431
FXUS66 KSEW 220205
AFDSEW

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

.SYNOPSIS...High pressure will move eastward tonight allowing for a
weak system to pass north of western Washington Sunday. While
northern portions of the area may see a chance for light rain, there
will be very little impact elsewhere. High pressure returns for the
start of the week with offshore flow and warmer temperatures
expected. Widespread rain is expected for the second half of next
week as a series of systems impact the region.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...Mostly clear skies with some
scattered high clouds here and there. High clouds associated with
the incoming system for Sunday already starting to show up over the
NW corner of the Olympic Peninsula and stretching northeastward to
start filtering into western Whatcom County as well. Will see these
clouds increase areawide overnight. Otherwise, forecast remains on
track with incoming dissipating system expected to impact the
northern tier of the CWA with precip chances remaining north of
Everett for the most part...except for some possible activity
running along the coast. For further details, please refer to the
Previous Discussion section below.

No evening updates planned to the forecast.

18

From Previous Discussion...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/...From Previous
Discussion...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 this evening before turning more
westerly tonight as the weak upper ridge flattens over the area.
Surface winds generally northerly, although near and north of the
Strait seeing winds more west to northwesterly. Light and variable
winds expected overnight before becoming southerly at less that 5
kts by late morning.

VFR conditions in place and expected to remain that way for most
terminals for the TAF period. High clouds will increase with
lowering cigs, however they are not expected to lower enough to
impact flight category. The exception to this will be those
terminals more prone to lower cigs, such as OLM, HQM and PWT...where
low level moisture will be enough for stratus/fog development,
resulting in MVFR to IFR conditions by early Sunday morning. While
OLM and PWT will likely return to VFR conditions by late morning,
HQM may remain socked in as approaching system draws closer.

KSEA...VFR conditions with increasing high clouds overnight and
Sunday. Stratus or low vsbys are not expected Sunday morning with
increasing high clouds. Cigs will slowly lower Sunday nearing MVFR
late Sunday evening into Sunday night. NW winds this evening ranging
4 to 8 kts will become light NE tonight and transition SW on Sunday.

18/JD

&&

.MARINE...High pressure will weaken over the area 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/18

&&

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

$$