054
FXUS66 KPQR 010445 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
945 PM PDT Sun Aug 31 2025

Updated aviation discussion.

.SYNOPSIS...Continued seasonable temperatures through the
holiday with morning incursion of marine clouds giving way to
breaks of afternoon sun. Building high pressure will yield a
warming trend with the potential for high temperatures reaching
the 90s and Moderate HeatRisk Tuesday through Thursday across
inland areas. Increasing chances of scattered showers and
isolated thunderstorms Tuesday and Wednesday, with drying and
unstable conditions leading to elevated fire weather concerns.

&&

.DISCUSSION...Sunday Afternoon through Saturday...The upper
level low pressure system that has been situated off of the
Oregon and Washington coast this weekend has moved slightly to
the north, with water vapor satellite imagery showing the center
just southwest of Vancouver Island. This has allowed for mostly
clear skies this afternoon with the exception of marine stratus
still lingering in SW Washington. Marine stratus is expected to
build along the coast again tonight with another push inland in
the early morning hours tomorrow before dissipating by late
morning. Temperatures today and tomorrow remain on the more
seasonable side with low to mid 80s for inland valleys and 60s
along the coast. However, wherever any stratus intrusion lingers
into the late morning or afternoon hours, temperatures may end
up lower than the forecast.

The main concern for this week is shifting from heat to
thunderstorms. Most ensemble members are now coming into
consensus that a compact low pressure system will move inland in
central California sometime on late Monday into Tuesday and then
move north towards Oregon. Uncertainty remains in the exact
timing of the low with some guidance indicating it will move
through Oregon into Washington by late Wednesday and others
indicating it will slowly move through northern California
through Wednesday and move through Oregon on Thursday. Either
way, the potential for thunderstorms over the mountains is
increasing for Tuesday into Wednesday, it`s just a matter of
exactly where and when the thunderstorms may happen. Latest
thinking is a 15-20% chance of thunderstorms possible over the
central Oregon Cascades on Tuesday and Wednesday afternoon and
evenings. However, depending on timing and trajectory of the
low, some guidance is hinting at the potential for nocturnal
convection spreading from the Cascades west to the Coast Range
and even over the Willamette Valley, either on Tuesday
night/Wednesday morning or Wednesday night/Thursday morning.
Confidence is extremely low in this solution and as such it is
not included in the forecast package, but it still bears
mentioning.

As this compact low pushes inland, the low pressure just
offshore will retrograde into the Pacific, and high pressure
currently settled over the Great Basin will be able to move
slightly west over western Oregon. This will bring drier air
back into central and western Oregon and Washington with
humidity values tanking quickly. This dry air plus the unstable
conditions from the compact low will lead to elevated to
potentially critical fire weather conditions, especially when
adding possibility of lightning to the mix. More information
about this is detailed in the Fire Weather discussion below.

Additionally, the high pressure spreading west will still allow
temperatures to rise to above normal for Tuesday into midweek.
Confidence is high in daytime temperatures peaking in the upper
80s to low 90s on Tuesday. Beyond Tuesday, significant
uncertainty remains in exactly how warm temperatures could be
due to the incoming compact low. However, each NBM run lowers
the high end of the forecast spectrum for temperatures. The 13z
NBM now only has a 40-60% chance of temperatures even reaching
90 degrees for inland valleys on Wednesday and Thursday, higher
chances south of Portland. Probabilities for reaching 95 degrees
are now only 10-25% Tuesday through Thursday. In fact, Tuesday
could now end up being the hottest day. Major HeatRisk has
mostly vanished from the forecast, only remaining in the
Columbia River Gorge and pockets of the Cascade foothills, where
overnight temperatures could remain on the warmer side in the
upper 60s to around 70. Widespread Moderate HeatRisk still
exists Tuesday through Thursday, which means those who are
vulnerable to heat could still be at risk for heat-related
illness and should still use caution.

For Friday into next weekend, expect temperatures to cool again
as troughing develops in the eastern Pacific. Ensemble guidance
is pretty messy in the details of the trough, but general
pattern indicates potential for cooler temperatures and
increasing moisture with a slight chance for showers, mainly
limited to the mountains at this time. -HEC

&&

.AVIATION...A marine stratus deck was beginning to fill back in
along the central OR coast late Sunday evening while remaining in
place over the north OR coast. This stratus deck will impact KAST
and KONP with MVFR cigs or lower tonight through 20-21z Tuesday
before low clouds scatter out and briefly lift to VFR before
trending down again towards 03-06z Tuesday. Confidence is lower
for inland terminals in regards to low cloud development. Current
expectations are for MVFR cigs fill in across the lower Columbia
tonight, most likely making it to KPDX and KTTD towards 14-15z
Tuesday, similar to what was observed Sunday morning. These two
terminals have the highest chance for MVFR cigs, with a 60%
chance at KPDX and an 80% chance at KTTD. Meanwhile, KHIO, KUAO,
KSLE and KEUG have a 40-50% chance. Any low clouds that do form
over inland valleys Monday morning should be short-lived, most
likely scattering out between 17-19z. Winds will generally remain
light at around 5 kt or less through early Monday afternoon.

PDX AND APPROACHES...VFR conditions will most likely give way to a
brief period of MVFR cigs towards 15z Monday (60% chance). If MVFR
cigs do materialize, except low clouds to scatter out and lift
back to VFR towards 18z Monday, similar to what was observed
Sunday morning. Light northwest winds around 5 kt continue. -TK

&&

.MARINE...Overall, fairly benign late summer-time weather
conditions continue through next week. Expect a weak area of low
pressure just off the coast to help maintain rather light winds
across the coastal waters through Monday and Monday night. From
there, confidence is high a ridge of surface high pressure builds
over the waters Tuesday and Wednesday increasing northerly winds.
Winds will be strongest in the afternoons and evenings Tuesday
through Thursday, especially to the south where the surface
pressure gradient will be the strongest. Peak wind gusts during
this early to mid week period get close to small craft conditions
(>21 knots gusts), mainly across the outer waters. Guidance keeps
the probability to meet small craft criteria around 20-35% during
the afternoon and evenings hours, Tuesday through Thursday. Seas
hold below 4 ft through Monday, with an increase in wind waves
building seas to 5-6 ft Tuesday through Thursday. -Schuldt

&&

.FIRE WEATHER...A compact low pressure system is expected to
move north from California into Oregon Tuesday into Wednesday,
which will pose a threat for thunderstorm development.

Anomalous southeasterly flow aloft ahead of the low,
along with synoptic support for ascent and increasing elevated
instability could favor convection along the Cascades, 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 abundant lightning and limited
rainfall through the afternoon and evening on Tuesday and
Wednesday. Even if thunderstorms do produce rain, lightning
strikes could reach well outside of any narrow rain cores. With
the relative humidity falling to afternoon minimums below
25-30% along the Cascades and receptive fuels remaining in
place, there is increased fire weather concerns. Additionally,
the unstable atmospheric conditions could lead to fire growth.
There is still significant uncertainty in the exact timing of the
low, and as such, there is uncertainty on whether thunderstorms
could continue into Wednesday or just impact the area on
Tuesday. A Fire Weather Watch has been issued only for Tuesday
from 2-8 PM PDT due to this uncertainty. -LP/HEC

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for ORZ689-690.

WA...None.

PZ...None.

&&

$$

www.weather.gov/portland

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