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000
FXUS66 KSEW 150315
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
815 PM PDT Sun Apr 14 2024

.SYNOPSIS...Weak high pressure overhead today before upper-
level troughing returns on Monday bringing cooler temps and showers
back to western Washington. Ridging will return later this week with
the potential for more warm and dry weather.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Current satellite
imagery this evening shows most of the stratus present over the
coast has pushed back to the Pacific waters. Mid and high level
clouds have begun spreading across the region ahead of our next
frontal system which is slowly approaching western Washington. The
previous discussion can be found below along with updated
aviation and marine discussions:

Timing still remains a little fuzzy as deterministic models
continue to waffle on timing...the GFS favoring a later frontal
entry than the ECMWF. Neither model seems to be particularly
impressed with precip amounts, each one showing the system to be
far weaker than in previous runs. Ensembles tending to agree with
these current reduction in expected precip amounts and both
deterministic and ensemble data all seeming to agree on letting
the chances for precip linger throughout much of Tuesday...with
ensemble QPF guidance even showing a bit of a bimodal split as
opposed to grouping precip all together as in previous runs.
Ultimately, current forecast tries to straddle the line of
incorporating the new data into the old trend...and this appears
to be the best way to go for now...keeping the earlier starting
time with the front making it to the coast at or around 12Z Monday
morning, then allowing PoPs to linger throughout much of Tuesday
with highest chances during the event still remaining over the
Cascades. Wednesday sees upper level ridging return to the area
with generally dry conditions. ECMWF and ensembles on the same
page, keeping the entire CWA dry, however the GFS wants to keep
some moisture present over the central Cascades. Unfortunately,
the NBM seems to have latched onto this as well, injecting some
slight chance PoPs where meteorologically there really should not
be any...but given that the range is 10-20 pct, ultimately there
really is not much of a difference.

Temps today still expected to be split with mid to upper 50s
expected along the coast and water adjacent locations while the
interior will top out in the mid to upper 60s. Monday and Tuesday
will see temps more uniform throughout the area as well as cooler,
with highs in the lower to mid 50s. Wednesday kick starts a new
warming trend in earnest with highs for much of the lowlands ranging
in the upper 50s to lower 60s.

18

.LONG TERM /THURSDAY THROUGH SUNDAY/...Dry conditions with warming
temperatures will remain the case for the bulk of the long term
forecast as upper level ridging will continue to be the dominant
weather feature. Deterministic models disagree on extent and
intensity and the GFS seems to insert a retrograding trough for
reasons that elude current reasoning. Ensembles definitely in
agreement for a prolonged dry stretch. Next system progged to enter
the area possibly Saturday morning or afternoon. Deterministic
models disagree and this disagreement has the trough either passing
to the south as a closed low with precip barely nicking the SW
corner of the CWA via the GFS or a weakening trough that fizzles out
while crossing according to the ECMWF. Ensembles have a wide range
of solutions, however the ensemble mean still suggests that both
Saturday and Sunday could see some, albeit minimal, precip. NBM
PoPs in the forecast remain in the Slight Chance to low-end Chance
range /10-30ish pct/...splitting the difference yet still conveying
low confidence...which would be the proper attitude toward the end
of the forecast period.

Daytime highs will continue to warm Thursday and Friday, the former
in the lower to mid 60s and the latter in the mid 60s to around 70.
Precip or no...the next system will bring temps down somewhat, with
highs both Saturday and Sunday in the upper 50s to mid 60s.

18

&&

.AVIATION...Westerly flow aloft as a cold front and upper low dig
southward into western Washington through the overnight. Onshore
flow continues at the surface with a convergence wind boundary
shifting northward, which is expected stall somewhere around PAE
tonight before a cold front begins diving southward across the area,
shifting northerly winds back southward across the Puget Sound
terminals. Mostly VFR conditions across the terminals this evening,
with the exception being HQM, where stratus has pushed back onshore
with MVFR CIGs. Elsewhere, CIGs will lower tonight, with a 30%
chance for MVFR CIGs by around 09Z through 20Z at the Puget Sound
terminals. Scattered light rain showers and drizzle likely much of
the day Monday across this region as a convergence zone with
enhanced shower activity sets up, favoring the northern half of King
County north of BFI according to latest HREF run, VSBY restrictions
look minimal, but included mention at most probable TAF sites.

KSEA...VFR this evening although high clouds are quickly thickening
from the north. South-southwest winds will prevail ahead of a cold
front that will dig southward across Western Washington tonight.
CIGs lower closer to MVFR thresholds after 09Z with a 25% chance of
MVFR or lower through 20Z as CIGS lift back to 3500-5000 foot range.
Light rain and showers expected with convergence banding through
much of the day after 10-12Z. Latest HREF guidance favors most
shower activity north of I-90, but cannot rule out brief VSBY
restrictions in scattered showers. Growing confidence in northerly
winds to shift just south of the airport with a southwest to
northeast wind shift possible in the 19-06Z timeframe with the
convergence boundary stalling somewhere in southern King County
(around 50% confidence in this wind shift). Best timing for this
potential northerly wind shift looks to be in the 22-03Z timeframe
Monday afternoon and evening.

Davis

&&

.MARINE...A cold front continues to dive south across British
Columbia, poised to cross the area waters late tonight into Monday
morning. Onshore flow continues, with gales through the Central and
East Entrance to the Strait of Juan de Fuca as of 8 PM. Gale
Warnings continue for the Central and East Strait until at 3 AM.
Small Craft Advisories remain in effect through 11 AM Monday across
the coastal waters and West Entrance to the Strait. Winds will ease
through the morning with Gales likely being dropped to Small Craft
Advisories before they quickly pick up again Monday afternoon. A
Gale Watch has been issued for the Central and East Strait along
with Admiralty Inlet Monday afternoon through early Monday night,
with Gales likely (80% confidence). Marginal chances (50% or less)
for brief SCA-level wind gusts Monday night through much of Tuesday
over the offshore waters.

Seas from 7 to 9 feet will increase closer to 10 feet tonight over
the coastal waters before gradually lower back to 7 to 9 feet
through the day Monday. Seas continue to lower through the week,
trending below 5 feet Thursday and Friday.

Davis

&&

.HYDROLOGY...No river flooding is expected through the next seven
days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 11 AM PDT Monday for Coastal Waters
     From Cape Flattery To James Island 10 To 60 Nm-Coastal
     Waters From Cape Flattery To James Island Out 10 Nm-Coastal
     Waters From James Island To Point Grenville 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville Out 10
     Nm-Coastal Waters From Point Grenville To Cape Shoalwater
     10 To 60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater Out 10 Nm-West Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Gale Warning until 3 AM PDT Monday for Central U.S. Waters
     Strait Of Juan De Fuca-East Entrance U.S. Waters Strait Of
     Juan De Fuca.

     Gale Watch from Monday afternoon through late Monday night for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 3 AM PDT Monday for Admiralty Inlet.

     Gale Watch from Monday afternoon through late Monday night for
     Admiralty Inlet.

&&

$$