805
FXUS66 KSEW 021635
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
934 AM PDT Sun Jun 2 2024

.SYNOPSIS...Atmospheric river pattern over Western
Washington today will continue through Tuesday, consisting of two
separate waves of heavier rain moving through the area. The first
will continue until early Monday morning and the second starting
Monday evening and continuing into Tuesday. This second wave will be
weaker than the first. Upper level ridging builds over the area
Wednesday and will remain in place through at least the weekend,
bringing dry and much warmer weather to the region.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...Not surprisingly, W WA pretty
socked in with clouds per satellite imagery given the incoming wet
system. Radar showing slow and steady eastward progress of initial
front which is currently just a little inland from the
coastline...however, there are some patches of pre-frontal showers
resulting in some light rainfall such as Everett, Port Townsend,
Shelton and Port Angeles. The transition to more consistent rainfall
still looks to occur in the late morning hours, possibly early
afternoon for locations east of the Sound. Forecast continues to be
on track, so see no reason for any morning updates. Will reiterate
key points for the short term forecast period before engaging in
more detailed discussion.

...Key Messages for Upcoming Active Weather Pattern...

* A late season atmospheric river pattern will develop today and
  continue with breaks through Tuesday.

* Heavy rainfall amounts of 3 to 5 inches in the mountains, with
  possible 6 inch event totals over the southern slopes of the
  Olympics, and 2 to 4 inches along the coast.

* 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.

The front currently moving in is only the first of two systems that
are expected throughout this event. Even though the front will pass
through W WA and move into E WA by late this afternoon/early this
evening, the parent upper level low moves in quickly thereafter,
ensuring persistent wet conditions tonight and much of Monday.
Latest deterministic forecasts having a difficult time showing it,
but ensembles clearly suggest perhaps a little bit of a break...or
at the very least an easing up of activity...come Monday evening or
Monday night, as there is the tiniest of gaps between the exiting
low and the next incoming front. That is not to say that the CWA
will dry out, but at the very least will see a dip in PoPs for a
narrow 6-8 hour window. The next frontal system makes its way into W
WA late Monday night/early Tuesday morning and will continue for
much of the day before tapering off Tuesday evening and Tuesday
night. Does not appear to be as much QPF with this system as with
the first one but this one-two punch of AR-style systems will still
merit keeping an eye on hydro concerns...which will be discussed
below. Also meriting vigilance will be burn scars within the area.
Although not expecting any significant activity there at this time,
continued monitoring will be necessary should precip amounts or
rates exceed those currently forecast. All of that said, the
inherited Flood Watch will remain in place without any alteration.

Along with precip, could see some breezy to locally windy conditions
with occasional gusts throughout the short term with each of these
frontal passages. Wind speeds do not look to merit any headlines at
this time, however this combination of heavy rains and increased
winds may allow for isolated instances of either branches or
entire trees being felled. Caution is encouraged.

As would be expected, temps will take a bit of a hit today when
compared to yesterday, with most locations only getting into the
upper 50s to lower 60s. Highs cool even further Monday as most
locations will sit in the upper 50s with the occasional spot hitting
60. A warming trend will kick off on Tuesday with highs returning to
the lower 60s. Overnight lows will be pretty static, with both
nights in the upper 40s to lower 50s...so not much of a diurnal
spread expected.

18

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...From Previous
Discussion...In a version of weather whiplash, an upper level ridge
builds over the area Wednesday with the ridge remaining in place
through the weekend giving Western Washington dry and increasing
warmer weather as we get to the weekend. 500 mb heights rising into
the lower 580 dms by Friday.
Low level flow never turns strongly offshore and model 850 mb
winds don`t turn easterly for much of the period Friday and
Saturday. There are a few ensemble solutions that put highs well
into the 80s Saturday but most of the solutions are are in the
lower to mid 80s. With the lack of strong offshore flow in the
models right now will stay with this idea and keep the warmest
temperatures Saturday in the mid 80s. Highs Wednesday in the mid
60s to lower 70s with 70s to lower 80s by Friday and the mentioned
mid 70s to mid 80s Saturday. Lots of the ensemble solutions are
showing an upper level blocking pattern developing over the
Eastern Pacific over the weekend. Blocks are hard to break down
this time of year. If this pattern does develop Western Washington
could be in for the longest dry spell since last August. Felton

