119
FXUS66 KSEW 011037 CCA
AFDSEW

Area Forecast Discussion...CORRECTED
National Weather Service Seattle WA
337 AM PDT Sat Jun 1 2024

.SYNOPSIS...Weak system moving through Western Washington today.
Rare late spring atmospheric river taking aim at the area
beginning Sunday. Rain, heavy at times for this time of year,
Sunday through Monday afternoon. Rain continuing into Tuesday but
with the atmospheric river weakening rainfall amounts will be
lighter. Upper level ridge building over the area Wednesday
through Friday for a quick transition to drier and warmer weather.

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

* Late season atmospheric river pattern will develop Sunday
  through Tuesday.

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

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

* Snow levels above 6500 feet will 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.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...Happy Meteorological Summer.
Certainly not summer like weather for the next few days. Currently
cloudy skies over Western Washington. Doppler radar has a large
area of light precipitation covering most of the area. Surface
observations reporting much of the precipitation indicated on the
radar is not reaching the ground. Temperatures at 3 am/10z were
in the 50s.

Weak system moving through the area this morning into the
afternoon hours keeping a chance of rain in the forecast. Rainfall
amounts will be light, less than a tenth of an inch. Cloudy skies
will keep high temperatures only in the upper 50s to mid 60s.

A little break in the action tonight with the leading edge of the
approaching atmospheric river still offshore by 12z Sunday. With a
130-140 knot jet core associated with the atmospheric river there
is a chance the rainfall could arrive a little earlier than the
models are indicating so have added chance pops to the interior
for early Sunday morning. Lows under cloudy skies in the 50s.

Wet day in store on Sunday as the atmospheric river takes aim at
the Pacific Northwest. Highest PWAT values moving into Oregon with
1.4 to 1.6 inch values along the Central Oregon coast but PWAT
values in Western Washington are still in the 1.0 to 1.2 inch
range. Air mass warming up with snow levels rising to 6500 to
8500 feet. 850 mb winds increasing during the day and by 00z
Monday will be in the 25 to 40 knot range out of the southwest.
This will increase the rain rates over the Olympics and Cascades.
Highs will only be a few degrees warmer than the lows, a couple of
degrees either side of 60.

Moderate to heavy rain continuing Sunday evening until the first
shortwave embedded in the flow aloft moves through Western
Washington. Rainfall rates will ease overnight in the lowlands but
west southwesterly 850 mb winds in the 20 to 30 knot range will
keep heavier rain continuing especially in the Cascades. Snow
levels remaining high so only the tops of the volcanoes will see
any snow. Lots of daily rainfall records for Sunday will be in
jeopardy. See climate section for more details.

Another wave embedded in the flow aloft moving through Monday
afternoon with rainfall rates increasing Monday morning before
dropping off significantly as the system moves through Monday
afternoon. Highs once again will be around 60.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Extended models in good
agreement with one more disturbance moving through Tuesday. This
feature is weaker than the first two with lower PWAT vales and
weaker winds aloft. Heaviest rain along the coast and over the
Olympics with lesser amounts over the Cascades and the interior.
Snow levels remaining elevated with most of the precipitation in
the mountains falling as rain.

Models also in good agreement with an upper level ridge building
quickly overt the area Wednesday and Thursday resulting in a quick
drying and warming trend. By Thursday highs in the 70s will be
common across the interior with highs near 70 along the coast.

Some differences in the models start to show up FRiday with the
ECMWF continuing to keep a high amplitude ridge over the area
while the GFS has the ridge further to the east with southerly
flow aloft over the area. This could introduce some mid level
moisture and combined with the slightly unstable air mass over the
higher terrain could produce some convection especially over the
Cascades late in the day. Ensemble solutions are not much help
with both model ensembles supporting the operational runs. Most of
the clusters supporting the drier solutions with the ridge further
to the west than the GFS solution so will keep Friday dry for now
with the warmer locations pushing 80 degrees. Felton

&&

.AVIATION...A weak upper ridge will continue to push eastward
across Washington this morning as an upper level trough approaches
from the west. Flow aloft will become west to southwesterly. At
the surface, a frontal system will move onshore and dissipate,
bringing light precipitation to the region. Most likely areas to
receive precipitation are the coast and the interior terminals
north of KPAE. VFR conditions continue to prevail this morning,
but expect ceilings to lower somewhat later this morning ahead of
the approaching weak frontal system. MVFR conditions will be
possible at times in light rain. Expect rain to spread inland from
the coast this morning.

.KSEA...VFR conditions early this morning. Ceilings still look
to gradually drop under increasing high and mid level clouds.
MVFR in light rain possible between 15Z-18Z this morning. Surface
winds northerly with occasional shifts to the northeast or northwest
4 to 8 knots. Winds then shift to light southerly through the
morning today. McMillian/14

&&

.MARINE...A frontal system moving across the coastal waters this
morning will move onshore and dissipate. A stronger front will
approach the area waters on Sunday, bringing the next round of
gusty winds to the region. Latest probabilistic guidance has a
50-70 percent chance of gusts occasionally reaching gale across
the coastal waters and eastern Strait of Juan de Fuca as the
frontal system moves across the area. However, guidance does not
suggest frequent gusts and maintains the strongest gusts south of
the coastal waters for now, so have held off on any gale watch
issuance for now. Another frontal system will then move across
the area waters on Monday, quickly followed by another on Tuesday
for an active period. High pressure then looks to build back into
the coastal waters on Wednesday.

Seas currently continue to hover between 2 to 4 feet offshore this
morning. Expect seas to build towards 4 to 6 feet over the weekend,
before building towards 8 to 11 feet early next week. 14


&&

.HYDROLOGY...This could be a record breaking event for river
levels around the area for this time of year. 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. Right now the forecast only has only one
river reaching flood stage, the Snoqualmie at Carnation with
additional rivers flowing out of the Central Cascades and Southern
Olympics reaching action stage. The flood stage for the
Snoqualmie at Carnation is 54.0 feet. The forecast is for the
river to crest right at flood stage. The Snoqualmie at Carnation
has never been above 52.5 feet this time of the year. The
Snoqualmie River at the Falls is currently forecast to crest
under flood stage but also at the highest level it has ever been
for this time of year.

The flood watch for Skagit, Snohomish, King, Pierce and Mason
counties remains in effect. Felton

&&

.CLIMATE...Here are some daily rainfall records for around the
area for Sunday June 2nd, 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.

Monthly rainfall normals for the entire month of June are, Seattle
1.45 inches, Olympia 1.46 inches, Bellingham 1.61 inches,
Quillayute 3.30 inches and Hoquiam 2.01 inches. It is certainly
possible that some locations could surpass their normal monthly
June rainfall in the first 3 days of the month. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flood Watch from Sunday morning 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-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...None.
&&

$$