577
FXUS66 KSEW 182229
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
329 PM PDT Sat May 18 2024

.SYNOPSIS...Cool and showery conditions will persist today with an
upper level trough situated over the region. The trough will move
east by Sunday afternoon allowing for upper level ridging to nudge
into Western Washington. Dry and warmer conditions are expected
Monday, before another system moves into the region on Tuesday for
a return to wet and unsettled conditions. Troughing over the
region then looks to bring unsettled conditions through much of
the next week.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...An upper level trough
sinking into the area from British Columbia today will continue
to bring cooler and showery weather to western Washington. Radar
this afternoon shows scattered shower activity across much of
western Washington. Breaks in the cloud cover and increasing
instability will yield a slight chance of thunder for the area
through this evening. Thunder chances then wane through the
evening hours. Expect shower activity to continue through this
evening and gradually taper through the overnight period into
Sunday. High temperatures today will generally trend a good 5 to
10 degrees below normal - topping out in the mid to upper 50s.

The upper trough axis will then shift east tonight, allowing for
showers to gradually taper on Sunday. Hi-res guidance indicates
only a few remnant showers persisting in the Cascades. Drier
northerly flow aloft Sunday afternoon will allow bring some
afternoon sun to western Washington. High temperatures on Sunday
will warm a few degrees and generally look to top out in the upper
50s to low 60s.

Weak upper ridging will nudge into the region on Monday for
drier and warmer conditions. Afternoon highs look to climb into
the mid to upper 60s for the interior. Highs along the coast will
remain in the upper 50s and low 60s.

The next upper level trough then looks to dig southward from
British Columbia and into western WA through the day on Tuesday.
Its associated surface front will move across the region, bringing
the next round of more widespread precipitation to western WA.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Cool and showery conditions
will persist into midweek as the aforementioned upper trough
continues to traverse southeastward across the Pacific Northwest.
The forecast details become a bit more uncertain late in the week,
though ensembles generally hint at troughing over the Pacific
Northwest persisting while a ridge of high pressure remains situated
over the Gulf of Alaska. Ensembles still struggling with the
depth of the trough over the Western US. However, with guidance
continuing to hint at troughing, have maintained cooler conditions
and chances of showers in the forecast for now through late week.

&&

.AVIATION...West/northwesterly flow aloft today as an upper level
trough moves through the region. Ceilings this afternoon bouncing
between VFR and MVFR with a broken cloud deck and scattered showers
across the region. Scattered showers will begin to diminish the
evening. Ceilings will remain through the early evening, with a
decent chance of more solidly MVFR conditions building in between
around 12-18Z Sunday. Cloud cover will break throughout the day
tomorrow with a return to VFR conditions.

Onshore winds continue through the TAF period, decreasing to light
overnight. Northerly through the Seattle area, southerly through the
remainder of the interior, then transitioning back to south through
the entire interior tonight.

KSEA...Ceilings between VFR and MVFR with light showers in the
vicinity. Showers diminishing by around 03Z. Better chance at
redeveloping MVFR CIGs between 12-18Z Sunday. Winds generally light
northerly this afternoon and evening, switching back to southerly
between 03-06Z Sunday. Winds remain light southerly through the
overnight and into tomorrow.

LH

&&

.MARINE...Broad high pressure over the NE Pacific will maintain
onshore flow across western WA through the weekend. Strongest winds
will be over the Strait of Juan de Fuca with Small Craft Advisory
winds extending through late tonight. The next frontal system looks
to move through the area on Tuesday, with another push of westerlies
down the strait possible with the potential for additional
headlines. Otherwise, winds look to remain fairly benign through the
coastal waters.

Seas around 6 ft through most of the forecast period, through the
frontal system on Tuesday may raise seas closer to 8 to 10 ft,
making waves steep with an 8 second period.

LH

&&

.HYDROLOGY...The daily hydrology discussion has ended until the
start of the next rainy season; it will only be updated as needed.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Sunday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$