618
FXUS66 KPQR 122157
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
257 PM PDT Tue May 12 2026

.SYNOPSIS...The arrival of a weather system tonight facilitates a
notable pattern change towards cooler/slight wetter conditions.
This is kicked off by the potential (15-35%) for a period of
high-based thunderstorms this evening into early Wednesday
morning, mainly I-5 corridor and eastward. After rain decreases
on Wednesday, showers linger at times through the end of the
week, mainly over the Cascades and coast/coast range. Then
another low from the Gulf of Alaska increases precipitation
chances(30-60%) to start the weekend while maintaining near to
slightly below normal temperatures.

&&

.DISCUSSION...Today through Monday...Broad southerly flow is in
place this afternoon with mid to high cloud cover streaming
across the region. Still, the added clouds haven`t been robust
enough to suppress temperatures too much, and as of 1500 this
afternoon we`re running about 3-8 degrees warmer across the
Willamette Valley and Portland metro than we were at this time
yesterday. Enjoy the conditions this afternoon if you`re a fan
of warmer weather, we likely won`t see temperatures in the 70s
and 80s for the remainder of the week.

The feature responsible for the added high cloud cover and
above normal temperatures is an approaching negatively tilted
shortwave trough which we`ll have to watch closely this evening
and overnight as it takes aim at the Pacific Northwest. While
it`s mainly beneficial from a precipitation/moisture standpoint
once we get to Wednesday morning, this approaching feature will
result in a period of increasing high-based instability this
evening through a good chunk of tonight with a 15-35% chance of
thunderstorms of the nocturnal variety. To add more of a
challenge, any thunderstorms that do develop, will likely not be
surface based and can be tough for models to properly resolve.
The primary question of thunderstorm coverage and frequency
remains, but there has been some added clarity with the latest
CAMs (Convective Allowing Models) this morning. Out of the
available CAMs, the 12/18z NAMNEST and 12z UW WRF are the most
bullish, with storm development starting around 6-8pm in Lane
County and rapidly spreading north along the Willamette Valley
through SW WA and across Cascades by 9-11pm. This presents more
of a "worse case" scenario in regards to convection but the
NAMNEST at least should be taken with a grain of salt as it can
sometimes run "hot" when it comes to these set-ups; the UW-WRF
does add some credence to the scenario it`s depicting however.
Otherwise, the remainder of the high resolution ensemble space
keeps development a bit delayed (10pm-midnight), more sporadic
over the lower elevations, and further north/east largely
placing activity in around the I-5 corridor in the north
Willamette valley/SW Washington eastward into the Cascades. As
the night goes on, the instability fueling convection continues
to progress eastward eventually bringing and end to
thunderstorm chances by the sunrise Wednesday. Just chances for
plain old rain and showers follow.

Later Wednesday morning into the early afternoon, the axis of
the shortwave trough will shift east of the area leading to a
decreasing precipitation the remainder of the day. The added
cloud cover, westerly flow, and cooler airmass likely pushes
high temperatures down into upper 50s to mid 60s for much of
the area, almost 10-20 degrees lower than the day prior across
the inland valleys. Looking towards Thursday and Friday, a more
zonal flow pattern emerges keeping temperatures near to below
normal, and shower chances in place across the coast, coast
range/Willapa Hills, and Cascades. Then late Friday into early
Saturday most deterministic and ensemble guidance shows another
shortwave trough dropping down from the Gulf of Alaska into the
region. This likely increases chances for precipitation across
the area and nudges high temperatures even lower into mainly
the 50s to near 60 Saturday afternoon. For those dreaming of a
return to daytime temperatures back in the 60s and 70s hope is
not lost as there is moderate confidence in a ridge of high
pressure beginning to build back overhead by early next week.
-99/42

&&

.AVIATION...Mainly VFR conditions Tuesday across NW OR and SW WA
except for along the coast. MVFR marine stratus continues along
the coast south of KAST under southwesterly surface flow. There
could be a few hours of lifting to VFR at KONP or fluctuations
between MVFR and VFR ceilings between 22z Tue - 03z Wed. Across
inland areas, westerly winds are increasing to around 8-10 kts.
Winds will then ease and become more south to southwesterly after
06z Wed.

An upper level low pressure system approaches the region Tuesday
night with a surface front pushing ahead. The front will move
through the region between 06-15z Wed. Ahead of the front, there
is a slight chance (15-25%) of scattered showers and thunderstorms
03-09Z Wed for all inland TAF locations. The chance for these
showers or thunderstorms aren`t high enough at any one location to
add them to the TAFs at this time, but any strong shower or
thunderstorm could produce heavy rain, small hail, lightning, and
gusty and erratic winds. A more stratiform band of showers then
pushes inland along and behind the front, beginning along the
coast around 06-09z Wed then moving east and reaching inland
terminals by 09-12z Wed. This band will be mainly east of the
Cascades by 18z Wed with scattered showers continuing through the
afternoon. There`s a 60-80% chance of MVFR ceilings south of KSLE
and a 30-60% chance at KSLE and north between 09-14z Wed. Winds
increase along and behind the front around 09-12z along the coast
and 12-15z inland. Expect winds around 8-12 kts with gusts up to
20-25 kts at times.

KPDX AND APPROACHES...VFR conditions with high clouds into Tuesday
night. There is a slight chance (15-25%) of scattered showers and
thunderstorms from 03Z-09Z Wed. Then a more stratiform band of
rain will impact the terminal beginning around 10-14z Wed with a
30-45% chance of MVFR ceilings. Scattered showers continue behind
this with occasional chances of MVFR conditions in showers. Winds
becoming west to southwest up to 8-10 kt, then becoming lighter
and more variable after 02z Wed. Winds become southwest again and
increase to 8-12 kts with gusts up to 20 kts after 11-14z Wed. -03

&&

.MARINE...Relatively benign conditions expected through the week
with predominately westerly winds, mainly under 10 kts. An upper
level trough and associated front moves through the waters on
Wednesday, which bring a 40-55% chance of occasional small craft
gusts up to 25 kt across all waters. This will mainly be when the
front is passing between 4 AM - 12 PM Wednesday. Not expecting
gusts to last long enough over any specific area to issue a small
craft advisory at this time, though we will continue to monitor
conditions. Winds remain westerly but decrease once again
Wednesday night. Seas generally below 8 ft around 10-12 seconds
through the majority of week, but could see seas close to 10 ft by
Friday/Saturday. -42/03

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.
&&

$$

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