035
FXUS66 KSEW 031039
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
339 AM PDT Wed Sep 3 2025

.SYNOPSIS...Upper level ridge building over the area from the
southeast while an upper level low offshore continues to drift
northwest today. Upper level ridge remaining over Western
Washington into Friday. Ridge shifting east Friday night. Upper
level disturbances spinning out of an offshore upper level low
moving through during the weekend. Upper level low weakening into
a trough with the trough moving over the area the first part of
next week.

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...Satellite imagery shows upper
level ridge building over Western Washington from the southeast
this morning. High level clouds embedded in the flow aloft over
the southern portion of the area with low clouds just along the
coast at 10z/3 am. Temperatures were in the mid 50s to mid 60s.

Upper level ridge will continue to build into the area from the
southeast today. Weak shortwave over Southern Oregon will move
northwest and go by to the south later today spreading high clouds
over Western Washington. Low cloud cover confined to the coast
early this morning with light surface gradients. Like the last
couple of mornings temperature dew point spreads small, two
degrees or less in many cases, so expect some stratus to form
over the interior around sunrise. The layer will be shallow and
dissipate by noon leaving sunny but hazy skies with some high
clouds. Highs a touch warmer today versus Tuesday, mostly in the
70s and lower 80s. The exception to this will be the Cascade
valleys and the foothills where highs will be in the mid 80s to
lower 90s. A heat advisory for these locations will remain in
effect.

Not much change in the pattern tonight into Thursday with the
upper level ridge remaining over the area. Surface gradients
going weakly onshore overnight creating a shallow marine layer
for early Thursday morning that will quickly get wiped out by the
morning sunshine. More high clouds streaming up from the southeast
so will word the forecast partly sunny. Lows tonight in the mid
50s to lower 60s. Highs Thursday similar to today, 70s and lower
80s for the interior, mid 80s to lower 90s in the Cascade foothill
valleys. Weak onshore flow knocking a couple of degrees off the
highs along the coast with readings near 70.

Upper level ridge axis drifting off to the east Thursday night and
Friday. Upper level low dropping down out of the Gulf of Alaska
near 45N/140W by Friday afternoon. Flow aloft turning more
southerly but first weak upper level disturbance embedded in the
flow aloft still south of the area by 00z Saturday. Will indicate
a slight chance of showers over the Southern Cascades in the
afternoon. Outside of that, another sunny day with above normal
temperatures similar to Thursday.

.LONG TERM /SATURDAY THROUGH TUESDAY/...Confidence in the extended
forecast over the weekend not very high this morning. There is
good agreement in the models that the upper level ridge will
continue to move east opening the door for shortwaves spinning
out of the upper level low to move over the area. This will lead
to more cloud cover and cooler temperatures. Models are very
inconsistent with the timing and the strength of the shortwaves.
ECMWF a little slower and weaker with the features versus the GFS.
Ensembles off of both models never indicate more than half the
solutions at any one time with precipitation so will go with
chance pops through the weekend. Air mass unstable enough
Saturday afternoon and evening to have a slight chance of thunder
over the Cascades. Highs cooling to the mid to upper 60s on the
coast and 70s inland Saturday and mid 60s coast and upper 60s to
mid 70s inland Sunday.

Models in better agreement for the first part of next week with
the upper level low weakening to an upper level trough and moving
over Western Washington keeping a chance of showers in the
forecast and continuing the cooling trend. Tuesday could feel like
a touch of fall with highs barely getting out of the 60s. Felton

&&

.AVIATION...VFR conditions across the interior terminals early this
morning. Marine stratus is expected to redevelop later this morning.
IFR/LIFR ceilings are expected with periods of reduced visibility.
There is some uncertainty regarding the extent of the stratus for
interior terminals. Confidence is highest for the northern interior,
such as KPAE and KBLI. However, confidence is low for South Puget
Sound and the Seattle area. Current guidance has a 15-25% chance of
seeing IFR ceilings for South Puget Sound/Seattle area, which are
around the same probabilities for conditions remaining VFR.
Therefore, due to equal chances and uncertainty, will keep IFR
ceilings in the prevailing line. For interior terminals that do see
ceilings lower, improvement is expected after 15z-18z to VFR, with
VFR persisting through the TAF period. As for the coast, latest
guidance does hint at some brief improvement into VFR between 21z
Wednesday through 00z Thursday. However, confidence is low at this
time. Light winds early this morning will increase this afternoon to
6-11 kt, with the strongest winds along the coast.

KSEA...VFR early this morning. Marine stratus is expected to move
inland later this morning between 13z-16z, bringing the potential to
for IFR ceilings to the terminal. Current probabilities of IFR are
around 20%. With equal chances of seeing VFR and IFR ceilings, will
keep IFR ceilings in the prevailing line. Ceilings may bounce around
at times. Any ceiling that does develop should clear by 17z/18z.
Otherwise, VFR for the remainder of the TAF period. Light northerly
winds early this morning will become northwesterly this afternoon,
increasing to 6-8 kt.

29

&&

.MARINE...Weak high pressure will remain over the waters through
Thursday. It will weaken Friday/Saturday as a low pressure system
moves over the coastal waters. Weak high pressure will rebuild over
the waters Sunday.

Westerly pushes through the Strait of Juan de Fuca each evening. The
strongest push will occur this evening. Have held off on issuing a
Small Craft Advisory for the Strait due to sustained wind speeds
being below 21 kt. However, may see an occasional gust greater than
21 kt (50-70% chance) at times in the central and eastern portions
of the Strait.

Combined seas 4-6 ft today through Thursday, decreasing slightly to
3-5 ft Friday into the weekend.

29

&&

.FIRE WEATHER...Warm and dry conditions continuing into Friday.
Shallow marine layer in the late night and morning will continue
to give excellent RH recoveries in the lowlands but with the
layer being shallow RH recovery will be poor in the mid-slopes and
ridges in the Cascades and Olympics. The poor recoveries, very
dry conditions and increasing instability will keep the Red Flag
Warning for fire weather zones 658 and 659 in effect today.
Conditions will be reevaluated this afternoon for a possible
repeat of the Red Flag Warning conditions Thursday and Friday.
Cooler conditions over the weekend with a deeper marine layer will
bring about an end to the threat. There is a chance of showers
over the weekend with possible thunderstorms over the Cascades
Saturday afternoon and evening but confidence remains low with
respect to the specifics. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Red Flag Warning from noon today to 9 PM PDT this 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.

&&

$$