537
FXUS66 KSEW 161016
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
316 AM PDT Mon Sep 16 2024

.SYNOPSIS...Upper level trough well to the south today while an
approaching front will stay north of Vancouver Island. Front
moving southeast tonight and Tuesday moving through Western
Washington Tuesday evening. Rain out ahead of the front spreading
over the area Tuesday. Weak upper level ridge Wednesday into
Thursday. Unsettled weather Thursday night through the weekend
with the ridge not strong enough to prevent systems from reaching
the area. High temperatures remaining below normal except for
today with highs near normal.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...Satellite imagery shows
mostly clear skies over Western Washington with the upper level
trough well to the south taking most of the cloud cover with it.
Some stratus banked along the west slopes of the Cascades along
with some patchy fog preventing skies from being totally clear. Nice
night up on the roof here at the office with clear skies and
light winds. Jupiter, Mars and Saturn all visible plus an almost
full moon. With the mostly clear skies a little range in the
temperatures at 3 am/10z with the cooler spots already in the mid
40s while temperatures in the Seattle area in the mid 50s.

Sunny skies for the most part today as the cold front along the
British Columbia coast moves slowly southeast and the upper level
trough continues to move south. The fog and stratus will dissipate
this morning. Even with plenty of sunshine, the cool start to the
morning will keep highs near normal, in the mid 60s to lower 70s.

Front continuing to move southeast tonight but still well
offshore by 12z Tuesday. Even the clouds out ahead of the system
not arriving until the early morning hours. Could be a colorful
sunrise Tuesday morning. Lows tonight in the upper 40s to mid 50s.

Front offshore near the coastline by 00z Wednesday with rain along
the coast in the morning spreading inland during the afternoon.
There is a chance the precipitation will not reach the Cascades by
late Tuesday afternoon. Strong jet digging south offshore behind
the front will slow down and weaken the feature as it gets close
to Western Washington. With plenty of cloud cover and rain highs
will be cool Tuesday, around 60.

Front trying to move through Western Washington Tuesday night but
with the digging jet offshore chance the front just dissipates
over the area. Upper level trough behind the front will keep
showers in the forecast overnight. Lows Tuesday night in the 50s.

Wednesday starting to look like Sunday with the upper level trough
digging south and drying northerly flow aloft developing late in
the day. Air mass unstable enough to keep a chance of showers in
the forecast. Temperatures will remain cool with highs in the
lower to mid 60s.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Confidence in the extended
forecast not very high this morning. Pretty good agreement in the
models Thursday with a flat upper level ridge over the area for a
dry day. Weak shortwave riding by to the north Thursday night
into Friday for a slight chance of showers.

Model solutions for the weekend very inconsistent. The GFS builds
an upper level ridge over the area for dry weather. The ECMWF has
a much weaker ridge with a long fetch of moisture extending all
the way back to the dateline. The first system embedded in this
fetch of moisture reaching Western Washington Sunday. Ensembles
show a little different story with the ECMWF solutions not as wet
as the operational run but the GFS solutions wetter than the
operational run. The trend in the ensembles from last night`s run
is there are more wet solutions this morning. With the lack of
confidence for now will stay with a broad brush mostly
cloudy/partly sunny chance of showers forecast through the
weekend. Felton

&&

.AVIATION...Light east to northeast flow aloft continues over
Western Washington today as an upper level low over northern
California shifts eastward into the Great Basin. The flow aloft will
shift to westerly late tonight as another upper trough and
associated frontal system approach the region. Light surface winds
and low level moisture will contribute to patchy LIFR/IFR in
fog/stratus across parts of the interior this morning. This will
scatter out by around 18Z for VFR areawide this afternoon and
evening. Increasing onshore flow tonight will produce widespread low
MVFR or IFR ceilings across Western Washington on Tuesday morning.

KSEA...Persistent E/NE surface winds overnight have helped keep the
temperature and dew point comfortably apart thereby reducing the
probability to less than 15% of LIFR conditions developing at the
terminal this morning. VFR continuing through this evening. Low MVFR
in stratus is likely by mid-morning Tuesday. Surface winds N/NE 7
knots or less becoming northwesterly this afternoon then backing to
south/southwesterly tonight.  27

&&

.MARINE...A broad surface ridge remains centered well offshore with
lower pressure over the interior. Onshore flow will increase
somewhat this evening. Winds may briefly get close to small craft
advisory levels in the central/east strait tonight. The surface
ridge over the offshore waters will weaken tonight and a trough and
associated front will shift into the coastal waters on Tuesday. This
system is unlikely to generate any headlines. The trough will weaken
as it gradually shifts southward into the Oregon coastal waters on
Wednesday. High pressure rebuilds offshore late in the week before
another front is expected over the upcoming weekend.  27

&&

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

$$