684
FXUS66 KSEW 211549
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
849 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.

&&

.UPDATE...No major changes to the forecast below. A small area of
showers were able to develop in the Strait of Juan de Fuca this
morning (most of these showers resided up into Canada. They may
make it into the San Juans, but guidance kills the showers very
quickly by late this morning. There is also areas of patchy to
dense fog in the Chehalis Valley along the I-5 corridor between
Olympia and Chehalis, with visibilities down to a half or a
quarter mile. This (along with other areas of low clouds/fog) will
clear out later this morning, with plentiful sunshine moving in
this afternoon.

HPR

&&

.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 today with flow
becoming a bit more zonal tonight as the weak upper ridge flattens
over the area. Generally VFR mid to high clouds across Western
Washington this morning, with lingering areas of fog or low stratus
near KHQM/KOLM and near KBFI. Fog and stratus will improve to VFR in
these areas mainly between 17-19z. Otherwise, clearing conditions
expected this afternoon and this evening. Mid and high clouds will
then increase later tonight into Sunday morning, with MVFR/IFR
developing early Sunday morning along the coast. Light winds this
morning will increase N/NW this afternoon.

KSEA...VFR cigs with widespread high clouds this morning. A
localized band of low stratus (generally 400 to 700 feet) is located
N of the terminal near BFI as of 1530z. Although stratus is not
expected to impact the terminal this morning, there may be a brief
period of LIFR/IFR cigs generally through 1700-1730z before VFR
continues. However, confidence is low in this occuring. Vsbys may
briefly drop in stratus as well with the lower cigs. Any stratus
will clear late morning with periods of high clouds this afternoon.
Light winds this morning will increase from N/NW this afternoon,
mostly between 5 to 8 kts. JD

&&

.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.
&&

$$