519
FXUS66 KSEW 292317
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
417 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.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY/...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/...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

&&

.AVIATION....W/NW flow aloft as a transient ridge tracks overhead.
However, we`ll see a brief switch to SW flow as a shortwave enters
before more northerly flow develops into Tuesday. VFR across area
terminals this afternoon with scattered showers. Convergence zone
activity will persist this evening. Hi-res ensemble guidance has the
bulk of the zone over KPAE before fizzling out late tonight. MVFR
CIGs are at play again late tonight into early Tuesday morning,
mainly for Puget Sound terminals. Any lingering low stratus should
burn off to VFR conditions by midday Tuesday. W/SW winds 5-10 kts
for most terminals this afternoon becoming lighter overnight.

KSEA...VFR so far this afternoon with scattered showers. SW surface
winds between 5-10 kt will trend more towards the S around 10-12z
Tuesday. MVFR is set to return late overnight (after 07z) before a
rebound to VFR around midday Tuesday with a switch to northerly
surface winds between 5-10 kt.

McMillian

&&

.MARINE...SCA remain in effect over the central and eastern strait
until tonight. Afterwards, a brief beak looks to set up but an
additional SCA over the same zones is likely needed for 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. Will continue to
monitor them if additional headlines are needed.

Seas 7 to 9 feet, then rising near 10 feet briefly into Tuesday as
the next weak surface low enters. Seas then look to subside to 3 to
6 feet Wednesday onward.

McMillian

&&

.HYDROLOGY...River flooding not expected in the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
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...Small Craft Advisory until 10 PM PDT this evening for Central
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S.
     Waters Strait Of Juan De Fuca.

&&

$$