472
FXUS66 KSEW 221042
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
342 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.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...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

&&

.AVIATION...Westerly flow expected aloft this morning as a weak
upper ridge flattens over the area. Flow aloft will become more
northwesterly again tonight into Monday as the ridge rebuilds
over the area. Surface winds generally light northerly early this
morning, but expect winds to transition back to southerly and
increase to 4-8 kts by late morning.

Current conditions across area terminals are VFR. Mid to high
level cloud cover will increase through the day today with
gradually lowering cigs as a frontal system moves into British
Columbia. Low level moisture will be abundant enough for some
stratus/fog development early this morning for terminals along the
coast and across the southern Sound (HQM, OLM, and PWT). This
could result in a few hours of MVFR to IFR conditions early this
morning. While OLM and PWT will likely return to VFR conditions
by late morning, HQM may remain socked in as the approaching
system draws closer. A few isolated showers will be possible
this afternoon across the far northern interior terminals,
generally from KPAE northward. Shower activity may briefly drop
ceilings and visibilities to MVFR at times. Ceilings will then
become more widespread MVFR at the area terminals as low clouds
push into the region overnight into Monday morning.

KSEA...VFR conditions with increasing mid to high clouds today.
Ceilings will slowly lower through the day, nearing MVFR by late
tonight. Light northerly winds will transition to the SW and
increase to 4-8 kts this morning. Latest NBM probabilistic
guidance indicates a 70 percent chance of ceilings lowering to
MVFR by Monday morning and a 30 percent chance of ceilings
lowering towards IFR. 14

&&

.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.
&&

$$