983
FXUS66 KPQR 160924
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
223 AM PDT Mon Jun 16 2025

.SYNOPSIS...Onshore flow will maintain seasonable temperatures
through much of the coming week. Chances for light rain return
to the coast Monday night into Tuesday and inland Tuesday night
into Wednesday, with very little to no measurable rain expected
for most inland locations. Transitioning to a cool and showery
weather pattern Friday into Saturday as a closed upper level low
settles over the region and brings more substantial rain
amounts to the area.

&&

.DISCUSSION...Now through Sunday night...An onshore flow regime will
remain in place this week. This will result in the continuation pf
seasonable temperatures with highs mainly in the 70s inland and 60s
at the coast, except Friday through Sunday when highs in the 60s are
likely for inland areas and 50s for the coast.

Satellite and surface weather observations from early Monday morning
depicted mostly clear skies and calm winds over northwest OR and
southwest WA under high pressure. Over the northeast Pacific, water
vapor imagery showed two areas of enhanced moisture associated with
separate frontal systems. The first front is already approaching the
coastal waters and will move towards the coast Monday night/Tuesday
morning, bringing a 20-40% chance of light rain for coastal
locations. Inland areas will likely stay dry as the first front will
be decaying upon landfall. The second front arrives Tuesday
night/Wednesday morning, bringing chances for light rain for coastal
and inland locations. That said, PoPs are low from Salem to Eugene at
around 10%, suggesting these areas will most likely stay dry.

After these two systems high pressure will build in temporarily,
bringing dry and mild weather on Thursday with a mix of sun and
clouds. The weather pattern will begin to change on Friday as a cool
upper level low settles directly over WA/OR. This low will linger
over the region through at least Saturday. During that time,
widespread showers are expected across all of western WA and western
OR. Models and their ensembles continue to suggest abundant moisture
will accompany this low, while soundings show steepening mid-level
lapse rates as cooler air aloft develops. If there are enough sun
breaks on Friday and/or Saturday, there would likely be enough
surface-based instability to warrant stronger showers and isolated
thunderstorms capable of producing heavy rainfall. NBM 6-hour thunder
probabilities currently range between 10-15% Friday to Saturday
during the afternoon/evening hours. NBM 48-hr probabilities for 0.25"
of rain or more have increased to 60-90% across all of northwest OR
and southwest WA, with the highest probabilities in the mountains and
the lowest probabilities in the Willamette Valley. Probabilities for
at least 0.5" of rain range from 20-40%, except 40-60% in the
mountains. Assuming this much rain falls, it would be the most
significant rain amounts observed during the month thus far.

On Sunday, the exact location of the low pressure system becomes more
uncertain. Most ensemble guidance from the GEPS/GEFS/CMC suggests the
low will progress eastward into Idaho and Montana, resulting in cool
northwest flow aloft for western WA/OR and mainly dry conditions.
However, around 20% of the total ensemble space suggests the low will
remain stuck in place over eastern WA/OR, resulting in a more
northerly to northeasterly flow regime aloft and higher chances for
continued rain showers. -TK

&&

.AVIATION...VFR conditions currently along the coast with small
patches of marine stratus beginning to show up on satellite.
Expecting stratus to become more widespread along the coast after
10-12z with MVFR/IFR CIGs, mixing out by 18z or so. Northwest
winds will remain below 5 kt through the overnight period, and
increase to 5-10 kt out of the west to northwest by the afternoon.
Another chance for marine stratus after 3-6z Tuesday.

Inland, VFR conditions are likely to persist throughout the
period, although few-sct stratus at 2-3 kft is expected to build
westward from the Cascades after 10z. There remains a 15-25%
chance of coverage increasing enough to result in MVFR cigs, most
likely at TTD. Cloud bases will lift as mixing deepens by 20-21Z.
Winds below 5 kt overnight will increase out of the west to
northwest at 5-10 kt by Monday afternoon.

*KTTD ASOS is currently INOP except for altimeter and will be AMD
 NOT SKED.

PDX AND APPROACHES...Mainly VFR conditions expected. Sct stratus
coverage at 2-3 kft is again expected to encroach from the west
after 10-12Z Mon, with around a 10% chance of MVFR cigs if stratus
coverage is higher than anticipated. Skies will trend clearer and
cloud bases lift above 3-4 kft by Monday afternoon as mixing
increases. Northwest winds around 5 kt overnight increase to 5-10
kt by the afternoon. -Batz/Picard

&&

.MARINE...A weak disturbance is expected to cross the region on
today, supporting a wind shift from northwesterly flow to
southerly flow late this evening and through Tuesday. A westerly
swell is also expected to move into the waters late Monday,
increasing seas from around 3 feet to around 5 ft at 10 seconds.
Combined seas still look to remain 7 ft or less through next week.
Winds remain southerly through Tuesday, gradually turning back to
the west/northwest on Wednesday. -Batz

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

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

&&

$$

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