119
FXUS66 KPQR 020511 AAC
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1011 PM PDT Mon Sep 1 2025

Updated aviation discussion.

.SYNOPSIS...Today is ending with comfortable, seasonable
weather and some breaks of afternoon sunshine. High pressure
will build over the region by midweek, leading to a warming
trend with valley highs likely reaching the 90s Tuesday and
Wednesday, possibly Thursday. This will bring Moderate HeatRisk
inland, especially where warm nights limit relief. Scattered
showers and a few thunderstorms are possible Tuesday and
Wednesday, mainly near the Cascades, with dry and unstable
conditions contributing to fire weather concerns.

&&

.SHORT TERM...Now through Wednesday Night...Skies have turned
partly sunny across much of the area following another round of
morning marine clouds. Temperatures are topping out in the 70s
to low 80s for most inland valleys, with cooler 60s to 70s along
the coast and higher terrain. Pleasant conditions continue
through this evening with lows in the 50s to low 60s.

Changes arrive Tuesday as high pressure builds north and a
compact low pressure system lifts in northern California. This
setup will draw in warmer air and also provide just enough lift
and instability for isolated showers and thunderstorms, most
likely over the Lane and Linn County Cascades. Any storms could
push north or west with outflow winds and cloud debris, and
given how dry the air is near the surface, lightning strikes may
occur without much rainfall. This trend continues into
Wednesday, though moisture increases and may allow for a few
wetter storms. Temperatures will climb quickly under this
pattern, with highs reaching the low to mid 90s in the
Willamette Valley and Gorge Tuesday and Wednesday, and 70s to
80s elsewhere. Warm overnight lows in the 60s will provide
little relief, especially in the Gorge where a Heat Advisory is
in effect Tuesday afternoon through Wednesday evening.
Conditions should begin to ease Wednesday night as the ridge
weakens slightly.

.LONG TERM...Thursday through Sunday...By late week, forecast
models increasingly show a shift toward troughing over the
northeast Pacific. While the details are uncertain, this would
bring temperatures down closer to normal and gradually increase
chances for showers, especially along the coast and Cascades.
This period would be summarized as warm but trending wetter.
-Hall/Picard

&&

.AVIATION...Low pressure in the northeast Pacific continues to
shift in with stratus just off shore. VFR everywhere. MVFR along the
coast after 10Z but will lift to VFR through the morning. Hot again
on Tuesday will cause winds to generally be northerly.
Increased speeds in the southern Willamette Valley after 22Z Tue.

PDX AND APPROACHES...VFR with a 30% chance of MVFR CIGs between 14-
17Z Tue. No other concerns. -Muessle

&&

.MARINE...
Weak surface low pressure remains in place southwest
of Vancouver Island and will gradually drift westward through
the middle of the week. Headed into Tuesday and Wednesday, high
pressure will begin to strengthen over the waters will low
pressure develops inland. This will increase the surface
pressure gradient, increasing northerly winds during the
afternoon and evening hours Wednesday and Thursday, especially
across the outer waters and southward. Peak wind gusts during
the mid-week period have decreased with probabilities for small
craft conditions coming down from yesterday, to around 10-20%.
Seas hold below ~4 ft through Tuesday morning, with an increase
in wind waves building seas to 5-6 ft at 7-9 seconds late
Tuesday through the end of the week. -Batz/Schuldt

&&

.FIRE WEATHER...A low pressure system is expected to move north
from California into Oregon Tuesday into Wednesday, which will
pose a threat for thunderstorm development. Southeasterly flow
aloft ahead of the low, along with support for ascent and
increasing elevated instability, could favor convection along
the Cascades. Instability most likely be highest in the
Willamette National Forest in Lane and Linn Counties. These
areas will be the most covered in the remainder of these
discussion. Elevated instability with fast lapse rates with
storms and/or outflow clouds propagating downstream over the
remainder of the forecast area. Model soundings also show
abundant dry air beneath any elevated convection, which could
allow for thunderstorms with dry lightning through the afternoon
and evening on Tuesday and Wednesday. Based on current model
outputs, there is a higher probability that thunderstorms that
do form, will be dry with high instability. In these types of
scenarios, pyroconvective activity is expected on ongoing fires
and fires that do start have the potential to have rapid spread.

Even in the case that thunderstorms become wet (producing heavy
rainfall) thunderstorms, lightning strikes could reach well
outside of any narrow rain core. Minimum humidity will fall to
25-30% along the Cascades, and in the 30-35% range in the lower
elevations. Additionally, gusty thunderstorm outflow winds
could result in unanticipated erratic fire spread, while high
instability could also lead to the growth of fires already on
the landscape. Due to increasing confidence in the presence of
thunderstorms and convective potential, a Red Flag Warning has
been issued for the Lane and Linn County Cascades (FWZ689-690).
-Muessle/Picard

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Heat Advisory from noon Tuesday to 9 PM PDT Wednesday for
     ORZ120>122.
     Red Flag Warning from 2 PM to 9 PM PDT Tuesday for ORZ689-690.
WA...Heat Advisory from noon Tuesday to 9 PM PDT Wednesday for
     WAZ209-210.
PZ...None.
&&

$$

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