124
FXUS66 KSEW 211000
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
300 AM PDT Mon Sep 21 2026

.SYNOPSIS...
Upper level ridge well to the east with an upper level trough
offshore today. Weak system moving by to the north this
afternoon. Convergence zone possible over the Central Puget
Sound late tonight into Tuesday morning. Next system spinning
out of the trough dissipating as it reaches the area Wednesday.
Stronger system arriving Thursday night into Friday morning.
Flat upper level ridge over Western Washington next weekend with
a warm front dissipating off the coast Sunday.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...
Satellite imagery looks almost identical to early Sunday
morning with low clouds and fog starting to form over the coast
and most of the lowlands. Many locations are already down to the
mid to upper 40s at 3 am/10Z with the remainder of the area in
the lower to mid 50s.

Like Sunday morning good set up for fog and low cloud
development across the area this morning. Light flow in the
lower levels combined with the longer nights and low level
moisture will produce areas of low clouds and fog. Tops near or
below 1000 feet with a late morning/midday breakout. Weak system
moving by to the north spreading middle and high level clouds
over the area this afternoon for partly sunny/mostly cloudy
skies. Highs remaining a little below normal, in the 60s and
lower 70s.

Southwesterly flow aloft continuing tonight and Tuesday with an
upper level trough offshore. Increasing low level onshore flow
with a possible convergence zone late tonight into Tuesday
morning over the Central Puget Sound producing some drizzle.
The increase in the onshore flow will limit fog coverage late
tonight but with plenty of low level moisture will still have
low clouds developing in the overnight hours. Low clouds
dissipating midday Tuesday with mostly sunny skies in the
afternoon. Lows in the upper 40s to mid 50s. Highs in the 60s.

Next system spinning out of the upper level trough offshore
falling apart as it reaches the coast early Wednesday morning.
What is left of the system moving through the Northwest
Interior later in the morning for a slight chance of showers. A
couple hundredths along the north coast will be the best this
system can do rain wise. Middle and high level clouds with the
system will make for a partly sunny/mostly cloudy day for the
interior. Lows in the mid 40s to lower 50s. Highs in the 60s.
Felton

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SUNDAY/...
Models in good agreement with a consolidated front reaching the
area late Thursday night/Friday morning. Rain out ahead of
the front spreading over the area Thursday afternoon into
Thursday evening. This system will have a little juice with it
with a half to one inch of rain along the coast, Olympics and
Central and Southern Cascades and a quarter to half inch over
the interior. Right now the North Cascades looks to be the
wettest location with one to one and a half inches of water.
Snow levels lowering to 6000 to 6500 feet giving the higher
elevations a few inches of snow. Upper level trough behind the
front showing signs of digging south and becoming slightly
negatively tilted Friday morning which could stall the system
over the area into Friday afternoon. Upper level trough moving
through Friday evening. Cool days Thursday and Friday with highs
only in the lower to mid 60s. Lows in the mid 40s to lower 50s.

Models continue to be in good agreement over the weekend with a
flat upper level ridge over the area. Warm front dissipates
before it reaches the coast Sunday. Post frontal onshore flow
and the lack of much ridge building will keep temperatures cool.
Highs remaining in the lower to mid 60s. Mornings will be on
the cool side as well with the coldest locations dropping into
the upper 30s. Lows in the 40s for most places.

Just a note the autumnal equinox is at 5:05 pm Tuesday the
22nd. Felton

&&

.AVIATION...
Otherwise mostly clear skies and light to calm winds will promote
continuing development of low stratus and fog this morning. It
remains most prolific across southwest WA and along the coast, but
will continue to develop across portions of the rest of the interior
this morning through around 15Z. Compared to yesterday, the coverage
of fog appears to be slightly less. Clouds will then begin to lift
and scatter between 17-20Z this morning for VFR conditions through
much of the remainder of the TAF period. Onshore flow will increase
tonight, allowing for another round of stratus to build late tonight
into early Tuesday, through primarily around or after 12Z Tuesday.
The highest probabilities will be along the coast and through the
central Puget Sound interior with convergence following the passage
of a dry cold front late tonight. Mostly calm winds will be westerly
8 kt or less during the day, except up to 10-15 kt through the
Strait of Juan de Fuca this evening.

KSEA...VFR early this morning with clear skies. Fog will develop in
the valleys near the terminal, through confidence is lower in it
forming directly over the terminal later this morning. Model
probabilities are in the 10-20% range for IFR or lower between 14-
17Z. VFR conditions then prevail with high clouds overhead. A dry
cold front will move through tonight, and behind it, weak
convergence will allow for MVFR cigs to develop overnight into
Tuesday morning, after around 10-13Z Tuesday. Light N winds 4 kt or
less will become more W/NW 4-6 kt after around 18Z. Winds become
light again tonight.

62

&&

.MARINE...
High pressure will weaken over the coastal waters this morning.
A weak cold front will traverse the area waters this afternoon into
early Tuesday, resulting in increased onshore flow. Winds will
increase this afternoon through the Strait of Juan de Fuca and will
reach Small Craft Advisory thresholds through late tonight. The
advisory was extended slightly to account for the potential of some
lingering gusts into early Tuesday morning.
Another push of westerlies through the Strait of Juan de Fuca is
forecast Tuesday evening, but guidance remains confident in the
strongest winds remaining on the Canadian side of the strait.A
weakening front then moves across the waters on Wednesday, with the
potential for another weather system moving through late Thursday
into Friday.

Seas of 3 to 5 feet will build to 7 to 9 feet tonight as a longer
period northwesterly swell arrives to the coastal waters.
Ensemble guidance has backed off on the probability for seas greater
than 9 ft (15-25% or less). Seas then subside to 6 to 8 feet Tuesday
afternoon and remain like so through the remainder of the week.

62

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 2 PM this afternoon to 3 AM PDT
     Tuesday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$