Z Tue.
KPDX AND APPROACHES...Post frontal air-mass will continue to
settle with high pressure slowly developing. Generally VFR, with
a 30-50% probability of MVFR conditions and 10-20% probability of
IFR or lower conditions developing around 12Z-14Z Monday.
Conditions expected to improve towards VFR starting around 16Z-18Z
Monday with northerly winds. /42
&&
.MARINE...Evening Update...Issued a Small Craft Advisory for all
waters and the Columbia River Bar through Monday evening.
Background swell looks to be running a touch higher than guidance
which is supported by observations from buoys 46029, 46050 and
46089. Will continue to monitor and adjust the forecast as new
observations and guidance is ingested. /42
Previous Discussion Follows...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 6-9 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...Small Craft Advisory until 8 PM PDT Monday for PZZ210-251>253-
     271>273.
&&
$$
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NWS PQR Office Area Forecast Discussion