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604
FXUS66 KSEW 310336
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
836 PM PDT Thu May 30 2024

.UPDATE...
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No changes to the overall forecast this evening. Please
see the aviation and marine sections below for updates to those
forecasts.
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&&

.PREV DISCUSSION... 
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/issued 306 PM PDT Thu May 30 2024/

.SYNOPSIS...Zonal flow and high pressure will dry out western
Washington the rest of Thursday. Mix of clouds of sun will
continue through Friday. A weak front will bring a chance of
showers Sunday. A much stronger late season atmospheric river will
bring moderate to heavy rainfall from Sunday into Tuesday. This
will cause some rivers to rise early next week, with flooding
possible. Temperatures will remain around average, with the
potential for a significant warm up late next week.
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&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...Synoptic flow aloft has
become more zonal in wake of yesterdays trough, now weakening over
Montana and the Rockies. This has allowed for high pressure to
sneak into the region, as a ridge builds to the west over B.C.
Canada. With some mid level moisture left, mix of low cumulus and
stratus will stick around today into Friday. High temperatures
will improve into the low to mid 60s Thursday, and mid 60s to low
70s by Friday (slightly above average). Winds will remain light
out of the north.

A weak upper level trough perturbation will immediately follow the
ridge as it moves east and out of the state Saturday. Expect a
weak occluded cold front to pass through during the first part of
the day. This will come with a slight chance of rain showers,
primarily in the north Cascades, with most likely a tenth to two
tenths of an inch of rain. Sky coverage will remain mostly cloudy
on Saturday elsewhere, with high temperatures dipping down to the
low and mid 60s.

Sunday begins the first of a multi-day late-season atmospheric
river event across the region. A strong shortwave trough
(especially for this time of year) is expected to dig down the
B.C. coastline Sunday. This will drive a 300 mb jet streak into
the Pacific coastline, driving an abundance of moisture advection
into the region. Snow levels will remain above 5,000 feet which
will keep precipitation as rain everywhere except the tallest
peaks. As a surface system approaches from the west Sunday
afternoon, periods of moderate to heavy rain bands are expected to
make their way into the state, beginning over the coast/Olympics,
and spreading inland into the Cascades. This heavy rain will
continue into Monday and Tuesday. Please see the long-term section
for additional details.

.LONG TERM /MONDAY THROUGH THURSDAY/...The previously mentioned
upper level trough and surface system will continue to spread
heavy rain inland, into Puget Sound and the Cascades through
Tuesday. The heaviest of the rain with this system will fall
Sunday night into Monday. Additional rain bands will be possible
rest of Monday into Tuesday, but rainfall rates are not expected
to be as high as Sunday night/Monday morning. Given the strength
of the system/synoptic forcing, thunder will be possible at times.
When the system wraps up Tuesday, rainfall amounts will range
substantially between lowlands and mountains. Puget Sound will
have a shot of totalling out at 1.25 to 2 inches (10-20% chance
this exceeds 2 inches), and the Cascade/Olympics will most likely
see 2 to 4 inches (10-20% chance exceeding 5 inches). Given the
nature of this system, combined with warm temperatures and soils,
people should be on guard for potential flooding (from urban areas
to rivers), and monitor for updates to the forecast, as well as
potential headlines. Please see the hydrology section for river
flooding details.

In addition to rain, a few periods of gusty southwest winds are
possible Monday and Tuesday afternoon. lowland areas will have
the potential to see wind gusts of up to 20 to 25 mph. The
windiest points appear to be west of I-5 from Everett to
Bellingham, where there is a 10% chance of winds gusting over 30
mph.

CPC guidance has been advertising a 50 to 60 percent above
average temperatures middle to end of next week, with the pattern
appearing to show strong upper-level ridging (as ensemble guidance
points to). This outlook goes beyond the scope of the forecast,
but the days 5 through 7 periods will see a noticeable warm up
into the 70s. Expect skies to clear out also by Wednesday through
the end of the work week.

HPR

&&

.AVIATION...
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A weak upper ridge is building into Western Washington
tonight with northwest flow aloft. VFR conditions in place and will
continue throughout the TAF period with the exception of HQM, which
could see some low level marine clouds push in overnight. Surface
winds northerly for most terminals with the exception of KCLM where
westerlies remain in place.

KSEA...VFR conditions currently and will continue throughout the TAF
period. Cloud bases continue to rise as atmosphere gradually dries.
Northerly winds 4-8 kts with minor variations to NW or NE at times.
18/McMillian
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&&

.MARINE...A broad surface ridge over the area will maintain
light onshore flow today into Friday. Winds have eased in the Strait
throughout the morning and early afternoon, so little concern for
any additional headlines there for the afternoon forecast package. A
weak front will move onshore early Saturday with little to no
impact. However, a secondary front on Sunday will be relatively
strong for early June and will likely require headlines for portions
of the the coastal waters. Post-frontal onshore flow on Monday could
be strong enough to require headlines for much of the inland waters
in addition to the coastal waters if model trends remain as they
are. 18

&&

.HYDROLOGY...
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A late season atmospheric river will bring
moderate to heavy precipitation from Sunday afternoon through
Tuesday across all of western Washington. Heavy rainfall rates are
possible over the southwest Olympics, and the Cascades. Total
rainfall amounts will range from 1 to 2 inches in the lowlands,
and 2 to 4 inches in the mountains. Snow levels above 5000 feet
will result in most precipitation falling as rain. This will
result in rivers running unusually high, with the Skagit,
Snohomish, Skykomish, White, and Skokomish Rivers potentially over
action stage. The Snoqualmie River potentially will reach minor
flood stage. This may impact any use of the river flood planes
that are normally dry this time of year. Rivers will crest
sometime between Sunday and Tuesday.

HPR
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&&

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

&&

$$