475
FXUS66 KPQR 312129
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
229 PM PDT Thu Jul 31 2025

.SYNOPSIS...Near average temperatures expected through Saturday,
turning slightly cooler Sunday into early next week. Chances for
showers and thunderstorms continue along the Cascades through this
evening. Mostly sunny conditions for Friday and Saturday, followed by
increasing clouds on Sunday with chances for showers and
thunderstorms over the Cascades late Sunday. General troughing and
onshore flow moderate temperatures Monday through Wednesday with
increasing chances for more widespread rain late Wednesday into next
Thursday, however uncertainty is high that far out in time.

&&

.SHORT TERM...Thursday afternoon through Sunday night...A low
pressure system continues to spin in the Gulf of Alaska, bringing
moist southerly flow aloft over the region. A 500 mb shortwave trough
embedded within this belt of southerly flow is currently moving
slowly northeastward over southwest and central OR while a closed
surface low resides over south-central WA. The aforementioned
shortwave trough is aiding forcing for ascent for showers and
thunderstorms where moisture and instability is maximized, which is
currently along the Oregon Cascades to the south of Santiam Pass.
Radar and lightning observations from 2 PM PDT Thursday showed a
cluster of nearly stationary thunderstorms occurring along a line
from Willamette Pass to Crater Lake National Park to Upper Klamath
Lake. Based on current trends and observations, suspect an isolated
thunderstorm or two will develop somewhere between Willamette Pass
and Santiam Pass later this afternoon before storms diminish with the
loss of daytime heating. The main hazards with these thunderstorms
will be cloud-to-ground lightning and locally heavy rain.

Skies become sunnier on Friday with temperatures a few degrees
warmer. Expect high temps between 84-89 degrees for the Willamette
Valley and Portland/Vancouver metro, and 80-85 degrees over the
Cowlitz Valley, Columbia River Gorge and Upper Hood River Valley.
Residual moisture and instability over the Lane County Cascades with
southerly flow aloft will bring a 15% chance of thunderstorms near
the crest Friday afternoon, with most of the convection staying east
of the crest.

Onshore flow increases on Saturday as southerly flow aloft
transitions to dry southwesterly flow. Expect near average
temperatures along with another mostly sunny day, aside from morning
clouds along the coast and lower Columbia. Cloud cover will increase
on Sunday as weak shortwave troughing approaches the
region. Temperatures are expected to be a few degrees cooler on
Sunday due to the increase in cloud cover and cooler temperatures
aloft; most locations will see high temperatures in the 70s, except
60s at the coast and near 80 degrees over the Willamette Valley. This
trough will also bring increasing chances (20-40%) for showers over
the Cascades and foothills late Sunday afternoon through Sunday
night. Cannot completely rule out isolated thunderstorms as well,
depending on the timing of this trough and the degree of instability.
NBM thunder probabilities currently range between 10-20%. -TK


.LONG TERM...Monday through next Thursday...The long term forecast is
highlighted by onshore flow, near average to slightly below average
temperatures for this time of year, and a mix of sun and clouds each
day with clouds being most prevalent during the morning hours. This
mild weather pattern is in response to a series of two or three
shortwave troughs that move over the area during the early to middle
part of next week, each one helping to deepen the marine layer. There
is also a 15-30% chance of light rain or drizzle with low marine
stratus along/near the coast Tuesday and Wednesday, mainly to the
north of Lincoln City. Inland areas are more likely to stay dry.

Chances for more widespread rain increase late Wednesday night into
next Thursday, however this is also when model spread increase
significantly and the forecast becomes much more uncertain. As of
right now, it appears two scenarios are possible heading towards
August 7-8. The first scenario would be continued upper level
troughing over the area, which would support cooler temperatures and
potential widespread rain. The second scenario would be the gradual
development of upper level ridging, bringing dry weather with warmer
temperatures in the mid 80s to low 90s. As of right now, the WPC`s
cluster analysis suggests there is around a 40-45% chance scenario
one verifies with troughing, and a 55-60% chance scenario two
verifies with ridging. If the first scenario verifies, widespread
wetting rains will be possible. The NBM is showing a 10-30% chance
for at least 0.25" of rain August 7-8, except around 5% across Lane
County. Hopefully the first scenario comes to fruition, as rain
amounts of this magnitude would help temporarily dampen fire season.
-TK

&&

.AVIATION...High confidence (>80% chance) that LIFR/IFR stratus
will prevail along the coast tonight into Friday morning. Expect
VFR conditions through the TAF period across the Willamette
Valley with SCT/BKN high clouds overnight. Marine stratus should
break out around 18-21z Fri with daytime heating, improving
conditions along the coast to VFR. Sunny skies inland.
Northwesterly winds around 5-10 kt across the region through the
TAF period.

PDX AND APPROACHES...VFR with high clouds. Northwesterly winds
less than 10 kt. -Alviz

&&

.MARINE...High confidence for dense fog returning tonight which
could reduce visibility to 1 NM or less; therefore, a Dense Fog
Advisory has been issued across all waters through noon Friday.
Otherwise, minimal changes to the overall weather pattern through
the remainder of the week. High pressure across the waters will
maintain north/northwest winds through early next week with gusts
up to 20 kt. Seas remain around 2-4 feet at 6-7 seconds.

Guidance is hinting at a pattern change Tuesday to Wednesday next
week as a trough approaches British Columbia and the Pacific
Northwest. This system would return southerly winds across the
waters, however, chances for small craft winds during that time
are currently around 20-30%. -Alviz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...Dense Fog Advisory from 11 PM this evening to noon PDT Friday
     for PZZ210-251>253-271>273.
&&

$$

www.weather.gov/portland

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