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233
FXUS66 KSEW 300328
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
825 PM PDT Mon Apr 29 2024

.SYNOPSIS...Cool upper level trough over Western Washington moving
east Tuesday. Brief upper level ridge Tuesday night. Systems
Wednesday night into Thursday and Friday night and Saturday could
move south of the area. Another front arriving early next week.

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.SHORT TERM /TONIGHT THROUGH THURSDAY/...
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Current IR satellite
imagery showing some cooling cloud tops at the time of this writing,
however radar trend continues to show any active cells waning in
strength. While no lightning strikes currently showing, there were a
handful of strikes in the Seattle metro area earlier in the
evening, even resulting in a couple of rumbles of thunder here at
the NWS office.

Scattered showers still out there over portions of W WA this evening
and as the upper level trough continues to stretch eastward, that
risk will remain throughout the night. That said, best chances for
thunder are mainly in the rearview, with game over expected after 10
PM PDT tonight. A secondary upper level low within the trough will
keep showers in the forecast during the overnight and into Tuesday
morning.

Inherited forecast remains on track with no evening updates needed
at this time. For forecast details, please refer to the Previous
Discussion section below.

18

From Previous Discussion...Satellite imagery shows upper level
trough over Western Washington this afternoon. Doppler radar has
numerous showers with the highest concentration over the Cascades
and the lowest along the coast. Temperatures at 4 pm/23z were in the
lower to mid 50s.

Upper level trough remaining over Western Washington tonight. Very
cold air aloft with 500 mb temperatures in the -30 to -32C range.
Lifted index values in the +2 to -2 range. With some afternoon
sunshine temperatures have risen above the convective temperatures
which are in the lower 50s. Have left a chance of thunderstorms
in the forecast for this evening. The thunderstorm threat will end
quickly once the sun goes down. Scattered showers continuing
overnight into Tuesday morning with the trough still in place. It
will be another cool for the end of April morning Tuesday with
lows in the mid 30s to lower 40s.

Upper level trough remaining over Western Washington Tuesday for a
repeat of Monday`s weather. Temperatures aloft slight warmer as
the trough begins to move east in the afternoon. Have left the
mention of thunderstorms out of the forecast Tuesday afternoon.
If the trough is a little slower to move east look for a chance of
thunderstorms to get added to the Tuesday afternoon forecast.
Highs once again below normal, in the mid and upper 50s.

Trough moving off to the east Tuesday night with a weak upper
level ridge moving over the area. Still a chance of showers
Tuesday evening as the daytime heating comes to an end but by
midnight the shower activity should be over. With the decreasing
cloud cover the colder location early Wednesday morning will be
near freezing. For the remainder of the area lows in the mid 30s
to lower 40s.

Wednesday looks to be the dry day of the short term with Western
Washington in between systems. Upper level ridge will shift east
during the day but the next system will just be crossing 130W at
00z Thursday. Temperatures aloft still a little cool but with some
sunshine high temperatures "warming" to the mid 50s to lower 60s.

Models in good agreement with the next system tracking south of
the area. The question is how far north with the rain associated
with the system extend. Will keep pops in the chance category with
likely pops along the coast. Increasing cloud cover will make lows
a little warmer, in the lower to mid 40s.

System continuing to move southeast Thursday with a weak upper
level trough over Western Washington. With the trough overhead
will keep some chance pops in the forecast for most of the area.
Good chance the Northwest Interior and the San Juans are dry.
Highs Thursday near 60.

.LONG TERM /FRIDAY THROUGH MONDAY/...From Previous
Discussion...Model solutions remaining inconsistent for most of the
extended period. There is a general agreement that a cold upper
level low will develop offshore Friday night. The ECMWF tracks this
low into Oregon Saturday, close enough for at least showers over
Western Washington. The GFS track is further west which would limit
the precipitation chances to the coast. The ECMWF keeps the low over
Oregon Saturday night into Sunday while the GFS has the low well
south of the area. Ensemble solutions mostly support the operational
runs. There is better agreement with a splitting system arriving
Sunday night into Monday. With the lack of consistency will have to
go with chance pops for most of the extended period. Pops a little
higher, in the likely category, with better agreement Monday.
Temperatures remaining below normal through the period with highs
in the mid 50s to lower 60s and lows in the 40s. Felton
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.AVIATION...
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.An active Puget Sound Convergence Zone has
maintained showers and isolated thunderstorms roughly along a KCLM-
KPAE line extending eastward into the Cascades. This will remain in
place into the overnight hours with scattered showers elsewhere.
Expect VFR to higher MVFR conditions this evening, with lower
ceilings nearest to PSCZ. Again expect MVFR ceilings to develop
through most of the Puget Sound terminals as well as along the coast
overnight as showers taper and the air mass begins to stabilize.
Surface winds ease overnight as low moves inland and low-level flow
turns northerly behind the low. Another round of showers and
isolated lightning again possible early in the day Tuesday with
predominately VFR ceilings areawide.

KSEA...Expect continued southerly surface winds and VFR ceilings
with the lingering PSCZ activity remaining well north of the
terminal. With the air mass stabilizing a bit overnight, expect a
return to MVFR ceilings after 10z through the morning. Northerly
surface winds likely to develop in the afternoon reaching 8-10 kt.
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.MARINE...
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Lingering gusty winds through the central and eastern
Strait of Juan de Fuca this evening, but winds already easing other
than near stronger showers. May again see a low-end advisory
strength push Tuesday evening through Wednesday morning. A series of
frontal systems will make their way into the region Wednesday night
into Thursday followed by another one Friday night into Saturday.
Seas will build toward 10 feet later Tuesday with the next low
moving into the waters, but then subside well below advisory levels
by Wednesday afternoon through the end of the week.
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.HYDROLOGY...River flooding not expected in the next 7 days.

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.SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Winter Weather Advisory until 2 AM PDT Tuesday for West Slopes
     North Cascades and Passes-West Slopes North Central
     Cascades and Passes-West Slopes South Central Cascades and
     Passes.

PZ...None.
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