738
FXUS66 KSEW 120330
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
830 PM PDT Fri Sep 11 2026

.SYNOPSIS...
Unsettled weather remains on track to move into western Washington
this weekend with a trough moving through. This includes a slight
chance of thunderstorms Saturday afternoon, along with a steadier
rain Saturday evening into Sunday. Many locations will see at least
half an inch of rain in total, with a higher potential for over an
inch of rain in the Cascades south of Interstate 90. Drier weather
will move in next week, with temperatures warming back up from
Wednesday onwards.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/...
An upper level low will dive southeastward this weekend from
B.C. and deepen as it comes ashore over the weekend. A surface
trough and low pressure will also progress inland this weekend
as well. The ensembles are indicating an abundance of moisture
available with this system compared to normal for this time in
September (as the ECMWF shows anomalies for precipitable water
and integrated vapor transport at 4.0 and 4.9 standard deviations
above normal). With the abundant moisture available for this
time of year, the region will see some beneficial rain this
weekend.

The timing of the precipitation will start Saturday, with the coast
seeing some wrap around precipitation to start the morning. As the
low progresses inland, more wrap around showers will begin to form
across the interior and the Cascades. The initial shower activity
has a 15-20% chance of thunder. While SPC has all of western WA in a
general thunderstorm risk for Saturday, the actual thunder may be
more concentrated into portions of the south interior and Olympics
with upslope/orographic lifting as the precipitation moves northwest.
Otherwise, the precipitation coverage will become more stratiform
going into Saturday night/Sunday, and cover primarily the Cascades
(south of I-90), and adjacent portions of the south interior.

Most locations will see at least 0.50" of rain with this system from
Saturday through Sunday evening. The bulk of the precipitation is
expected again in the south Cascades, where there is a 60-80% chance
of rainfall exceeding 1" and 1.50". Rain rates are not expected to
exceed 0.25" to 0.35" an hour with this system (except potentially
in thunderstorms), which should keep hydrologic impacts minimal
(except for some ponding). Winds this weekend will remain generally
light, but could get a little breezy Sunday across portions of the
interior, and along the Cascade Crest with gusts up to 20 mph. High
temperatures will remain in the upper 60s to around 70, with lows in
the mid 50s.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Ensembles have the upper level low/trough moving out of the region
late Sunday, with northwest flow aloft increasing. There could be a
few remaining showers in the Cascades going into Monday morning.
Otherwise, expect conditions to dry out through the day for
most, with more clouds than sun keeping temperatures the same.
Another trough will pass through early Tuesday, however the
models cap any moisture with this trough moving through. Wednesday
is where the models have an upper level high building over the
Pacific, with a split difference as to how long the high remains
offshore going into late next week. A couple models even show
an omega block over the Pacific. Regardless, most will see a
warm up with the surface pressure centered over southern B.C.,
and lower pressure to the southwest off the coast. Highs in the
lowlands will likely climb back into the mid/upper 70s (with
perhaps a few 80s).

HPR

&&

.AVIATION...
Southwesterly flow aloft in place over W WA as an upper level low
digs southward over the eastern Pacific. This will turn more
southerly as the feature reaches the OR coast Saturday afternoon.
South to southwesterly winds at the surface becoming northerly with
speeds less than 5 kts or going light and variable overnight.
Northwesterly to westerly winds starting Saturday afternoon with
speeds 5-10 kts as incoming front approaches.

Widespread VFR conditions remain in place and will persist
throughout the TAF period, even though cigs will lower as frontal
system begins to approach not so much overnight but more in the late
morning to afternoon hours Saturday. Rainfall associated with this
system may bring cigs down to MVFR at times, but these will generally
be isolated and as such lack the confidence for TAF inclusion.
Timing of precipitation for the coast will start around 20Z and the
interior terminals mainly after 00Z Saturday. Additionally, there is
a threat for isolated thunderstorms Saturday afternoon/evening.
Highest chances (20%) will be over the Olympics. As for the interior
terminals, there is a 10-15% from PAE southwards. Chances for
thunderstorms will subside Saturday night.

KSEA...VFR today and Saturday. Precipitation chances will increase
Saturday afternoon, with precipitation likely impacting the terminal
after 00Z. There is a slight chance (10%) for isolated thunderstorms
in the afternoon and evening. Southwesterly winds this afternoon
will shift northerly by 06Z at 3-5 kt. Northerly winds will increase
to 5-7 kt Saturday afternoon. Southwesterly winds return Saturday
evening after 00Z.

18/29

&&

.MARINE...
High pressure will continue to weaken as a low pressure system moves
across area waters towards Oregon into Saturday. Behind the system,
high pressure will begin to rebuild over area waters and strengthen
through the middle of the week. Westerly winds will increase through
the Strait of Juan de Fuca with the potential to reach small craft
thresholds Saturday evening and Sunday evening. Latest probabilities
are around 40-55% on Saturday and  70-85% on Sunday. Seas generally
around 4-6 feet this weekend, decreasing slightly early next week to
3-5 feet.

29

&&

.FIRE WEATHER...
Onshore moisture will increase this weekend as a trough moves across
western Washington. The initial precipitation Saturday afternoon
will be more showery (with a 15-20% chance of thunder), but coverage
will become more stratiform late Saturday night into Sunday,
increasing the chances of a wetting rain for most (especially the
south Cascades). As a result, the fire weather concerns will remain
minimal going through the weekend and early next week. High pressure
will build in south B.C. by midweek, with lower pressure offshore at
the surface. The pattern will become drier with warmer temperatures,
and additionally some models are showing brief breezy east to
southeast winds Wednesday evening/Thursday morning. A few Cascade
locations may see RH values dip into the 20s, though there
remains some uncertainty in the ensembles at this time as to how
strong and long the high pressure will remain late next week.

HPR

&&

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

&&

$$