019
FXUS66 KPQR 230520 AAB
AFDPQR

Area Forecast Discussion...UPDATE
National Weather Service Portland OR
1020 PM PDT Tue Sep 22 2026

Updated Aviation discussion

.SYNOPSIS...Seasonably cool and dry weather continues though at
least Wednesday night, before a frontal system moves onshore
Thursday into Friday, bringing widespread chances for rain. Dry
weather returns Saturday, with increasing uncertainty in the
forecast Sunday into next week.

&&

.SHORT TERM...Now through Wednesday Night...Largely dry weather
and slightly below-normal temperatures are expected through
Wednesday night as upper-level troughing over the Gulf of
Alaska approaches the coast while reinforcing onshore flow over
the region. A typical overnight inland intrusion of low clouds
from the coast and areas of fog within sheltered valleys
followed by clearing through by the day will continue, with high
clouds otherwise increasing ahead of the approaching system on
Wednesday. A weak initial wave on the leading edge of the
trough will bring low chances, 10-20%, for light rain showers
Wednesday morning along the south Washington and far north
Oregon coasts. -36


.LONG TERM...Thursday through Tuesday...A fall-like frontal
system will affect the region Thursday into Friday as an upper
shortwave rounding the base of the trough acts to amplify the
pattern and supports the deepening of a surface low over the
Northeast Pacific. An initial period of steady rain will reach
the coast most likely early Thursday afternoon, before pushing
inland to the I-5 corridor by the mid-afternoon. Steady rain
will give way to a hit-or-miss distribution of rain showers late
Thursday evening into Friday morning before conditions tend
drier through Friday afternoon and evening. Temperatures will
fall to some 10 degrees below normal behind the frontal passage
on Friday, when locals on the coast may struggle to reach 60
degrees and inland areas the mid 60s in the afternoon.

Average forecast rainfall amounts from 5 PM Thursday through 5
PM Friday are generally 1-1.25 inches along the coast, in the
Coast Range and Willapa Hills, and in the Cascades, with 0.75-1
inches in the Willamette Valley and 0.5-0.75 inches east of the
Cascade crest in Hood River County. These ranges do not capture
the full uncertainty in potential rainfall amounts, however, as
there is a 10% chance of less than 0.3-0.4 inches as well as a
10% chance of 1.5-2.5 inches across portions of the region. Some
of this variance may be due to uncertainty in the position of
the surface low as it weakens and moves onshore; the majority of
mid-range ensembles take a track into the central Oregon coast,
but a minority instead show a track toward the Columbia River,
which could allow for a longer duration of moisture transport
and convergence overhead. Ensemble means support an IVT values
near the weak/moderate atmospheric river threshold, however
given the progressive overall nature of the system and the
largely dry antecedent conditions, this early-season AR is
unlikely to yield any flooding concerns. The uncertainty in low
locations also introduces the potential for locally strong winds
along the coast if the low center tracks nearby, specifically
15-25% chances for winds reaching 40 mph at some point on
Friday behind the frontal boundary. These winds would not
typically result in impacts to coastal communities, but the
early-season timing will see more leaves on trees, which could
allow some trees or branches to come down where the winds are
the strongest.

Cool air aloft behind the frontal boundary may also support
shallow convection on Friday, although the occurrence and
coverage of any thunderstorms may hinge on how quickly skies
clear Friday, with earlier or more widespread sun breaks
allowing for more instability to develop by peak heating in the
afternoon. Mean surface-based CAPE of around 100 J/kg is
expected at this point, but there is a 10% chance of CAPE as
high as 200-250 J/kg which could support more widespread thunder
chances. Given this uncertainty, thunderstorm chances are
currently assessed to be 15-25% across the region, peaking in
the afternoon on Friday, with storms capable of producing
locally heavy rainfall rates and gusty, erratic outflow winds.

Behind the departing system, there is good agreement of drier
and marginally warmer weather returning on Saturday as brief
shortwave ridging passes overhead. There continue to be low
chances for morning low temperatures of 36 degrees or cooler in
the Upper Hood River Valley on Saturday and Sunday mornings,
specifically around 5% near Odell and 25-35% near Parkdale.
Additional troughing which may move onshore Sunday into early
next week will maintain persistent low chances, 5-25%, for
light rain across the region. -36

&&

.AVIATION...Through 06Z Thursday...West to southwest flow aloft
will persist through Wednesday. Predominately VFR conditions
expected through this evening. Marine stratus likely returns to
the coast with chances for MVFR to IFR CIGs increasing to over
50% by 8-10z Wed. Probs for MVFR stratus inland late tonight into
tomorrow morning have decreased slightly, but will maintain a few
hours of MVFR CIGs at inland terminals in the TAFs between 12-18z
Wed. A decaying front will approach the coast early Wed morning,
which could bring some light rain or drizzle to KAST briefly
between 12-18z Wed. Otherwise, expect increasing mid to high
level clouds streaming across the airspace tonight into Wednesday.
Diurnally driven winds generally remain northwest around 5-10 kt
or less.

KPDX AND APPROACHES...VFR conditions expected to persist through
at least 12z Wed with scattered clouds. There is around a 30%
chance for MVFR stratus to return, mainly between 13-17z Wed.
Otherwise, expect VFR to persist with increasing mid to high level
clouds. Northwest winds expected around 3-6 kt through the TAF
period. /DH

&&

.MARINE...High pressure will maintain predominately northerly
winds across the waters through Thursday afternoon. An exception
would be Tuesday afternoon to Wednesday morning, when winds may
turn more westerly to southwesterly for the waters north of Cape
Falcon with the passage of a weak front. A relatively stronger
frontal system will then move over the coastal waters late
Thursday into Friday, bringing increasing northwesterly winds and
rain. Chances for small craft advisory level wind gusts of 21 kt
or stronger are around 60-80% with this system with the highest
probabilities over the northern waters to the north of the
Columbia River Bar. There is also a 15-25% chance for max wind
gusts of 34 kt or stronger, as some guidance suggests a stronger
area of low pressure is possible over the coastal waters with this
system. However, the most likely outcome is for a relatively
weaker low pressure system with peak gusts somewhere in the 20-25
kt range.

Seas will most likely range between 6 and 9 ft through Friday,
but are not expected to be particularly steep at any point in time
due to dominant wave periods of 10 to 13 seconds. However, there
is a 15-20% chance seas will peak at 10 ft or higher late Thursday
night into Friday if the aforementioned area of low pressure
comes in stronger than what most guidance suggests. Seas decrease
to around 5 to 6 ft at 9 to 10 seconds for the weekend. -03/23

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...None.
WA...None.
PZ...None.

&&

$$

www.weather.gov/portland

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