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614
FXUS66 KSEW 221520
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
820 AM PDT Sun Sep 22 2024

.SYNOPSIS...Flat upper level ridge over Western Washington with
weather system moving by to the north tonight. Ridge strengthens
Monday before moving off to the east Tuesday. Western Washington
still on the backside of the ridge Tuesday with thermally induced
surface trough along the coast in the morning for a short spell
of offshore flow. Thermally induced trough moving inland later
Tuesday. Cold front arriving Wednesday with another system later
Thursday into Friday.

&&

.UPDATE...
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No major changes to the forecast below. Sunny start for
most areas for the first day of fall. The disturbance north in
British Columbia is still on track to bring rain showers late
today and into tonight (starting in the north Cascades and Neah
Bay areas, spreading southward towards Puget Sound and coastal
areas). See discussion below for further details.

HPR
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&&

.SHORT TERM /TODAY THROUGH TUESDAY/...
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Satellite imagery shows
high clouds over the northern portion of the area this morning.
Winds are calm but so far no fog seen on the satellite imagery or
in the surface observations. Temperatures at 3 am/10z ranged from
the mid 40s in the Southwest Interior to the upper 50s in the
Seattle metro area.

Flat upper level ridge over Western Washington today with a weak
shortwave trying to move by to the north late in the day. Dry this
morning with some high clouds along with patchy fog. Clouds
thickening up over the northern portion of the area this afternoon
with a chance of showers along the north coast and near the
Canadian border. Highs today similar to Saturday, in the mid 60s
to lower 70s.

Shortwave stalls to the north tonight keeping the chance of
showers in the forecast for the coast and the interior from about
Everett northward. Mostly cloudy skies for the remainder of the
area. Lows in the 50s.

Upper level ridge strengthening Monday with 500 mb heights in the
mid 580 dms by 00z Tuesday. This will push the shortwave well
north of the area. Surface gradients remaining northwesterly so
even with the warming temperatures aloft highs will just be a
couple of degrees warmer than today, mid 60s to mid 70s.

Thermally induced surface trough along the Central Oregon coast
Monday expanding northward to the Washington coast by early
Tuesday morning. Low level offshore flow developing keeping lows
in the 50s.

Thermally induced surface trough moving inland during the day
Tuesday. Cross Cascade gradients never turn negative combined
with the upper level ridge moving east will keep highs from
getting too warm. Warmer locations like the Southwest Interior
highs pushing 80 degrees while most of the remainder of the area
stays in the 70s. Not a very big spread in high temperatures this
time of year. The normal high for Seattle Tuesday is 69 degrees
but only 12 out of the previous 79 years ( 15 percent ) has there
been a high on the 24th 75 degrees or warmer.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Extended models showing
good consistency this morning. With the upper level ridge well to
the east by Wednesday this opens up the door for a cold front to
move through the region. Current timing has the front moving
through Western Washington Wednesday afternoon. Highs cooling into
the lower to mid 60s.

Cool air mass behind front for Wednesday night. Air mass unstable
enough to a chance of evening thunderstorms in the Cascades. In
the lowlands, shower activity will taper off overnight. Lows
around 50.

Next frontal system arriving Thursday into Friday keeping rain
chances in the forecast Thursday with likely pops Thursday night
into Friday. Upper level ridge trying to build behind the front
Friday night into Saturday for the possibility of a dry weekend.
Felton
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&&

.AVIATION...
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Westerly winds continue through today before
transitioning more northwesterly tonight into Monday as a ridge
builds back over the region. VFR mid and high clouds across Western
Washington this morning with patchy lower cigs. VFR will continue
into this afternoon with increasing mid and high clouds. Cigs will
lower later this afternoon into tonight as a frontal system weakens
and moves into British Columbia. Cigs will initially lower along the
coast to LIFR/IFR this evening, then slowly lower across Puget Sound
tonight into Monday morning. Widespread MVFR/IFR expected Monday
morning. In addition, light rain will be possible tonight into early
Monday, mainly along the Olympic Peninsula and areas from Snohomish
County northward. Light winds this morning will become more S/SW
this afternoon into tonight.

KSEA...VFR cigs with increasing mid to high clouds through this
afternoon. Cigs will slowly lower tonight, becoming MVFR generally
between 08-10z. There is also approximately a 30% of IFR cigs Monday
morning. Light, variable winds this morning will become S/SW at 4 to
7 kts this afternoon. JD
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&&

.MARINE...High pressure over the coastal waters will weaken today
as a frontal system moves into British Columbia. High pressure
will then build back into the region Monday and Tuesday while a
thermally induced trough expands northward along the Oregon coast.
This will allow for flow to turn weakly offshore and northerly. A
frontal system will then move into the coastal waters on Wednesday,
bringing the return of onshore flow. Onshore flow will strengthen
in the wake of the front Wednesday evening and may bring small
craft strength westerlies to portions of the central and eastern
Strait of Juan de Fuca. Another, stronger frontal system looks to
approach the area waters Thursday into Friday.

Seas of 4 to 6 feet will continue today, before building to near
7 feet across the outer coastal waters on Monday. Seas will then
build later Tuesday into Wednesday, with GEFS probabilistic wave
guidance indicating a 70 to 80 percent chance of seas approaching
9-10 feet on Wednesday across the coastal waters. Seas then look
to build even more next Friday, with GEFS guidance hinting at a
60-80 percent chance of seas building to greater than 12 ft
across the coastal waters. 14

&&

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

$$