917
FXUS66 KSEW 212056
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
156 PM PDT Sat Sep 21 2024

.SYNOPSIS...A beautiful day across western Washington will
continue Saturday as high pressure passes over the region. A weak
disturbance on Sunday in Canada will bring the chance of light
rain showers in the northwestern corner of the state Sunday. High
pressure will build back into the region Monday and Tuesday as an
upper level ridge shifts east, turning the flow slightly offshore
and warming temperatures into the mid and upper 70s. Widespread
rain is expected second half of next week with a couple of frontal
systems.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...A weak upper-level transient
ridge continues to slide eastward over the state and Canada today.
It strengthened a little bit on models but weakens as it exits the
region late today. Surface high pressure is centered over the
Strait of Juan de Fuca for today. In turn, cloud coverage and fog
burned off very quickly this morning, and satellite is just
showing small areas of high cirrus this afternoon over the western
portion of the state. The quick cloud/fog burn off has allowed
temperatures to climb into the mid and upper 60s, and a couple
spots may be able to break 70 this afternoon (post daytime
heating). And so, the 21st of September was never meant to be a
cloudy day.

The next weather maker is already visible on satellite.
Significant cloud coverage over Canada will slowly make its way
into western Washington late tonight into Sunday. This comes from
a weak upper level trough that digs into southern British Columbia
(but struggles to maintain intensity down into Washington due to
a ridge building into the trough behind it). The rain shower
chances as a result are limited to the northwestern part of the
state, primarily affecting Neah Bay and the north Cascades.
Rainfall totals will be light, with the north Cascades and Neah
Bay areas receiving the most with a quarter of an inch, and
remaining areas along the coast/Cascades and Olympics receiving
several hundredths. Areas not receiving showers will see increase
cloud coverage Sunday.

The upper level ridge previously mentioned will shift inland over
the region. The ridge tilts eastward, and a thermally induced
trough will build along the coastline from the south Monday into
Tuesday. These features (put together) will turn the flow pattern
slightly offshore, especially on Tuesday. Sky coverage will clear
for most areas east of the Olympics (clouds will stick around
along coastal areas). There will be more widespread low 70s on
Monday, and these will increase more into the mid and upper 70s
Tuesday (couple 80s possible in the South Interior). HeatRisk
remains minor with this short increase in temperatures.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...The upper level trough
(for Tuesday) is expected to give way to a large-scale trough that
arrives Wednesday. This is expected to drape a cold front into all
of Washington, dropping high temperatures back into the mid 60s.
Widespread precipitation is likely with this frontal passage, with
Wednesday appearing to be the wettest day in the extended. It
appears that locally heavier rainfall rates will be possible in
the Olympics/coastal areas, and north Cascades Wednesday morning.
A second trough will bring another chance of additional rain
Thursday into Friday, some of which may be heavy at times in the
mountains. For the second system, elevations above 7,000 feet may
see snow mix in with the rain, especially as the cooler air drops
the snow level towards the end of the week. Saturday appears to be
drier at this time, although shower chances will remain post
trough/front for any convergence activity that trails the system.

HPR

&&

.AVIATION...Northwest flow aloft will continue into this evening
with flow becoming westerly tonight as the weak upper ridge flattens
over the area. VFR high clouds continue across Western Washington
this afternoon. High clouds will increase further tonight into
Sunday ahead of a weakening frontal system. There also may be areas
of patchy fog or low stratus, mainly near OLM/HQM/PWT early Sunday
morning, but confidence low with the increasing higher clouds.
Otherwise, cigs will lower on the coast Sunday morning with
increasing mid clouds into the interior Sunday afternoon. Light rain
may also increase near the BC border later Sunday afternoon as well.
N/NW winds this afternoon will become light tonight and transition
light S/SW for most on Sunday.

KSEA...VFR conditions with periods of high clouds this afternoon.
High clouds will increase further tonight into Sunday. Stratus or
low vsbys are not expected Sunday morning with increasing high
clouds. Cigs will slowly lower Sunday evening towards MVFR late
Sunday evening into Sunday night. NW winds this afternoon ranging 5
to 9 kts will become light NE tonight and transition SW on Sunday. JD

&&

.MARINE...High pressure will weaken over the area into tonight for
continued light flow. A frontal system will move into British
Columbia on Sunday. High pressure will then build back into the
region Monday and Tuesday, allowing for flow to turn weakly offshore
and northerly into midweek. Onshore flow then returns and increases
midweek as a front moves into the Coastal Waters. A stronger frontal
system may move towards the area then late Thursday into Friday.
Westerly winds may approach Small Craft Advisory criteria Wednesday
night.

Seas of 4 to 6 feet will continue into Sunday before building to
near 7 feet for the outer Coastal Waters Sunday night. Seas will
then build later Tuesday into Wednesday. GEFS probabilistic wave
guidance suggests a 70 to 80 percent chance of seas approaching 10
feet on Wednesday for the Coastal Waters. Seas may then build
further next Friday as well. JD

&&

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

$$