624
FXUS66 KSEW 021636
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
936 AM PDT Tue Sep 2 2025

.SYNOPSIS...Upper level high pressure will remain over western
Washington, resulting in a warming and drying trend through at
least midweek. Daytime temperatures during this time frame will
be well above normal. Some slight cooling and potentially more
active weather may be possible by the end of the week.

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...Latest satellite imagery
shows almost the reverse of 24 hours ago, with marine clouds
effectively making down the Strait and impacting northern
portions of the CWA...although not so far north as Skagit and
Whatcom counties...while mid-level clouds are more the story for
locations south of Snohomish county and Forks. Much of the area
continues to see temp/dewpoint spreads of only one or two
degrees, so will likely see stratus fill in many of the areas
yet to experience it...it will just occur an hour or two later
than initially forecast, perhaps as late as just before sunrise.

As stated in the Synopsis, the upper level ridge will be the
dominant weather feature for the entire short term period, resulting
in warming temperatures and drying conditions. The confounding
aspect of the forecast continues to be daytime high temperatures.
NBM continues to advertise highs both today and Wednesday that would
have an impact to HeatRisk, thus warranting the inherited Heat
Advisory. However, observed highs the past couple of days have been
considerably lower than what the NBM has been advertising, shaking
confidence in the aforementioned headline. As such, have undercut
the model to take into account the seeming gap between reality and
NBM output. Will retain headline at this time, as even though with
cooler forecast highs, daytime temps will still be unseasonably warm
and headline does an excellent job of pointing that out. Fire
Weather impacts will be addressed below.

Models remain consistent on showing a shortwave embedded in the
ridge that will bring some relief for Thursday. While not
necessarily reflected by numbers in the forecast, as again there is
some reliance on the likely still too warm NBM, temperatures will
start to trend downward with this feature, if only a little at
first, as temps Thursday will slip just a few degrees from those of
Wednesday, still ranging in the interior from the mid 70s to around
80.

18

.LONG TERM /FRIDAY THROUGH MONDAY/...Consensus on the upper level
low over the Pacific and its track remains lacking, however there is
enough agreement pointing toward the ridge shifting eastward enough
to allow the associated upper level trough to influence W WA
weather. Not only does this mean the cooling trend will continue
throughout the long term, with daytime highs returning to the more
seasonable yet still a touch warmer than average mid 70s. Models are
also advertising some slight chance PoPs, although at this time
these chances are relegated to the terrain of the Cascades. Given
the degree of uncertainty of the actual positioning of the low, it
is difficult to have any degree of confidence in solutions present
within the forecast itself, simply the transition to a slightly more
active weather pattern is enough for now until a proper consensus
can emerge.

18

&&

.AVIATION...Upper level low just off the coast this morning will
move west northwest through Wednesday. Upper level ridge over
Idaho building into Western Washington tonight. The ridge will
remain over the area Wednesday. Southerly flow aloft today
becoming southeast tonight and light Wednesday. Light flow in the
lower levels becoming onshore tonight into Wednesday.

Low stratus in the IFR category did fill in this morning across much
of the interior and along portions of the coasts. Many terminals
this morning are still reporting either IFR with CIGs around 500-800
ft, with pockets of Puget Sound and the South Interior observing
mist or fog this morning (with visibilities down to as low as 1/2
SM). The fog/mist is only expected to last through late this morning
as temperatures slowly rise, with full scattering now likely to take
place early this afternoon. North winds of 5 to 10 kt are expected
across most terminals today (may be a touch breezy with a weak push
down the Strait of Juan de Fuca with winds up to 10-15 kt). Another
similar setup is expected Wednesday morning (right now in the
interior 20% chance of MVFR with a 10% chance of IFR and lower -
this increases to a 70% chance along the coast for MVFR and IFR, and
a 40% chance of LIFR).

KSEA...Fog and stratus formed over the terminal this morning - still
continuing to see visibilities of 1/2 SM continues. Expectation is
for the low fog/mist to clear by 17-18Z, with scattering now taking
place early this afternoon around 19-20Z. Winds have picked up out
of the north 4 to 8 kt this morning, and will increases to 5 to 10
kt this afternoon, decreasing back to 4 to 8 kt Wednesday morning.
Currently a 30% chance of MVFR Wednesday morning (25% for IFR and
10% chance of LIFR with NBM guidance).

Felton/HPR

&&

.MARINE...Weak high pressure will remain over the coastal
waters through Thursday. The high will weaken Friday and
Saturday with a low pressure system moving into the offshore
waters. Lower pressure will remain east of the Cascades through
the weekend.

Light onshore flow developing tonight and continuing into the
weekend. Diurnal westerly pushes through the Strait of Juan de
Fuca each day with the Wednesday night push having the best chance
of reaching small craft advisory speeds. Felton

&&

.FIRE WEATHER...Gradually hotter temperatures and very dry
conditions will continue to develop each of the next couple of days.
These conditions, coupled with light offshore flow during the day,
will likely result in elevated fire conditions around Wednesday in
the Cascades. These factors will also yield poor to moderate RH
recovery in the mid-slopes and ridgetops as the inversion
strengthens overnight, followed by good mixing in the afternoon
hours. All of this in mind, collaboration with surrounding offices
and an eye to the unstable conditions this would create and the
already dire state of fuels, have opted to go with a Fire Weather
Watch for the Cascade zones of 658 and 659 for Dry and Unstable
conditions Wednesday. This will merit further watching though. If,
as stated above, temperatures do not get as warm as the forecast
suggests, this headline may no longer be needed. Continue to monitor
forecasts for the latest temperature and RH expectations. Showers
with possible isolated thunderstorms are forecast late in the week
as monsoonal moisture increases across parts of the region, though
confidence remains low in the specifics.

18

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Fire Weather Watch from Wednesday afternoon through Wednesday
     evening for West Slopes of the Central Cascades Generally
     above 1500 Feet-West Slopes of the North Cascades
     Generally above 1500 Feet.

     Heat Advisory until 5 PM PDT Thursday for Foothills and
     Valleys of Central King County-Foothills and Valleys of
     Pierce and Southern King Counties-Foothills and Valleys
     of Snohomish and Northern King Counties-Foothills and
     Valleys of Thurston and Lewis Counties-Foothills and
     Valleys of the North Cascades.

PZ...None.

&&

$$