629
FXUS66 KSEW 201002
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
302 AM PDT Sun Sep 20 2026

.SYNOPSIS...
High pressure aloft with light flow in the lower levels through
tonight. Ridge weakening Monday as a weak system moves by to
the north. Another weak system will brush the northern portions
Wednesday. Upper level trough developing offshore Wednesday
night and Thursday with a more organized system arriving
Thursday night or Friday.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...
Satellite imagery shows low clouds and fog along the coast with
low clouds and fog filling in over the interior lowlands at 3
am/10Z. Temperatures were in the mid 40s to mid 50s.

Western Washington on the backside of an upper level ridge today
with light flow in the lower levels. Temperature dew point
spreads are already down to two degrees or less early this
morning. With the light low level flow expect fog and low clouds
to form over all of the interior lowlands by sunrise. The layer
will be shallow but as yesterday reminded us, the sun this time
of year is losing its strength ( equivalent to late March ) so
even with a shallow marine layer it going to take into the
afternoon hours to dissipate. Highs today in the 60s.

Little change in the pattern tonight. Upper level ridge moving
east Monday with weak low level flow continuing through the
day. Low level moisture over the area once again tonight into
Monday morning combined with the light low level flow for
another round of low clouds and fog Monday morning. Weak system
riding by to the north later in the day throwing some middle and
high level clouds over the area so even after the marine layer
dissipates it will still be mostly cloudy. Lows tonight in the
mid 40s to lower 50s. Highs Monday once again in the 60s.

West southwesterly flow aloft Monday night into Tuesday with 500
mb heights dropping as an upper level trough develops offshore.
Weak system spinning out of the trough approaching the coast
late Tuesday. A little more flow in the lower levels reducing
the morning fog coverage but plenty of low level moisture around
for low cloud formation. Like Monday by the time the marine
layer dissipates in the afternoon middle and high level clouds
ahead of the next system will be spreading over the area
making for another mostly cloudy day. Highs in the 60s. Lows
Tuesday morning in the mid 40s to lower 50s. Felton

&&

.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
Models in good agreement with the weakening system reaching the
coast Wednesday morning dissipating over the interior in the
afternoon. Chance of showers along the north coast Tuesday night
and over the entire northern portion of the area Wednesday.

Stronger upper level trough digging south out of the Gulf of
Alaska Wednesday night and Thursday. Models having a little
trouble on the timing of a developing system in the base of the
trough then moving into the area. The timing ranges from
Thursday afternoon to Thursday night/Friday morning. With the
trough digging offshore there is a chance this feature could
stall over Western Washington for a bit before kicking out to
the east. Ensembles favor the later timing with chance pops
Thursday night and chance/likely pops Friday. Upper level trough
moving through Friday night keeping chance pops in the
forecast. Weak split flow aloft off the coast Saturday for a
good chance at a dry day.

Highs in the 60s Wednesday and Thursday and lower to mid 60s
Friday and Saturday. Lows in the mid 40s to lower 50s. There is
a chance that Seattle could see the first low in the 40s since
June 8th ( 49 degrees ) Saturday morning. Felton

&&

.AVIATION...
West-southwesterly flow prevails over the area with broad troughing
over the northeast Pacific. Otherwise mostly clear skies early this
morning with light winds has promoted the development of fog in the
valleys across western Washington. Stratus will continue to
slowly build along the immediate coastline, as well. Several
area terminals, including OLM, are already experiencing LIFR
conditions due to fog, and this will continue to slowly develop
across the remaining area terminals through around 11- 14Z this
morning. The fog should begin to lift and start to burn out from
17-20Z. Following, skies will remain mostly clear with VFR
conditions through the remainder of the day today. The pattern
looks to repeat with another round of fog and low stratus once
again early Monday morning. Light to calm winds this morning
will remain N/NW 4- 6 kt during the day today.

KSEA...VFR conditions early this morning. Fog will likely develop
over the terminal with LIFR conditions as early as 13-14Z, then
slowly lifting and clearing out between 18-20Z. VFR conditions
prevail through the rest of the day. Another round of low clouds and
fog are possible again after 12Z Monday. Light N winds 4 kt or less
will increase to around 4-6 kt today. Winds remain light tonight
into Monday morning.

62

&&

.MARINE...
High pressure will continue to persist over the offshore waters
going into early next week. A weak front will graze the area just to
the north late Monday into Tuesday, which will allow for increased
onshore flow Monday night. Probabilities for Small Craft Advisory
winds through the Strait of Juan de Fuca increase to 60-90% Monday
afternoon into early Tuesday. Another push is expected Tuesday
evening, but probabilities decrease and are more confined to the
Canadian side of the Strait. Additional systems may move through the
waters Wednesday and towards the end of the week but widespread
stronger winds are not likely at this time.

Very calm seas of around 2 to 3 ft will increase late tonight into
Monday to around 7 to 9 ft as longer period northwesterly swell
arrives the coastal waters. Seas look to remain around 6 to 8 ft
Tuesday through the middle of the week.

62

&&

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

&&

$$