897
FXUS66 KSEW 311557
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
857 AM PDT Sun Aug 31 2025

.UPDATE...Monsoonal moisture aloft has brought some small
isolated convection over Grays Harbor County this morning. An
isolated thunderstorm threat will likely continue throughout the
morning over the Olympic Peninsula and the Cascades, with some
isolated showers possible elsewhere as an upper level low
continues to churn offshore and create instability over our area.
Generally, the thunderstorm threat will continue to remain limited
over the interior throughout the rest of the day.

&&

.SYNOPSIS...An upper level low off the coast will keep temperatures
seasonable again today, with slight chances for showers and possibly
an isolated thunderstorm or two over the North Cascades this
evening. Upper level ridging returns Monday, bringing another round
of hot and dry conditions with well above normal temperatures are
expected by mid-week.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...Latest satellite imagery
showing some scattered mid clouds moving northward over W WA this
early morning. Out west over the Pacific, the center of circulation
for the parent upper level low is fairly apparent. This feature once
again is expected to keep temps more seasonable, akin to yesterday
but maybe a degree or two warmer.

Models have latched back on to the potential for some scattered
showers starting in the late morning and the risk stretching into
the early evening hours. The area with the best chances /and even
those are just 10-20 pct, so not terribly convincing/ seems to be
over the northern half of the Olympic Peninsula and from Everett
northward. They also re-introduce isolated thunderstorms into the
mix but this too is limited to the Cascades of Whatcom, Skagit and
Snohomish counties. Given the back and forth in the models, not too
much confidence in this activity developing, but at the same time
both CAPE and LIs from latest runs certainly indicate that the
potential is there...as such, feel this isolated/slight chance
wording is likely the best fit for the non-zero chance that likely
exists.

The retreat of the upper low back to the Pacific waters and the
building of upper level ridging starting on Monday remains
consistent...so while temps will rise on Monday by a few degrees,
the more noticeable heating will be Tuesday as highs in the interior
start to return to the lower to mid 80s. Locations closer to water
and coastal areas will still linger in the 70s.

18

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...The ridge will remain the
dominant weather feature for much of the long term, however, models
are starting to advertise some mitigating features that could start
to rein in temperatures by the end of the week. This is going to
depend largely on the track of the upper low that retreated from the
area as stated above. Models show a fairly uniform northerly trek
for the system and as it moves, it could bring in a couple of
shortwave disturbances with the first being early Thursday morning.
While this feature is expected to be dry, some strengthening
northerlies/onshore flow will help to actually trigger a cooling
trend in daytime highs. Temps look to peak on Wed, with mid to upper
80s for interior lowlands...maybe as high as 90 for the Cascade
valleys...before falling back into the lower to mid 80s for Thu. A
secondary shortwave nearly 24 hours later will have a similar
effect, bringing temps down further to the upper 70s to around 80.
Saturday raises some interesting questions, as while there is
general agreement on a transition over to a more troughy pattern,
the orientation of the trough will play a big role as to whether or
not W WA could see the return for some chances for precip. No
consensus has emerged at this time, so confidence remains low.
However, this looks to at least keep the cooling trend intact, with
daytime highs a degree or two cooler than Friday.

18

&&

.AVIATION...Southerly flow into Monday as an upper low stalls
offshore of the Western Washington coast. A mix of low, mid, and
high clouds across the area this morning. Areas of LIFR stratus
continue along the immediate coast, and localized areas of Puget
Sound, mainly near KPAE. In addition, MVFR cigs are observed over
southern Puget Sound. There is also increasing mid and high clouds
due to monsoonal moisture aloft, with a few showers developing over
the Olympic Peninsula, and Cascade Foothills. Although the
thunderstorm threat remains limited, will need to monitor any
embedded thunderstorm within localized showers today, mainly for the
Olympic Peninsula and King CO northwards. Otherwise, cigs are
expected to improve for the interior by midday to VFR, with stratus
lingering along the coast. Another round of stratus will move inland
tonight into Monday morning for widespread cig reductions. Light
winds this morning will become more westerly this afternoon.

KSEA...VFR conditions this morning. Multiple lower stratus decks
exist in central Puget Sound this morning, as observed by the LIFR
cigs at PAE and MVFR cigs at KRNT. There remains a low chance of
stratus reaching the terminal through 18z. Otherwise, HZ aloft will
continue this afternoon with VFR mid and high clouds. Although dry
conditions are expected, will need to monitor any VCSH this morning.
Light NW winds this morning becoming more W this afternoon. JD

&&

.MARINE...An onshore flow pattern will continue into next week.
Onshore flow will vary in intensity, with westerly pushes through
the Strait of Juan de Fuca each evening. An SCA remains in effect
this evening for the central and eastern Strait of Juan de Fuca.
Onshore flow will then continue into midweek.

Although primarily dry conditions are expected, there is less than a
10% chance of thunderstorms into Sunday as well.

Seas of 2 to 4 feet for the Coastal Waters through Tuesday will
build to 4 to 6 feet by midweek.

15

&&

.FIRE WEATHER...A slight chance of dry thunderstorms is forecast for
the north Cascades this evening thanks to a low pressure system
parked offshore sending a weak impulse into the area. While this
will steepen lapse rates and offering some ascent, it will likely be
weak and perhaps more terrain driven. With no significant source of
moisture, there is some concern over dry lightning strikes, however,
given the poor dynamics and waffling of model runs as to this
prospect, confidence remains low. Any storms that happen to develop
will be capable of gusty and erratic winds as a by-product of the
steep lapse rates and downdraft CAPE, as well as lightning on dry
fuels. The probabilities are capped at 10-15%.

Later this week, temperatures will be climbing and humidity dropping
off, with poor overnight recoveries. Temperatures will be well into
the upper 80s to mid 90s, with near 100 degrees in the Cascade
valleys as periods of light offshore flow develop during the day.
Elevated concerns for hot, dry and unstable conditions will persist
through much of the mid to late week period.

21/18

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 5 PM this afternoon to 2 AM PDT Monday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$