-- Discussion containing changed information from previous version are highlighted. --
619
FXUS66 KSEW 020339
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
839 PM PDT Sat Jun 1 2024

.SYNOPSIS...Weak system moving through Western Washington this
afternoon. A rare late spring atmospheric river will take aim at
the area beginning Sunday morning, with rain, heavy at times,
through Monday afternoon. Rain will continue into Tuesday but
with the atmospheric river weakening rainfall amounts will be
lighter. An upper level ridge will quickly building over the area
into Thursday through Friday for a transition to drier and warmer
weather.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...

-- Changed Discussion --
...Key Messages for Upcoming Active Weather Pattern...

* A late season atmospheric river pattern will develop Sunday
  through Tuesday.

* Heavy rainfall amounts of 2 to 4 inches along the coast and 3 to
  5 inches in the mountains during this timeframe.

* Moderate to heavy rainfall of 1 to 2 inches across the lowlands
  during this timeframe.

* Snow levels above 6500 feet will promote additional runoff with
  several area rivers flowing out of the Cascades forecast to
  enter action or even minor flood stage. A Flood Watch remains in
  effect across portions of western Washington, with the extension
  into Lewis County.

A weak frontal system will continue to move across western
Washington this afternoon for some light rain showers mainly
across the Olympic Peninsula and the northern portion of the area.
High temperatures this afternoon will remain in the upper 50s.

Tonight, a much stronger frontal system will move into the Pacific
Northwest as a shortwave trough gets ejected from a closed low
over in the Gulf of Alaska. An atmospheric like pattern starts to
develop as the jet pushes a plume of moisture from the Pacific
right along the coastline. Rain will start to spread inland late
tonight into Sunday morning, with rainfall rates gradually
increasing into the afternoon. By Sunday night, rainfall rates
will generally be around 0.05 to 0.10 inches per hour.. it is
something to note that generally these rainfall rates will be
largely focused in the southern slopes of the Olympics and in the
Cascades. Model PWAT values around 1.0 to around 1.2 inches and
increasing low to mid level southwesterly winds will only enhance
the precip rates over the Olympics and the Cascades.

Heading into Monday morning, it is definitely looking like a wet
and soggy commute, with 24 hour rain totals by Monday morning
looking to be around 0.50 to 1.0 inches for the lowlands, 2 to 3
inches in the Cascades, and at least 3 inches in the Olympics.

Rainfall rates will start to ease by Monday afternoon, but wet
conditions will continue with some breezy winds as a shortwave
trough moves overhead western Washington. Cooler air aloft will
help develop weak instability, with the possibility for a few
lightning strikes Monday afternoon into Monday evening. High
temperatures will remain in the upper 50s, even lower 60s on
Monday.

A quick break in the action Monday night before the atmospheric
river pattern continues with another round of moderate to heavy
rainfall through Tuesday. This disturbance looks a little bit
weaker than the previous one on Monday, with weaker PWAT values
and weaker southwesterly winds aloft... although winds at the
surface will pick up ahead of the front, for breezy conditions
across the area. Again, the heaviest of the precip will be
focused in the Olympics and in the Cascades, with lower values in
the lowlands. Total amounts into Wednesday morning look to be
around 2 to 4 inches along the coast, 3 to 5 inches in the
Cascades and the Olympics.. and around 1 to 2 inches for the
interior lowlands.

It is important to note that especially with the start of June,
people will be in the mountains for recreation. Small streams and
rivers will be running unusually high and still remain cold, so
be weather aware and check if you do go. Always have ways to get
watches or warnings.

.LONG TERM /THURSDAY THROUGH SUNDAY/...After the funnel of
moisture earlier on in the week, model guidance is indicating in
an upper level ridge quickly building into the Pacific Northwest,
for warmer and drier conditions into the end of the week. Heading
into Thursday, afternoon high temperatures generally in the 70s
can be found pretty much throughout most of the area, even along
the coast.

Uncertainty does exist heading into the weekend.. as the GFS
ensemble pushes the ridge further east.. with a frontal system
brushing by the area on Friday. Although the EC and the ENS
members keep the ridge pretty much parked over western Washington.
Taking a look at the cluster guidance, it does look more
uncertain on which solution remains victorious. Given the
uncertainty, did not deviate much from the NBM, putting afternoon
high temperatures in the mid to upper 70s.

Mazurkiewicz
-- End Changed Discussion --

&&

.AVIATION...
-- Changed Discussion --
Southwesterly flow aloft as the next frontal system
approaches the region. Expect mostly VFR conditions through the
interior, with lowering ceilings toward the coast and northwestern
portions of the area as the front pushes eastward. Expect a return
of lower clouds, additional rain, and gusty southerly winds tonight
and especially Sunday across the region.

KSEA...VFR conditions continue with southwesterly surface winds and
a few few scattered showers. Ceilings will gradually lower further
with increasing rain coverage late tonight with steadier rain and
MVFR ceilings likely after 12z through much of Sunday.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
Relatively benign conditions across the waters this
evening, but expect much more active conditions starting late
tonight and early Sunday as the next frontal system approaches the
area. This next front will bring increasing winds with advisory
strength winds across the coastal waters by daybreak Sunday. There
remains around a 40% chance of a burst of gale gusts if a coastal
jet develops ahead of the front, with the best chances south of
Point Grenville. This would be most likely within a few hours of
noon Sunday, and as such have issued advisories and added this
wording for the southern coastal zones. Give this confidence level
and that gales are not expected to be sufficiently widespread if
they do develop, will refrain from any gale headlines at this time.
Meanwhile, seas will also sharply build in response to the winds
with this front and will be rather steep Sunday and into Monday.

An SCA remains in effect for the eastern Strait of Juan de Fuca for
pre-frontal southeasterly winds as well. It`s possible this will
need to be extended to other interior zones, but confidence remains
lower for now and will take another look with the next update.

The pattern remains active with additional frontal systems bringing
rounds of stronger winds later Monday into Tuesday. This will bring
steadily increasing seas over the coastal waters, with seas expected
to build into the 10 to 13 ft range early in the week.
Meanwhile, wave periods remain rather short (around 8 seconds),
maintaining steep seas. High pressure is then likely to build late
in the week.    Cullen/McMillian
-- End Changed Discussion --

&&

.HYDROLOGY....The Sunday through Tuesday forecasted rain totals
are still 3 to 5 inches for the Olympics and the Cascades, with
the possibility of 6 inch bullseyes along the southwest slopes of
the Olympics. Snow levels will be in the 6000 to as high as 8500
foot range for most of the event. West southwesterly flow aloft
favors the heaviest rainfall to be along the southwest slopes of
the Olympics and the Central Washington Cascades. Several rivers
flowing out of the Central Cascades and Southern Olympics are
forecast to reach action stage. The flood watch for Skagit,
Snohomish, King, Pierce and Mason counties remains in effect, with
the addition of Lewis County.

Felton/Mazurkiewicz

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
WA...Flood Watch from 5 AM PDT Sunday through Wednesday morning for
     Bellevue and Vicinity-East Puget Sound Lowlands-Everett and
     Vicinity-Hood Canal Area-Lower Chehalis Valley Area-
     Olympics-Seattle and Vicinity-Southwest Interior-Tacoma
     Area-West Slopes North Cascades and Passes-West Slopes
     North Central Cascades and Passes-West Slopes South Central
     Cascades and Passes-Western Skagit County.

PZ...Small Craft Advisory from 8 AM to 11 PM PDT Sunday for East
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 AM Sunday to 5 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.
-- End Changed Discussion --

&&

$$