&&

.AVIATION...Southwesterly flow will persist aloft as an upper
level low over the Gulf of Alaska digs southward towards the coast
of southern BC today. At the surface, an approaching frontal system
will bring periods of moderate to heavy rain, lower ceilings, and
gusty winds to the area today. Steady rain is moving into the
coastal terminals already and will advance into the Puget Sound
terminals by 18z.  Southerly surface winds will increase through the
mid to late morning hours, with gusts around 25 kt expected.
Widespread MVFR conditions expected for much of the day, with
periods of IFR to LIFR possible in moderate to heavy rain. With
stronger southerly flow just off the surface and initially weaker
surface winds, expect areas of LLWS at many of the terminals until
the stronger winds mix down to the surface in a more uniform
distribution of speed.

KSEA...Expect predominantly MVFR ceilings with some fluctuations
this morning ahead of main rain line. After 18z, steadier rain moves
into the terminal area with low ceilings and visibility restrictions
expected. Meanwhile, gusty southerly surface winds increase through
the afternoon and into the overnight period. LLWS concerns mostly
late afternoon with strong low level winds and somewhat weaker winds
at the surface, though this concern may fade as surface winds
increase toward 00z.                         Cullen

&&

.MARINE...More active conditions commence across the area waters
today as a strong frontal system approaches the area. Winds have
already started to increase towards small craft strength across the
coastal waters this morning. Guidance does still hint at the
possibility of a few isolated gusts to gale, especially for the
coastal waters south of Point Grenville should a coastal jet develop
ahead of the front. Gusts to gale will also be possible in the
vicinity of the front, but guidance still suggests that these will
be short-lived, with more of the widespread, persistent gales
remaining south of Cape Shoalwater. Thus, have maintained the small
craft advisories in effect for the coastal waters. Seas will sharply
build in response to the winds with this front and will be rather
steep today and will persist into Monday.

Small craft advisories also remain in effect for the eastern Strait
of Juan de Fuca, as southeasterly winds are expected to ramp up
ahead of the front this morning. Expect winds to then transition to
westerly in the wake of the frontal passage late tonight into
Monday. Though winds look marginal, have also issued an SCA for post-
frontal westerlies for the central Strait. May have to extend SCAs
with additional pushes expected down the Strait through early this
week.

SCAs have also been issued for the Northern Inland Waters and
Admiralty Inlet, as latest guidance indicates SCA strength winds
should be widespread in these zones. Have held off on Puget Sound
for now, but will need to monitor this through the day.

Overall, expect the pattern to remain active through the first half
of the week, with yet another frontal system expected to move across
the region on Tuesday. Latest guidance suggests the winds with this
next system could be stronger, with more widespread gales possible.
Seas are expected to build towards 10-13 ft. Wave periods look to
remain rather short, near 8 seconds, and will maintain steep seas at
times.

High pressure then looks to build into the coastal waters near
midweek for overall calmer conditions the second half of the week.
High pressure will persist into Thursday and looks to interact with
lower pressure inland through late in the week. 14

&&

.HYDROLOGY...From Previous Discussion...Today 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 7000 to
8500 foot range for for the first wave of the atmospheric river and
just a little lower for the second round. 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 river with the best chance to
reach flood stage is the Skokomish River in Mason county. Even with
some of the rivers only reaching action stage, these levels would be
records for this time of the year. The flood watch for Skagit,
Snohomish, King, Pierce, Mason and Lewis county remains in effect.

In addition to the high flows on area rivers, the water
temperatures in the rivers continues to be very cold. Most rivers
flowing out of the Cascades are still in the mid 40s to lower 50s
for water temperature. Felton

&&

.CLIMATE...Here are some daily rainfall records for around the
area today, Seattle 0.48 inches in 2001, Olympia 0.68 inches in
2010, Bellingham 0.48 inches in 1962, Quillayute 1.63 inches in
2010 and Hoquiam 1.35 inches in 1962. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch 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 PM this evening to 2 AM PDT Monday
     for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 2 AM PDT Monday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 8 PM PDT this evening for Admiralty
     Inlet-Northern Inland Waters Including The San Juan Islands.

     Small Craft Advisory until 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.

&&

$$