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FXUS66 KSEW 261028
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
328 AM PDT Sat Jul 26 2025

.SYNOPSIS...Weak upper level trough offshore with low level
onshore flow through the weekend. Strong upper level ridge over the
middle of the county will begin back building into the Pacific
Northwest Monday while a deep upper level trough remains anchored
over the Gulf of Alaska. Flow aloft becoming southerly in the
middle of next week bringing up the possibility of convection
over the Cascades.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...Satellite imagery shows band
of high clouds over the northern portion of the area with stratus
inland only as far as Shelton at 10z/3 am. Temperatures a little
brisk for July with Bremerton and Olympia in the upper 40s.
Remainder of the area in the 50s.

How far east the stratus gets is the forecast problem this
morning. Strong consensus in the models that stratus will get
east of Puget Sound by 15z. Example HREF has 80-90% chance of
ceilings at Seattle-Tacoma airport. With the stratus only to
Shelton this late in the morning don`t think the stratus will
advect east of the Sound. Stratus will instead form in place. The
high clouds over the area are hindering that process right now but
there looks to be a break in the high clouds soon giving the
stratus enough time to form. The layer will be much thinner than
the marine layer Friday morning. In addition onshore gradients are
weaker this morning so there will not be any reinforcing of the
layer. End result of all this high clouds plus stratus giving
mostly cloudy morning. Stratus deck dissipating earlier for sunny
afternoon. With the weak upper level trofiness overhead
temperatures aloft are a little below normal so even with the
afternoon sunshine highs will also be a little below normal, in
the mid 60s to mid 70s over the interior. Light onshore flow
throughout the day will keep highs in the lower to mid 60s coast.

Not much change in the pattern tonight into Sunday. Upper level
trough weakening a little over the area with light onshore flow in
the lower levels. Marine layer again will be thinner leading to
more sunshine Sunday. Highs getting back to near normal with
mostly 70s for the interior and mid 60s coast. With the cooler
air mass aloft it will be another crisp morning for late July with
the colder locations like Bremerton and Olympia dropping into the
upper 40s. Lower to mid 50s for the remainder of the area.
Forecast low for Seattle 53 degrees. The last time Seattle had a
low in the 40s the last week of July was 49 degrees on July 31,
2002. The time before that 48 degrees July 28th, 1959.

Upper level trough anchored in the Gulf of Alaska digging a little
south Sunday night and Monday while the large upper level ridge
over the middle of the country starts to back build towards the
Pacific Northwest. Low level flow still light onshore Sunday night
into Monday but only strong enough to produce a very shallow
marine layer Monday morning confining stratus to the coast, Strait
of Juan de Fuca and Lower Chehalis Valley. The marine layer will
dissipate by late morning. Under sunny skies temperatures warming
again with 70s and lower 80s in the forecast for the interior and
mid to upper 60s coast.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Extended models in good
agreement with the large upper level trough over the Gulf of
Alaska digging south with the corresponding upper level ridge
building over Western Washington Tuesday and Wednesday. Flow aloft
becoming southerly Tuesday night and remaining southerly at least
through Thursday. The models are still having trouble pin
pointing, as one would expect, a small closed low or shortwave
developing off the California coast getting embedded in the flow
aloft. A lot of uncertainty how this pattern will evolve Wednesday
through Friday but the consensus pops over the Cascades are
slowly increasing. For now will go with a broadbrush slight chance
of showers each day for the Cascades Wednesday through Friday.
For the lowlands mostly sunny days through Thursday with highs a
little above normal, in the 70s to mid 80s. Increasing low level
onshore flow Friday cooling highs a few degrees, back into the
upper 60s and 70s for the interior. Highs on the coast mostly in
the mid to upper 60s with lower 70s possible Tuesday and
Wednesday. Felton

&&

.AVIATION...VFR at all terminals early this morning for the time
being. Some stratus is revealing itself along the Kitsap peninsula
and may impact PWT in the next few hours. Unlike yesterday, the
gradients have relaxed enough to limit an expansive stratus cover
this morning. Model probabilities are around 40 to 60 percent
confident in seeing MVFR cigs in Puget Sound this morning, but are
not apt to keep it around much past 17Z to 18Z. Surface flow this
morning will be southwesterly for most terminals, becoming northerly
in the afternoon. A very brief period of breezier winds is possible
between 20Z and 23Z tonight through the Strait of Juan de Fuca,
impacting CLM.

KSEA...VFR presently at the terminal with light northerly flow.
Winds will become southwesterly after 12Z, persisting as such
through much of the day. Northerlies are likely to return after 06Z
tonight. Cigs and vsbys will be the challenge this morning, as
the surface gradients have lessened comparatively to yesterday,
which will limit as much of a stratus field. That said, model
guidance is about 40-60% confident on seeing at least MVFR cigs
beginning around 12Z, with limited, brief periods of IFR.
Scattering is likely to begin around 17Z for a VFR afternoon.

21

.MARINE...An upper level trough will bring increasing onshore flow
today over the region. The trough will remain in place through
Saturday before moving inland. A larger low will dip down from
Alaska offshore through next week.

For this weekend, the onshore marine pushes may bring stratus and
mist/fog over portions of the waters this morning, namely over the
inner coastal waters. Tonight, there will be a very brief period of
15-25 kt winds, mostly in the Central Strait of Juan de Fuca between
20Z-23Z. Given the limited area and time, a SCA was not posted, but
the chance for a period of breezier conditions remains in the
forecast.

Seas will remain 5 feet or less throughout the weekend through next
week.

21

&&

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

$$