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FXUS66 KSEW 010916
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
212 AM PDT Mon Sep 01 2025

.SYNOPSIS...Upper level ridging will build back into western
Washington starting today and continuing through much of this week.
This will result in dry conditions and a warming trend into at least
mid-week, with daytime temperatures well above normal. Some gradual
cooling and potentially more active weather is possible by the end
of the week.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...Latest satellite imagery
shows widespread marine stratus throughout the entire western half
of the CWA as well as taking over the south Sound area as far north
as the Seattle metro area. However, locations north of Bothell all
the way up to the Canadian border have not seen any such development
at the time of this writing. The one-degree temp/dewpt spread at
Paine Field as well as some isolated patchy fog at Burlington
suggest this stratus field might spread further northward by the
time the sun rises later on this morning. Zooming out away from W
WA, a fair amount of overnight convection continues east of the
Cascades. General trend is that this activity is easing as the
morning progresses and the majority of it is remaining well east of
the area. A single cell, at the time of this writing, is approaching
the far NE corner of the Cascades in Snohomish county...and while
this may also offer a glancing blow to the Cascades of Skagit and
Whatcom counties, no significant impacts nor rainfall amounts are
expected.

Instead, the big story for the short term will be the upper level
ridge that will build over the area, allowing temperatures to warm
throughout the period and the persistence of dry conditions. Daytime
highs will climb each day: mid to upper 70s today, upper 70s to
lower 80s Tuesday and lower to mid 80s Wednesday...with Cascade
valleys poised to break into the lower 90s. That said, HeatRisk is
expected to be a concern over the next couple of days ahead, as the
coverage of Moderate /Orange/ increases Tuesday and becomes
widespread on Wednesday. Cascade valleys will see areas of Major
/Red/, but these patches remain isolated for the most part. One
potential saving grace keeping this particular event apart from more
recent ones is the fact that overnight lows will still manage to
range from the mid 50s to around 60 while prior events saw overnight
lows solidly in the 60s.

18

.LONG TERM /THURSDAY THROUGH SUNDAY/...Even though the upper level
ridge remains in place, an upper low over the Pacific will start
flexing some influence over W WA in the long term. Initially, a pair
of shortwaves both Thursday and Friday will help to foster a slight
increase to onshore flow, taking the edge off of temperatures both
days and signaling the start of a cooling trend: Thursday seeing
highs in the upper 70s to lower 80s and Friday ranging in the mid
70s to around 80. While no consensus has been reached with regards
to the path this low will take, models remain in agreement that the
associated troughiness will be the main driver in W WA weather for
the remainder of the forecast period. This feature could churn in
moisture to the area as early as Friday afternoon or hold off until
late Saturday/early Sunday. The lack of any degree of consensus
keeps confidence low with regards to any precip chances related to
this. However, the impact on temps is a point of agreement, where
models suggest a return to highs in the lower to mid 70s. These
would still be slightly above seasonal norms, but far more in line
than the temperatures seen during the short term.

18

&&

.AVIATION...Mostly MVFR to IFR across the western Washington
terminals early this morning as low stratus continues to expand
inland across the lowlands. The marine layer will likely burn off
across most of the inland terminals by 18z-21z, but is favored to
linger along the Pacific Coast for most of the day. VFR conditions
will prevail across the interior this afternoon and evening with
scattered high clouds. Another round of low stratus is expected to
fill in across the region early Tuesday morning, with most inland
terminals seeing MVFR/IFR ceilings redeveloping around 9z-12z Tues.
Surface winds through the period will remain light 10 kt or less
through the period with light onshore flow.

KSEA...IFR ceilings in low stratus have developed over the terminal
early this morning, and deteriorated conditions will likely prevail
through the late morning. VFR conditions are favored to return
around 19z-20z, with scattered high clouds through the remainder of
the afternoon and evening. Another round of MVFR/IFR low stratus is
on track to redevelop early Tuesday morning around 9z-12z. Light
southwest surface winds around 5 to 8 kt will veer W/NW by the mid
to late afternoon and remain northerly through Tuesday night.

15

&&

.MARINE...A deep low pressure system will remain stationed offshore
through the week with high pressure building inland. This will
generate continued onshore flow with north to northwest winds over
the offshore waters and westerly pushes through the Strait of Juan
de Fuca each evening over the next several days. Westerly winds
through the Strait will continue to be monitored for the potential
of reaching Small Craft criteria. North to northwest winds will also
increase mid-week over the outer coastal waters.

Seas will remain between 2 to 4 feet over the coastal waters through
Tuesday, before building to 4 to 6 feet through the remainder of the
week.

15

&&

.FIRE WEATHER...Dry conditions and rising daytime highs, coupled
with already very dry fuels, will raise fire weather concerns
throughout the first half of this week. 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.
Atmospheric stability will need to be monitored, as steeper lapse
rates resulting in unstable conditions could lead to Red Flag
conditions...but the main concern for this would likely be on Wed
under peak high temps. The upper level shortwave for Thursday could
foster this instability, but by then temps take enough of a cut that
RH values could be the limiting factor. Earlier model runs suggested
that isolated thunderstorms could be possible with this shortwave,
however latest runs have backed away from this prospect.

18

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.

&&

$$