738
FXUS66 KPQR 271807 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1107 AM PDT Wed Aug 27 2025

...Updated aviation discussion...

&&

.SYNOPSIS...Active thunderstorm day with dozens of strikes in
the area this morning. low pressure will linger over the area
shifting northward through the day. Expect thunderstorms to
continue through the afternoon impacting the Cascades mostly.
Warming and drying on Thur-Fri until another active low aloft
moves inland bringing a return of monsoonal moisture.

&&

.MORNING UPDATE...This morning continues to be active as an upper
level low sits directly overhead, driving showers and thunderstorm
activity across northwest Oregon and southwest Washington. Elevated
instability, moisture, and lift from the upper low will support a 15-
25% chance for thunderstorms through late this afternoon. Went ahead
an expanded the coverage of PoPs and thunder across the area. Main
threats from any passing thunderstorms include lightning, gusty
outflow winds, and heavy downpours. Based on the latest 12z Salem
sounding and other mesoscale analyses, winds aloft from 700-300 mb
are very light. This could lead to thunderstorms moving very slowly.
If this were to happen, localized urban and small stream flooding is
possible, especially in low-lying and poor drainage areas. And of
course, when thunder roars, go indoors! -Alviz

&&

.SHORT TERM...Today...It has been an active morning as
thunderstorms have initiated over almost all of the northern
portions of the forecast area. Radar shows a mesoscale low
sitting right over northwest Oregon which is wrapping monsoonal
moisture into the area. This low coincides with a vorticity max
aloft and a jet streak which is providing coupled instability.
Throughout the Clark County Lowlands, rainfall totals thus far
from thunderstorms equate to around 0.25 inch per half hour or
so. These precipitation rates may produce localized urban
ponding of water. However, storms appear to be moving faster. As
the storms ride up and over the Coast Range, they are being
orographically lifting and even more lightning is being
initiated. The Coast Range Foothills near Forest Grove are being
lit up like a Christmas tree with lightning. Through the
morning, probability for lightning decreases.

This monsoonal moisture and enhanced instability will be the
main weather risk for the day. High resolution models are
depicting CAPE values in excess of 600 J/Kg in the southern
Washington Cascades and northern Oregon Cascades. By 5 PM, the
central Oregon Cascades will see CAPE values in excess of 1100
J/kg. Both of these values are exceptionally high for this area.
This level of instability combined with warm air in the region
and moisture will bring another round of storms this afternoon.
Overall the NBM Prob Thunder outputs have run lower than desired
given the pattern, but the general location of the Cascades
seeing the highest probability is depicted well. Overall, going
to be another active weather day.

The last challenge of the day will be the temperature forecast.
The forecast has fluctuated quite a bit as cloud cover has been
throwing wrenches our way. The more dense and widespread the
cloud cover, the lower the temperatures. Overall, looking at
just above normal seasonable high temperatures, and warmer
overnight temperatures. Overall, not looking at any widespread
Moderate to Major HeatRisk today, but will see localed evolved
HeatRisk in the northern Willamette Valley and the urban areas
around Vancouver and Portland.     -Muessle


.LONG TERM...Thursday through Monday...Thursday through
Friday, the aforementioned weak upper low will continue to
advect northward over Washington and eventually over Vancouver
Island. There is strong consistency between our global ensemble
models in regards to this pattern, but based on the
deterministic 500 mb heights, the pattern transition has slowed
slightly. Even with this being the case, upper level shortwave
ridging will develop which will maintain warm and dry conditions
over our area through Friday. Will generally see temperatures
rise back into the 90s for inland locations, with humidity
dropping - especially in the southern portions of the forecast
area. Simultaneously, a large scale upper level low will slowly
shift southeastward through the northeast Pacific.

