578
FXUS66 KPQR 121027
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
327 AM PDT Tue May 12 2026

.SYNOPSIS...A warming trend persists into Tuesday as high
pressure re-builds overhead, albeit temporarily. Precipitation
chances then return this evening as a trough approaches the
West Coast. Can`t rule out a period of high-based thunderstorms
this evening into Wednesday morning (15-25% chance) as well.
After lingering showers decrease Thursday, another low from the
Gulf of Alaska maintains precipitation chances along with
temperatures trending slightly cooler for the latter part of
the week and into the weekend.

&&

.DISCUSSION...Today through Monday...Mostly sunny/clear
conditions today as an upper level ridge of high pressure
remains over the region. For most locations daytime, high
temperatures today will likely be the warmest of the upcoming
week, reaching into the low to upper 60s along the coast, low
70s to low 80s the Cascades, and upper 70s to mid 80s for the
inland valleys/lower Cascade valleys. The past few model runs
continue to relatively cool the area off today. Thus decreasing
the moderate Heat Risk threat across the Portland/Metro and I-5
corridor. The latest NBM has lowered the probabilities for
temperatures to exceed 90 degrees, giving the Portland metro
only a 5-10% change to do so while the rest of the inland
valleys are less than 5%. So, at least heat related impacts
should remain a bit more muted than previously anticipated as a
result.

As the evening approaches, a shortwave trough will take aim at
the Pac NW coast line. This approaching trough will result in
an increase in overall instability through late tonight/early
Wednesday morning. The pattern as a whole is resulting in a
15-25% chance of thunderstorms of the nocturnal variety, which
does result in a challenging forecast. To add more of a
challenge, any thunderstorms that do develop, will likely not
be surface based. The primary question for today is going to be
where are the thunderstorms likely to start? CAMs (Convective
Allowing Models) do show a favorable pool of CAPE (Convective
Available Potential Energy) along the Central and Southern
Oregon Cascades as well as the Southern Washington Cascades,
with values around 300-1800 J/kg. Now, it should be noted that
this spread does include the NAM Nest, which is known to run
rather "Hot" here in the Pac NW. The more conservative CAMs are
showing CAPE values in the 300-500 J/kg range, with most CAMs
also showing around 20-75 J/kg of CIN (Convective Inhibition).
Overall, conditions are favorable for thunderstorm development
starting late this afternoon through late tonight/early
Wednesday morning, with a likely starting location along the
Cascades, with a general storm motion of south to north.

Wednesday morning/early afternoon, the shortwave trough will be
east of the area which will usher in a period of more stratiform
rainfall. The added cloud cover and cooler airmass on Wednesday
afternoon pushes temperatures into upper 50s to mid 60s for
much of the area. Looking towards the latter part of the week, a
more zonal flow pattern looks to emerge, with perturbations
within the upper level flow. This looks to maintain a cooling
trend for the region through at least Friday. However, looking
towards the weekend and into the start of next week, most
ensemble members continue to show another trough dipping down
from the Gulf of Alaska by the end of the week around
Friday/Saturday. This will maintain chances for precipitation
across the area and bring lower 500 mb heights, resulting in
temperatures cooling down to or even below seasonal normals.
/42-99

&&

.AVIATION...High pressure will maintain predominately VFR under
mostly clear skies through around 00Z Wednesday, except for the
central Oregon coast. Southwest flow aloft increases as the upper
level ridge shifts east and an upper level trough over the
Pacific approaches the region. A southerly wind reversal along the
coast will push marine stratus into KONP around 12Z Tuesday as
chances for IFR conditions increase to around 80-90% and chances
for LIFR are around 60-70% between 12Z-16Z Tuesday. Stratus may
reach as far north as KAST with around a 15-25% chance for MVFR
conditions between 12Z-18Z Tuesday. There is also around a 5-10%
chance that stratus pushes into the southern Willamette Valley
after 12Z Tuesday. Light northwest winds turn southwesterly on
Tuesday.

There is a slight chance (15-25%) of showers and thunderstorms
after 03Z Wednesday for all inland TAF locations. Thunderstorm
chances decrease after 09Z Wednesday, but the chance for showers
spread across the airspace. Showers and a mixture of VFR/MVFR
conditions persist through the remainder of the TAF period.

KPDX AND APPROACHES...VFR expected through Tuesday with increasing
high level clouds. Northwest winds become light overnight,
becoming southwesterly around 00Z Wednesday. There is a slight
chance (15-25%) of showers and possible thunderstorms after from
03Z-09Z Wednesday. Showers and a mixture of VFR/MVFR conditions
persist through the remainder of the TAF period. /42

&&

.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. Will continue to monitor the
situation as there remains a fair amount of uncertainty. 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

&&

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

&&

$$

www.weather.gov/portland

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