Washington State Forecast Discussion

Western Washington


003
FXUS66 KSEW 230357
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
857 PM PDT Fri Sep 22 2023

.SYNOPSIS...Upper level troughing building over the NE Pacific
will kickstart an extended period of wet weather. Rain chances
return on Saturday as a weak front enters followed by additional
disturbances into the upcoming week. Fall-like temperatures are
expected.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...Forecast remains largely
on track tonight. A weak system will move into the region,
bringing light rain to western Washington starting early Saturday.
Clouds ahead of the system have already started to push inland
along portions of the coast tonight. The remainder of the previous
discussion follows below, with updates made to the aviation and
marine sections. 14

For tonight, upper-level troughing will begin to progress into
the region. Deterministic guidance is showing a subtle shortwave
sweeping across the area late overnight. Mostly dry conditions are
expected tonight but a weak front looks to draw near the coast
into the early morning hours on Saturday with PoPs increasing.
Overnight lows are forecasted to be in the mid 40s to mid 50s.

As alluded to, a weak front is on tap to enter on Saturday. Light
widespread rain is in the forecast along with much cooler
temperatures, right on cue for the first day of fall. Widespread
60s are expected, anywhere from 8-12 degrees cooler than the
previous day. Rain will break into showers Saturday evening before
conditions gradually dry out briefly. Overnight lows are again to
be in the 40s and 50s.

After somewhat of a reprieve expected late Saturday night into
early Sunday, more rain is inbound. A deep low center over the NE
Pacific will scoop up copious amounts of moisture under strong
southwesterly flow. Here, an atmospheric river is on track to
make landfall along the WA/OR coast by Sunday evening, spreading
further inland by Monday morning. Ensemble and deterministic
guidance shows anomalously high PWAT values >1.00" during this
time. Widespread rainfall is forecasted to begin and continue
well into Monday. Current 24-hr rainfall forecast for Monday
includes 1-3" for the coast and Olympics. 0.5-1.0" throughout the
interior lowlands with values in the lower end influenced by
potential rain shadowing. 1-2" is in the forecast for the
Cascades. Note, confidence in exact totals is still on the lower
end at this time as the forecast could change in subsequent
updates. However, confidence still remains high on this being the
first prolonged moderate to heavy rainfall event in a while.
Breezy conditions are also in the forecast, particularly for the
coast with gusts upwards of 35-45 mph beginning Sunday night
through Monday.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Rain chances will remain in
the forecast through much of the upcoming week. NBM v4.1 has
anywhere between 85-95% chance of the coast seeing rainfall >1.0"
for a 24-hr period ending 5 AM Wednesday morning. 50-70% for much
of the Cascades and 25-45% through the lowlands. Troughing will
begin to push inland by Wednesday. However, ensemble guidance
remains in disagreement on the upper-level pattern going into
Thursday/Friday. Nevertheless showery and cool conditions are
favorable to remain.

McMillian

&&

.AVIATION...Light westerly flow aloft continues with a shift to
southerly flow expected overnight. This will maintain generally
VFR conditions under mostly clear skies for the interior this
evening. Increasing mid cloud deck will move over the region
tonight by 09z ahead of an approaching front. This will bring some
lower MVFR to IFR ceilings to the coast early evening. Light rain
will begin to spread across the region by 14z at the coast and far
north, and more solidly across the Puget Sound terminals later in
the morning, with additional rain through the day.

KSEA...VFR evening with clear skies and light southwesterly
winds. Increasing mid-level clouds with southerly winds increasing
to 10 kt by 16z. MVFR ceilings increasingly likely from 18z
onward through the period with light rain at times. Cullen

&&

.MARINE...A very strong low pressure system will rapidly intensify
and move into the area Sunday into Monday. NBM ensemble guidance
has continue to trend stronger with winds, with a 80-95% chance of
gale force sustained winds and a 55-75% chance of storm force
gusts across the outer coastal waters for Sunday night into early
Monday morning. Given this trend, have issued a Storm Watch for
the outer coastal waters for the aforementioned period, and a Gale
Watch for the inner coastal waters. Additionally, gales likely develop
in the Strait of Juan de Fuca as the strong pressure gradient
develops across the strait. Gales are likely to continue across
the coastal waters into Monday. Winds should ease up beginning
Tuesday and remain below thresholds through next week.

Seas will begin to build tomorrow to 5 to 7 ft Sunday with the first
frontal system, then building Sunday night into Monday up to 15-18
ft, first for the outer waters Monday then reaching the coastline
Tuesday. Steep waves will also develop across Grays Harbor bar
with a small craft advisory also issued there. LH/12

&&

.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...Gale Watch from Monday afternoon through Tuesday afternoon for
Grays Harbor Bar.

Small Craft Advisory from 11 AM Sunday to 8 PM PDT Monday for
West Entrance U.S. Waters Strait Of Juan De Fuca.

Gale Watch from Sunday morning through Monday morning for
Central U.S. Waters Strait Of Juan De Fuca-East Entrance
U.S. Waters Strait Of Juan De Fuca.

Gale Watch from Sunday morning through Monday evening for
Coastal Waters From Cape Flattery To James Island Out 10 Nm-
Coastal Waters From James Island To Point Grenville Out 10
Nm-Coastal Waters From Point Grenville To Cape Shoalwater
Out 10 Nm.

Storm Watch from Sunday morning through late Sunday night for
Coastal Waters From Cape Flattery To James Island 10 To 60
Nm-Coastal Waters From James Island To Point Grenville 10
To 60 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater 10 To 60 Nm.

Gale Watch from late Sunday night through Monday evening for
Coastal Waters From Cape Flattery To James Island 10 To 60
Nm-Coastal Waters From James Island To Point Grenville 10
To 60 Nm-Coastal Waters From Point Grenville To Cape
Shoalwater 10 To 60 Nm.

&&