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164
FXUS66 KSEW 100339
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
839 PM PDT Mon Sep 9 2024

.UPDATE...
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Forecast on track this evening with morning low clouds
Tuesday. Cool upper level trough moving southeast down the
British Columbia coast reaching Western Washington Wednesday.
Showers associated with the trough starting Tuesday night and
continuing through Wednesday. Trough moving east of the Cascades
Wednesday night with drying northerly flow aloft behind the trough
Thursday. Current forecast has the trends covered. No update this
evening. More detailed discussion below with updated aviation and
marine section. Felton
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.PREV DISCUSSION... 
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/issued 243 PM PDT Mon Sep 9 2024/

.SYNOPSIS...A series of systems will move across the region this
week, bringing increased onshore flow, below normal temperatures,
and shower chances to western Washington at times.

.SHORT TERM /TONIGHT THROUGH THURSDAY/...Zonal flow aloft and
increased onshore flow in the low levels allowed for marine
stratus to push inland into the interior this morning. Satellite
imagery this afternoon shows stratus still draped across much of
the interior and shows clouds scattering out very slowly. Stratus
will continue to lift and gradually scatter through the remainder
of the afternoon, but can expect some areas to stay cloudy through
the rest of the day. Afternoon high temperatures will top out in
the 60s and 70s across the region.

A trough will then sink south from the Gulf of Alaska and deepen
offshore on Tuesday. Onshore flow will continue across western
Washington, allowing for another marine push to usher in another
round of stratus on Tuesday morning. Mid to high level clouds will
increase throughout the day ahead of this next system and
afternoon highs are expected to top out in the upper 60s to low
70s. Showers look to start spreading inland along the coast by
Tuesday evening.

The upper level low will then move inland across the region on
Wednesday, sweeping a frontal system across western Washington.
Temperatures will cool even more, with afternoon highs expected to
to be in the 60s area-wide. This system will bring the next best
chance of wetting rains to the region, mainly along the coast,
central and south Sound, and central Cascades. Rainfall amounts
generally look to be around a quarter of an inch for most lowland
locations, though may trend closer to half an inch for areas along
the coast and in the mountains. Higher elevations of Mount Rainier
will even see a few inches of snow accumulations as more fall-
like conditions return.

A brief break in the unsettled conditions will be possible on
Thursday as the upper level low moves off to the east and a
shortwave ridge nudges into the region. Showers may linger across
the mountains, but will generally remain light. Afternoon high
temperatures will remain cool and are expected be in the 60s to
low 70s.

.LONG TERM /FRIDAY THROUGH MONDAY/...Ensemble guidance is in
decent agreement of another trough moving into the Pacific
Northwest over the weekend, however there remains some uncertainty
in regards to its amplitude. Regardless, this will continue to
promote below normal temperatures and chances of showers across
western Washington through much of the long term. 14
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&&

.AVIATION...
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Southwesterly flow aloft as a weak trough moves over
western Washington.

Satellite imagery is beginning to show the redevelopment of stratus
along portions of the Pacific Coast, as well as some remaining
stratus along the Cascade foothills.

Scattered clouds will remain tonight before filling back in between
09-12Z and dropping to IFR to LIFR at times through the early
morning hours, before again rising back up to MVFR after sunrise and
breaking up to VFR in the early afternoon. Mid and high level cloud
cover will increase tomorrow ahead of an incoming frontal system
that will approach the area tonight. Rain will be arriving along the
coast as we reach the end of the TAF period (around 03-06Z
Wednesday).

Southwesterly winds will continue to east this evening becoming
mostly light and variable. Winds will then turn back to
southwesterly early this morning and strengthen to 4 to 8 kt
tomorrow.

KSEA...VFR conditions this evening. Stratus will build back in early
tomorrow morning, by around 10-12Z Tuesday. Ceilings will likely be
low-MVFR to IFR, but could dip into LIFR at times between 12-16Z.
Stratus will lift and slowly scatter out Tuesday afternoon. Winds
southwesterly will ease becoming light this evening. Winds will
remain southwesterly and increase to around 4 to 8 kt tomorrow.

LH
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&&

.MARINE...Winds will continue to be northwesterly across the coastal
waters tonight. Winds will switch to southwesterly late Tuesday
night ahead of an incoming frontal system that will move through the
area waters early Wednesday morning. There is a chance for some
Small Craft Advisory winds Tuesday evening (30-60%) in the Central
Strait of Juan de Fuca as the system begins to approach the area.
Winds switch back to northwesterly Wednesday and remain like so
through the end of the week.

Seas 4 to 6 ft will continue through tomorrow before rising to 6 to
8 ft behind the frontal system Wednesday into Thursday before
subsiding back to around 4 to 5 ft through the end of the week.

LH

&&

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

$$