737
FXUS66 KPQR 312213
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
313 PM PDT Mon Aug 31 2026

.SYNOPSIS...After one final afternoon of summer-like conditions
today, an unusually strong frontal system by early September
standards takes aim at the Pacific Northwest for the middle of
the week. In juxtaposition to the last few weeks/months the
Wednesday into Thursday time-frame features ample widespread
rainfall, locally breezy/gusty winds, and a 15-30% chance for
pop-up thunderstorms (mainly Thursday). Time to clear those
rain gutters and dust off the ole` rain jacket. Conditions
trend slightly drier by Friday into the weekend but cooler than
normal temperatures and shower chances persist. Those eager for
fall-like weather rejoice!

&&

.SHORT TERM...Now through Tuesday Night...This afternoon an
upper-level ridge of high pressure which was overhead earlier in
the day is quickly shifting eastward as a weakening closed low
over the eastern Pacific dives towards the northern California
coastline. Deterministic and ensemble guidance are in good
agreement showing the offshore pressure gradient, which has
been driving our localized easterly winds today, weakening and
eventually reversing back onshore this evening. This should help
to push lingering wildfire smoke over portions of the
Willamette Valley back towards the Cascade crests, albeit slowly
at first.

Headed through Tuesday the aforementioned upper-level closed low
transitions into an open wave and eventually tracks into the
OR/CA border, increasing cloud cover and bringing a chance
(15-45%) for light shower activity. Both will help to suppress
daytime temperatures across region with highs on Tuesday only
reaching into the low to mid 70s for inland valleys, 60s for the
coast, and mid 50s to mid 60s for the Cascades. Winds also
begin to turn more southerly, generally remaining relatively
light with gusts below 15-20 mph. An exception will be the
Columbia River Gorge and Hood River Valley, where westerly gap
winds Tuesday afternoon appear prominent with gusts approaching
25-35 mph. -99

.LONG TERM...Wednesday through Sunday...It appears we`ll be
entering Meteorological Fall (Sept. 1st onward) with a bang as
a strong frontal system tees up the Pacific Northwest for
Wednesday. Deterministic models and their respective ensembles
continue to show a much deeper upper-level low pressure
dropping down out of the Gulf of Alaska and into the Pacific
Northwest Wednesday morning and afternoon ushering in
widespread moderate rainfall, cooler temperatures, and locally
breezier/gustier southerly winds. Following the frontal
passage, there`ll also be a 15-30% chance for thunderstorms. At
this point it`s almost a sure bet (85-95%+) rainfall amounts
exceed 1 inch from the coast to the crests of the Cascades
between 5 AM Wednesday to 5 AM Friday. Chances for exceeding 2
inches during this same timeframe have increased with the latest
run of the NBM and sit around 40-70% for the coast and our
higher terrain features - 5-20% chance along the I-5 corridor.
For many locations the amount of rainfall forecast over the
course of just a few days will get close to or exceed our
monthly climatological average for September which really
emphasizes the anomalous nature of this system. It`s much more
akin to a front you`d see in late October or November minus the
colder temperatures at the surface.

As mentioned above, southerly winds likely increase as the
frontal boundary moves onshore Wednesday allowing for a period
of breezy conditions, especially along the coast, although
exactly how strong wind may get is a point of uncertainty.
Models soundings from the NAM, GFS, ECMWF, etc. all point to a
low level jet feature just ahead and along the cold front with
850mb winds of 55-65 knots approximately 4,000-5,000ft off the
surface from the coast into the I-5 corridor. It`s just a matter
of whether there is a forcing mechanism to mix these winds
closer to the surface. Given the current model sounding profiles
which don`t appear readily conducive to to mix these high winds
to a lower elevation, it would take a modest convective element
right along the cold front to push these strong winds to the
surface - would give it a 10-20% chance at the moment.
Otherwise, winds should be largely pressure gradient/synoptic
driven and based on the EPS ensemble and NBM, confidence is
moderate for southerly wind gusts in the 35-50 mph range across
the coast, highest beaches/headlands, and gusts between 20-35
mph along I-5 corridor. While these wind gusts may not seem
particularly strong inland in the context of what we normally
experience annually, the early season nature of this event may
present exacerbated impacts. At least winds quickly decrease
behind the frontal boundary Wednesday evening.

Thursday through Saturday/Sunday the core of the upper-level
low remains over the Pacific Northwest with a convective post-
frontal environment lingering over the CWA. Given the quick
intrusion of cooler air aloft and decent daytime heating this
time of year increasing atmospheric instability is favored along
with showers and chances (15-30%) for thunderstorms focused
Thursday afternoon. Current vertical shear values in the post-
frontal airmass do not support storm organization (weaker pop-
up cell storm mode) however given the placement of the surface
low pressure Wednesday evening through Thursday an isolated
waterspout and/or cold air funnel cloud or two can`t be ruled
out with any stronger convection. Any passing thunderstorms may
produce lightning, heavy rain, erratic winds, and small hail.
Chances for showers likely linger Friday through the weekend as
the upper-level low weakens, transitions to a trough, then
stalls along the coastline. Temperatures remain below normal as
well. -99/42

&&

.AVIATION...Through 18z Tuesday...VFR conditions expected to
remain in place across the airspace into this evening at coastal
terminals, and into tonight inland. Largely clear skies will
persist through today, with winds rising to 5-10 kt inland and
to 8-12 kt along the coast. Weakening offshore flow early will
see east winds through the Columbia Gorge and north winds along
the Willamette Valley begin to shift out of the west to
southwest as onshore flow is re-established by this afternoon.

This evening and tonight, an onshore push will see 70-80%
chances of IFR cigs at coastal terminals after 02-04z Tue, and
60-80% chances of MVFR cigs at inland terminals after 10-12z
Tue, increasing overnight from south to north. Winds shift out
of the south to southwest this evening and overnight ahead of an
approaching frontal system over the Northeast Pacific.

KPDX AND APPROACHES...VFR conditions expected into tonight.
Northwest winds of 5-10 kt shift southerly this evening as a
marine push brings 70-80% chance of MVFR cigs after 12z Tue. -36

&&

.MARINE...Upper-level ridging continues to shift inland while
being replaced by an upper-level low over the Northeast Pacific.
Breezy north to northwest winds persist over the waters this
afternoon, with gusts to 25 kt most widespread within 10 NM of the
southern WA and far northern OR coasts including near the mouth of
the Columbia River. A Small Craft Advisory remains in effect in
these areas through 8 PM PDT until gusty winds ease this evening.
Meanwhile, largely wind-drived seas continue at 4-6 ft.

A potent frontal system looks to traverse the waters midweek, with
winds backing out of the south to southwest by early Tuesday
morning ahead of its arrival. Winds will then build through
Wednesday afternoon, reaching 30-35 kt. Gusts as high as 40-45 kt
are most likely from 8 AM through 2 PM PDT Wednesday across the
majority of the waters, and from 11 AM through 5 PM PDT Wednesday
within 10 NM of the coast. There is a 20-40% chance of isolated
storm-force gusts within 20 NM of the coast. Seas are expected to
reach 10-13 ft during this period, with only up to a 5-10% chance
that significant wave heights exceed 15 ft. Behind this system,
there are 10-20% chances for thunderstorms across the waters on
Thursday. Winds and seas will then steadily ease through the
remainder of the week and into the weekend. -36

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...Small Craft Advisory until 8 PM PDT this evening for PZZ210-
     251.
     Gale Watch from Tuesday evening through Wednesday evening for
     PZZ210-251>253-271>273.
&&

$$

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