394
FXUS66 KPQR 042202
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
302 PM PDT Thu Sep 4 2025

.SYNOPSIS...High pressure inland will continue to drive hot
weather across much of the region through Friday, with
widespread Moderate HeatRisk inland. Low chances of afternoon
and evening Cascade thunderstorms continue today, becoming more
widespread Friday and Saturday. A shift in the pattern will
then bring cooler and wetter weather over the weekend and into
the middle of next week.

&&

.SHORT TERM...Now through Saturday Night...A compact and
weakening upper low located just offshore of Washington per
latest water vapor satellite imagery will continue to slowly
move northward while broad, upper ridging over eastern OR/WA and
the Great Basin continues to break down. The break down of the
ridge as well as the departing low, is able to provide
instability for higher elevations. As a result CAMs are showing
showers and thunderstorms developing along the Cascades through
this afternoon and into the evening. Currently, satellite is
showing convective clouds already developing along the crest of
the Cascades. Also, the Medford Radar does have a few cells
developing around the Douglass/Klamath County border and moving
northward. Mid-level winds remain out of the south/southeast,
so storms developing over the higher terrain could track toward
the foothills or Willamette Valley locales. Overall, storm
tracks are expected to follow a similar pattern when compared to
what was observed yesterday (Thursday afternoon 9/3/2025).

Minimum surface humidity looks to remain slightly above
critical values, generally 28-35% for the Cascades with inland
locations in the 40-50% range. So, fire weather concerns do not
rise to the level of Red Flag Warnings, however isolated dry
thunderstorms, particularly among receptive fuels in the
Cascades, do pose some risk for new fire starts.

Overnight cooling to lows in the mid 50s to mid 60s tonight
will see adequate relief from the heat and only Minor to
Moderate HeatRisk for most. The Heat Advisory for the Columbia
Gorge Hood River Valley therefore remains in effect only through
8 PM this evening.

Friday and Saturday, temperatures continue to trend slightly
cooler as the aforementioned ridge weakens while at the same
time, an upper level shortwave trough moves inland over the
OR/CA border and swings northward towards Washington. This
feature looks to provide more robust forcing for convection,
with CAMs suggesting higher coverage of potential thunderstorms
over our CWA. Models currently are showing a noticeable
increase in CAPE for Friday and Saturday afternoons with values
around 1500-1700 J/kg with the NAM being the very typical
outlier with 2000-3000 J/kg of CAPE. Mean PWAT Values for both
Friday and Saturday are around 1-2" which would reduce the dry
lightning threat. However, it should be noted that for areas
east of the Cascade Crest, PoPs are relatively low (25% or
less). Therefore, while showers are in the forecast, the
likelihood of precipitation reaching the ground is minimal. /42

.LONG TERM...Sunday through Thursday...As the weekend comes to a
close and next week approaches, cooler and wetter weather are
in the forecast. A closed upper low will develop just offshore
over the northeastern Pacific through the weekend before moving
inland early next week. This will result in a rapid shift to
cooler and wetter conditions. Daytime highs in the upper 60s to
upper 70s on Sunday will cool into the mid 60s to mid 70s by
Monday and through the middle of the upcoming week. Nearly all
long-range ensemble members now depict accumulating rainfall
across northwestern Oregon and southwestern Washington. NBM
12-hour precipitation total probabilities are showing a 30-60%
probability of 0.25 inches of precipitation each 12 hour block
Monday through Wednesday, with the higher probabilities south of
Salem, OR. So, to summarize, cooler temperatures are expected
for the forecast area for the majority of next week. Also,
showers are also expected with areas likely to see at least 0.25
inches of precipitation each day. /42

&&

.AVIATION...Low level northwesterly flow over the region under
southerly flow aloft will keep conditions generally in VFR
thresholds for inland TAF sites and IFR/LIFR thresholds at coastal
taf sites through 00z Saturday. While conditions are temporarily
VFR at KAST, expect conditions to deteriorate rapidly this
evening. Once conditions settle into IFR/LIFR thresholds along the
coast, there is a >80% chance of IFR/LIFR persisting at KONP and
KAST until at least 19-20z Friday and any break thereafter would
likely only be a few hours. Farther inland, there is a 20% chance
that low end MVFR to high end MVFR ceilings develop for 1-3 hours
centered around 15z at KPDX and KTTD.

Expect smoke to produce widespread reductions down to 1-3 miles of
visibility in the Cascades of eastern Lane County through 00z
Saturday. Northeasterly flow in the Columbia River Basin on Friday
will bring smoke from eastern Washington into the Columbia River
Gorge and Cascades stretching from Mt St Helens southward through
Mt Jefferson. It is unclear if visibilities will drop below 6SM at
this time in that region, but suspect if they do, it won`t be by
much.


PDX AND APPROACHES...Low level northwesterly flow under southerly
flow aloft will produce predominantly VFR conditions through 00z
Saturday. There is a 20% chance that low end MVFR to high end
MVFR ceilings develop for 1-3 hours centered around 15z at KPDX
and KTTD. There is also a 10% chance for an isolated shower or
thunderstorm to impact the terminal towards 00z Saturday.

&&

.MARINE...Surface high pressure over the northeast Pacific will produce
northerly winds through Friday and areas of fog across at least
the inner waters through tonight. A switch to southerly winds
will occur late Friday night into Saturday with building southerly
winds Sunday into Monday. There is a 25% chance that southerly
wind gusts up to 25 kt will develop across the waters early next
week. Seas will generally hold in the 3-5 ft range into early next
week. There is a 25% chance of seas climbing above 5 ft during
the middle next week with those probabilities climbing to ~50% by
the end of next week.


&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Heat Advisory until 8 PM PDT this evening for ORZ120>122.

WA...Heat Advisory until 8 PM PDT this evening for WAZ209-210.

PZ...Dense Fog Advisory until 3 AM PDT Friday for PZZ210-251>253.

&&

$$

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