Very little change in the weekend forecast at this point. Another
round of instability is expected as this next upper low moves
over the Pacific Northwest. At this point it is unclear of
exactly the impact, but there will likely be wrap around
monsoonal moisture once again and cooler daytime temperatures.
The jet stream is projecting a jet streak moving directly
overhead on Saturday afternoon with stronger southwesterly flow.
This type of scenario could produce another round of
thunderstorms. However, the issue is that this low is moving
further north toward far northwestern Washington and British
Columbia, and there may not be enough moisture for thunderstorms
or rain in general. Coming out of prolonged high pressure with
a generally very dry air mass, it would take a much stronger low
with a lot more moisture to really get us some widespread rain.
NBM chances for measurable rainfall (0.01" or greater) over 48
hours ending 5 AM Sunday are low - only 10-20% along the coast
and Portland northward. South of Portland and the Cascades are
closer to 10-15%.

The decaying low will push northward leaving dry air trailing.
We are in store for what appears to be a drier start to next
week with less probability for thunderstorms, and a greater
probability of warming with low humidity. -Muessle/Alviz

&&

.AVIATION...Expect prevailing VFR conditions and high clouds
through the TAF period for most inland terminals. For coastal
terminals, low stratus will keep MVFR/IFR conditions present
through most to all of the TAF period. Best chances to see
improved conditions will be between 21Z Wednesday and 02Z
where chances for VFR/MVFR conditions is 20-40% for KONP and
70-90% for KAST. Otherwise, expect winds around 4-9 kt varying
from southerly to northwesterly.

Stray showers and thunderstorms remain possible through the TAF
period. Thunderstorms will have the potential to produce small
hail, heavy rain, and gusty outflow winds. Expect convection to
gradually lift northward through this afternoon, eventually
becoming limited to southwest WA.

PDX AND APPROACHES...VFR conditions and high clouds through the
period. Light southerly winds around 5 kts or less. Stray showers
or thunderstorms remain a possibility through this afternoon.

-Batz/Hall

&&

.MARINE...Earlier this morning, thunderstorms were observed
over the outer coastal waters just west of Newport, OR. Stray
showers and thunderstorms remain possible through this morning
and into the afternoon. Stronger thunderstorms will have the
potential to produce small hail, heavy rain, and gusty outflow
winds.

Looking ahead, expect benign conditions to persist, with seas
subsiding even more on Thursday, most likely down to 2 to 4 ft.
Note winds are expected to become southerly on Thursday in
response to weak low pressure over the coastal waters drifting
northward, however wind gusts should stay below 10-15 kt. Winds
should veer to the northwest by Friday afternoon, but should
remain light at 10 kt or less and seas should remain between 2 to
4 ft. Both winds and seas are forecast to stay below small craft
advisory criteria for the foreseeable future. Additionally,
another round of dense marine fog persists over the coastal
waters, so a Dense Fog Advisory remains in effect for all marine
zones until 11 AM today to cover the threat of reduced surface
visibilities down to 1 NM or less at times. -Hall/TK

&&

.FIRE WEATHER...Low pressure continues to spin over the area
causing thunderstorms to form, and increasing the instability
within the area. Convective parameters over the Cascades have
increased to levels that are high for the area. This means that
the atmosphere will be very unstable with a high probability for
pyroconvective behavior. This present instability combined with
the monsoonal moisture in the area will create conditions
primed for more thunderstorms. While thunderstorms will be wet,
outflow winds up to 35 mph are expected. Additionally, lightning
may strike outside of this rain core producing an almost dry
lightning effect on critically dry fuels. The combination of
these two components have lead to Red Flag Warnings over all of
the Cascades from the Willamette Natl Forest to the Gifford
Pinchot. Chances for a wetting rain remains low, generally less
than 15%, and despite high humidity recovery tonight, near-
critical relative humidity of 30-40% is expected Wednesday
afternoon. Lowest humidity will be in the central Oregon
Cascades - especially within the valleys. -Muessle/Picard

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 9 PM PDT this evening for ORZ688>690.
WA...Red Flag Warning until 9 PM PDT this evening for WAZ634-635.
PZ...None.
&&

$$

www.weather.gov/portland

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