005
FXUS66 KPQR 021100
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
400 AM PDT Tue Sep 2 2025

.SYNOPSIS...Inland high pressure building through midweek will
bring hot weather to the region with valley high temperatures
near 90 degrees, overnight lows in the 60s, and widespread
Moderate with some areas of Major HeatRisk. An approaching upper
low will support scattered showers and isolated thunderstorms in
the afternoon and evening today and Wednesday, mainly near the
Cascades. A pattern change will then favor cooler and possibly
wetter weather by the weekend.

&&

.SHORT TERM...Now through Thursday Night...The upper-level low
responsible for the relatively cool weather over the holiday
weekend has begun to retrograde westward into the Pacific, while
upper ridging is quickly building from the Great Basin into the
Interior Northwest. Meanwhile, another compact upper low
offshore of central California early this morning will move
inland over northern California this evening, and into western
Oregon early Wednesday morning. This approaching low will bring
chances for showers and thunderstorms, most likely to the
Cascades through the afternoon and evening today and tomorrow.

As mid-level temperatures overhead reach 23-25C, surface
temperatures will once again climb above normal across the
region. Highs this afternoon will most likely reach the upper
80s to low 90s across inland valleys, the mid 60s to mid 70s
along the coast, and the hottest temperatures in the mid to
upper 90s through the Columbia Gorge and along the Hood River
Valley. Additionally, overnight lows will remain relatively
warm, only falling to the mid to upper 60s in the central and
northern Willamette Valley and Cascade foothills, upper 60s to
low 70s through the Gorge to the Hood River area, and mid 50s to
low 60s elsewhere. As the aforementioned low approaches from
the south, slight cooling aloft as well as potentially increased
cloud cover may act to limit heating, resulting in a few
degrees of cooling by Wednesday and Thursday. If cloud cover
exceeds expectations, or wherever rain showers occur,
temperatures will more likely to fail to reach forecast highs.

The combination of hot daytime and warm overnight temperatures
will result in widespread Moderate HeatRisk inland, as well as
local areas of Major HeatRisk, most notably in the Columbia
Gorge and Hood River Valley, where a Heat Advisory remains in
effect from noon today through 9 PM Wednesday. Those without
access to cooling or who will be spending time or working
outside should take precautions to prevent heat illnesses.

The approaching upper low will additionally provide synoptic
support for ascent as well as increase elevated instability
initially to areas south of Salem today, spreading northward to
southwestern Washington by Wednesday. Conditions supportive of
sub-cloud evaporation may favor isolated dry thunderstorms
initially forming in the Cascades, while valley areas are less
likely to break capping. Southeast flow aloft could carry storms
and/or outflow clouds over the remainder of the forecast area,
contributing to the chance of cooler temperatures on Wednesday.
As the low weakens and lifts northward by Thursday, chances for
further thunderstorms will be limited. For more information on
dry lightning concerns, see the Fire Weather discussion. -Picard


.LONG TERM...Friday through Monday...By late week, long-range
ensembles continue to favor a pattern shift to upper-level
troughing over the Northeastern Pacific. Details remain
uncertain, but a broad upper low offshore will see temperatures
trending downward to below-normal values by early next week, as
well as favoring wetter weather. At this lead time, the highest
chances for rain showers look to be along the Cascades and the
coast this weekend into early next week. -Picard

&&

.AVIATION...Satellite imagery and surface observations show
marine stratus slowly developing along the coastline as of
10-11z this morning with an increasing likelihood for degraded
flight conditions at spots like KAST/KONP after 12-14z.
Fortunately any stratus/fog likely lifts to VFR by 18-21z. At
inland sites the decrease in westerly flow compared to yesterday
leaves the threat of MVFR CIGs due to pockets of sunrise
stratus a bit more dubious, but currently probabilities for MVFR
or lower CIGs only sit at 20-30% in the Portland/Vancouver
metro through the Willamette valley during the 13-17z timeframe.
Otherwise, high confidence in VFR conditions the rest of the
TAF period.

PDX AND APPROACHES...Generally VFR conditions with a 25-30%
chance of MVFR CIGs between 14-17Z this morning. Winds stay
fairly light out of the northwest. -Schuldt

&&

.MARINE...Fairly benign late-summer like conditions are expected
through the rest of the week into the weekend. A weak surface
low pressure off southwest Vancouver Island gradually
retrogrades westward the next several days as in conjunction
with high pressure strengthening across the coast waters. This
progression will tighten surface pressure gradients allowing for
increasing northerly winds during the afternoon and evening
hours, especially on Wednesday and Thursday. At least peak wind
gusts during the mid-week period likely sit in the 15-20 knots
range with probabilities for small craft conditions only around
10-20%. Seas around 3-4 ft this morning kick up to 4-6 ft this
afternoon due to an increase in wind waves and a fresh
northwesterly swell. Expect seas to hold in this 4-6 ft range
through Friday before backing off slightly over the weekend into
next week. -Schuldt

&&

.FIRE WEATHER...While high pressure builds inland, an area of
low pressure tracking northward from California into western
Oregon on Wednesday and western Washington by Thursday will pose
a threat for dry thunderstorms. Southeasterly flow aloft ahead
of the approaching low will support ascent and increase elevated
instability, while dry air remains near the surface with
relative humidity falls to minimum values of 20-30% in the
Cascades, and 30-40% in the Cascade foothills today. This
combination will favor showers and thunderstorms initiating
along the Cascades this afternoon and evening, before moving
northwestward toward foothill and valley areas. Cloud bases
around 10 kft will allow for evaporation in the sub-cloud layer,
which would yield instances of both dry lightning as well as
gusty outflow winds at the surface which could cause unexpected
erratic fire growth. Slower moving storms will be more likely
to see rain reach the ground, but lightning can still reach the
ground well away from any narrow rain shaft. Given dry and
receptive fuels, lightning could produce numerous new ignitions
today, with the highest concern in the Willamette NF, where a
Red Flag Warning remains in effect for Fire Weather Zones 689
and 690. High instability can also favor pyroconvective activity
at ongoing fires.

As the low continues to track northward into Wednesday,
confidence is somewhat lower in the continued threat for dry
lightning as the low weakens and humidities rise slightly. The
Red Flag Warning in the Willamette NF has been extended through
9 PM Wednesday as high instability is expected to continue
affecting fires already on the landscape in southeastern Lane
County despite the lower risk of additional thunderstorms. To
the north, a Fire Weather Watch has been issued for Fire Weather
Zones 634, 635, and 688 spanning the Mt. Hood and Gifford
Pinchot NFs from noon to 9 PM Wednesday. -Picard

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for ORZ688.

     Heat Advisory from noon today to 9 PM PDT Wednesday for
     ORZ120>122.

     Red Flag Warning from 2 PM this afternoon to 9 PM PDT
     Wednesday for ORZ689-690.

WA...Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for WAZ634-635.

     Heat Advisory from noon today to 9 PM PDT Wednesday for WAZ209-
     210.

PZ...None.

&&

$$

www.weather.gov/portland

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