357
FXUS66 KSEW 211103
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
403 AM PDT Sat Sep 21 2024

.SYNOPSIS...Weak upper level ridge over Western Washington through
Tuesday with weather systems moving by to the north. Low level
flow turning offshore Tuesday with the upper level ridge shifting
east. Ridge continuing to move east Tuesday night with a front
reaching the area Wednesday night. Another system will arrive
Thursday night or Friday.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...Satellite imagery shows just
some high clouds over Western Washington early this morning with
areas of fog over Eastern Grays Harbor county. Temperatures at 3
am/10z were in the mid 40s to mid 50s.

Upper level ridge over Western Washington today with a shortwave
riding over the top of the ridge into Central British Columbia.
The ridge will strengthen a little this afternoon pushing the high
cloud cover mostly north of the area. Highs will be near normal,
mid 60s to lower 70s.

Do you remember? Will be not much in the way of clouds to be
chased away tonight as the ridge remains intact. Lows in the mid
40s to mid 50s.

Ridge weakens a little Sunday. This will allow the next shortwave
moving into British Columbia to get a little closer to Western
Washington. Increasing clouds during the day with a chance of
showers over the northwestern portion. Even with the increasing
middle and high level clouds highs will still be in the mid 60s to
lower 70s.

Shortwave dissipating as it tries to move southeast into the
northern part of Western Washington Sunday night. Chance of
showers along the coast and over the interior from about Seattle
northward. Cloudy skies will keep lows in the lower to mid 50s.

Ridge rebounds Monday with 500 mb heights in the mid 580 dms range
by 00z Tuesday. Thermally induced trough south of the area along
the Central Oregon coast with northwesterly surface gradients over
Western Washington. The northwesterly surface gradients will keep
highs in check, in the upper 60s to mid 70s. A few degrees above
normal.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Models in good agreement
that Tuesday will be the warmest day in the forecast package.
Tuesday could very well be the warmest day Western Washington
sees until next spring. Thermally induced trough moving up the
coastline with the low level flow turning offshore. Upper level
ridge axis is to the east at this point but Western Washington
still on the backside of the ridge. Highs mostly in the 70s with
the warmer locations taking a run at the low 80s.

Warm spell will be a one day run with the thermally induced trough
moving inland later Tuesday and the upper level ridge continuing
to move east through Tuesday night. This will open the door to a
pair of weather systems. The first one arriving later Wednesday
will be splitting as it moves into Western Washington. Even with
the front splitting a vast majority of the ensemble solutions on
both the GFS and ECMWF are wet. Have upped the pops from the blend
into the likely category for Wednesday afternoon and evening.
Next system staying together better with a consolidated jet aimed
at the Pacific Northwest Thursday night into Friday. Highs
lowering into the lower to mid 60s Wednesday through Friday.
Felton

&&

.AVIATION...Northwest flow aloft will continue through much of
the day before turning more zonal tonight as a weak upper level
ridge starts to flatten out across the region. Winds are light and
variable for most terminals early this morning, but expect a
general shift to northerly for the majority of the TAF sites
through the morning hours. Winds then look to increase to 4-8 kts
by late morning.

Conditions primarily remain VFR across the majority of the
terminals early this morning with some cirrus streaming in
overhead. Terminals may see some FEW to SCT low clouds at times,
but are generally expected to remain VFR. However, a few locations
that typically trend towards lower ceilings, such as OLM and PWT,
may see MVFR to IFR conditions emerge by 12Z in patchy fog.
Widespread VFR conditions are expected to return areawide by 18Z.

KSEA...VFR conditions with high clouds at times. Patchy fog and
stratus may develop in localized areas of Puget Sound on Saturday
morning, and while FEW to SCT low clouds are possible, lower cigs
are not expected to move into the terminal. Light and variable
winds early this morning will transition to northerly through the
morning, increasing to 4-8 kt between 18-21Z. Winds will then
ease again tonight into Sunday. 14

&&

.MARINE...High pressure over the coastal waters will maintain
generally light flow across the region today. The ridge will
weaken on Sunday as a frontal system moves into British Columbia.
High pressure will then build back into the area Monday into
Tuesday, allowing for flow to turn weakly offshore by early next
week. Onshore flow will then return and strengthen by midweek as a
frontal system moves into the coastal waters. Westerly winds
along the central and eastern Strait of Juan de Fuca may approach
small craft criteria in the wake of the system Wednesday night
into Thursday.

Seas are currently hovering between 5 to 8 feet over the coastal
waters and will continue to subside towards 4 to 6 feet today.
Seas will then build back towards 7 feet across the outer coastal
waters Sunday night into Monday and persist at this range through
early next week. GEFS probabilistic wave guidance indicates
roughly a 70-80 percent chance of seas across the coastal waters
approaching 9-10 ft Wednesday into Thursday. 14

&&

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

$$