709
FXUS66 KSEW 142111
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
211 PM PDT Sat Sep 14 2024

.SYNOPSIS...An upper level trough stationed over the region today
will pivot southward over the weekend, maintaining cloudy skies,
cooler temperatures, and shower activity. Conditions will dry out
on Monday under split flow before another system moves into the
Pacific Northwest mid-week. Ridging is favored to return towards
the end of next week, bringing potential for for drier and warmer
conditions.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...An upper level trough will
continue to drop southward from British Columbia throughout the
weekend, maintaining unsettled and cooler conditions. A surface
front will continue to slide across western Washington this
afternoon, pushing a line of showers and isolated lightning
activity eastward. Rainfall totals will be fairly light, with a
quarter inch or less across the lowlands and locally higher
amounts closer to half an inch over higher terrain. While dense
cloud cover is limiting the instability as this system moves
through, a few more lightning strikes cannot be ruled out (10% to
15% probability) this afternoon in stronger showers that develop,
and the highest chance remains over the North Cascades towards the
Canadian border. Onshore flow behind the frontal passage enhances
the chances for convergence showers to develop over the central
Puget Sound later this evening, which may generate locally heavy
rain. Elsewhere, cloud cover will stick around with another marine
push bringing in low stratus later tonight.

Precipitation chances across western Washington decrease into
Sunday as the upper level low dives southward towards the
California coast. Wrap-around moisture from the parent low may
generate light showers over the Cascades, while the lowlands and
coast will see sun breaks as drier air moves inland. High pressure
will continue to amplify inland from the west on Monday,
generating split flow aloft and allowing conditions to dry out
ahead of the next incoming system. Temperatures through the
short-term will stay on the cool side for mid September, with
most lower elevations seeing highs in the low to mid 60s and lows
in the upper 40s to low 50s.

.LONG TERM /TUESDAY THROUGH SATURDAY/...Models continue to show
good agreement over another trough dropping southward on Tuesday,
bringing in another round of cooler temperatures and widespread
showers. Ensembles favor slightly higher rainfall amounts with
this system, with over an inch along the coast and over the
western slopes of the Olympics. This system will once again dive
to the south along the Pacific Coast late Tuesday into Wednesday,
with showers and cloudy skies lingering throughout the day
Wednesday. Lowland temperatures will stay below normal in the low
to mid 60s under the influence of this system.

Thursday and Friday bring in more uncertainty, with the potential
for a weak shortwave trough over southern British Columbia to
generate light shower activity and more cloudy skies over western
Washington. Long-range ensembles favor ridging over the Pacific
Northwest towards the end of next week paired with low level
offshore flow, which would allow conditions to clear out and warm
up. While current forecasts return temperatures to seasonal
normals, a warm start to the fall season cannot be ruled out.

Lindeman

&&

.AVIATION...An upper level low continues to dig southward along the
B.C./WA coastline this afternoon. The flow aloft will become more
southwesterly out ahead of the trough, before northerly flow makes
its way down to the surface for Sunday.

Western Washington continues to see post frontal showers behind a
cold front that went through earlier this morning. The heaviest
showers have been been able to reduce VISs/CIGs down to IFR briefly
passing over terminals (observed at KCLM and KHQM). The shower
activity will continue eastward from the coast/San Juans into Puget
Sound this afternoon, as a convergence zone is expected to setup
over King/Snohomish Counties. The brief heavy downpours will be the
primary concern for terminals impacted, along with erratic wind
shifts with gusts to 20 kt, and a possible lightning strike.
Additional showers may linger in Puget Sound through as late as 10Z.

CIGs continue to remain in MVFR for a majority of the region
(although improvements to VFR have been observed in the Southwest
Interior out ahead of showers approaching from the coast). For areas
still remaining under MVFR, CIGs have still be lifting throughout
the afternoon, and more areas will see VFR CIGs after 21Z (bases
around 4,000 ft). However areas impacted by showers will see
decreases in CIGs/VIS temporarily down to IFR. Lower MVFR/IFR CIGs
will redevelop overnight. Winds this afternoon will remain out of
the southwest 5 to 10 kt (gusts to 15-20 kt possible near Strait of
Juan de Fuca), with variable northerly winds possible with
convergence zone showers. Winds will diminish to under 5 kt
overnight, and pick up out of the north during the day Sunday 5 to
10 kt.

KSEA...MVFR CIGs continue to lift this afternoon (may reach VFR
shortly after 21-22Z). Showers are expected to arrive around the
terminal after 22Z, and may be convergent in nature (moderate to
heavy rain downpours may reduce VIS/CIGs briefly to IFR, and wind
shifts/gusts to 20 kt from the north). MVFR CIGs will redevelop
overnight (20% chance of IFR in the morning), before scattering out
later in the day. Winds Southwest 5 to 10 kt, shifting to the
northeast 4 to 6 kt after 02-04Z, before picking up out of the north
5 to 10 kt during the day Sunday.

HPR

&&

.MARINE...Post cold front showers continue across inner waterways
this afternoon as an upper level low digs over the region, and
tracks eastward. Brief downpours from these showers may reduce
visibilities to mariners. Gusty west winds will work their way into
the central/east Strait of Juan de Fuca early this evening, with
sustained winds around 21-23 kt (gusting to 25-28 kt). The small
craft advisory will continue in this area through 2 AM PDT.
Otherwise, winds will remain out of the northwest post-front over
the coastal waters, with the gustiest winds in the far outer coastal
waters (Sunday into Monday, and Tuesday into Wednesday up to 20 kt
gusts).

Seas 5 to 7 feet Saturday through Tuesday, dropping to 3 to 5 feet
before picking back up briefly to 6 to 8 feet with a disturbance
moving through Wednesday/Thursday.

HPR

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 4 PM this afternoon to 2 AM PDT Sunday
     for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$