546
FXUS66 KSEW 010327
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
827 PM PDT Fri May 31 2024

.SYNOPSIS...A weak cold front will continue to move inland
tonight, bringing light rain to the region through Saturday. A
series of strong frontal systems will cross the region Sunday
through Tuesday, bringing in heavy rainfall across western
Washington associated with an unseasonably late atmospheric river
pattern. Many rivers across the region remain under a Flood
Watch. High pressure looks to build across much of the western US
for the second half of next week, brining much warmer and drier
conditions.

...Key Messages for Upcoming Active Weather Pattern...
* Late season atmospheric river pattern will develop Sunday
  through Tuesday.
* Moderate to heavy rainfall of 1-2 inches across the lowlands
  during this timeframe.
* Heavy rainfall amounts of 2-4 inches across the coast and
  mountains during this timeframe. 60% chance for 3-day totals to
  exceed 3 inches ending Wednesday morning.
* Snow levels above 4500 feet will will promote additional runoff
  with several area rivers in the Cascades forecast to enter
  Action or even Minor Flood Stage. A Flood Watch remains in
  effect across portions of western Washington.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...Light rain will spread
inland tonight as a weak shortwave trough moves across southern
British Columbia, driving a weakening cold front across western
Washington. Rain will continue to move eastward over the Olympic
Peninsula tonight and inland towards the Cascades through the
morning with light accumulations generally a tenth of an inch or
less. Otherwise, mostly cloudy conditions will persist through the
morning, with a few breaks of sun possible in the afternoon as a
weak mid- level ridge attempts to build across the region. High
temperatures will fall below normal into the low to mid 60s,
kicking off cooler conditions for the next several days as an
atmospheric pattern develops.

A much stronger frontal system will cross the Pacific Northwest
on Sunday as a shortwave trough ejects from a deep closed low over
the Gulf of Alaska. Rain will spread inland well ahead of the
warm front associated with this system by early Sunday morning,
leading the way to a wet Sunday through and through. Rainfall
rates gradually increase through the day Sunday, reaching 0.05-0.1
inches per hour Sunday night, though NBM deterministic forecast
rainfall rates keep 0.1 inch per hour rates confined to the
Cascades and Olympics through the duration of this atmospheric
river event. Peak rainfall rates are on track to occur Sunday
night into early Monday morning for much of the region, making for
a soggy and slow Monday morning commute. Models continue to show
24-hour rainfall totals through Sunday night of 1-1.5 inches
across the lowlands (60% chance for 1-day totals of at least 1
inch by 5 am Monday, with lower chances to around 30% at lower
elevations near Puget Sound) and 2-3 inches across the Cascades
and Olympics (45% chance for 1-day totals of at least 3 inches by
5 am Monday across the southwestern Olympics and Cascades in
Snohomish County).

Though rainfall rates will ease throughout the day Monday, breezy
and wet conditions will linger through the day as additional
rounds of rain move across western Washington. A few lightning
strikes also cannot be ruled out (20% probability) Monday
afternoon across western Washington as weak instability develops
due to cold air aloft associated with the strong shortwave trough
moving overhead. Additional 24-hour rainfall amounts from Monday
morning through Tuesday morning will range from half to three
quarters of an inch over the lowlands, one to one and a half
inches over the Olympics, and one to two inches over the Cascades.

Ensemble guidance brings a brief lull in rainfall to the lowlands
late Monday night, but will provide little relief to the wet
conditions as the atmospheric river pattern continues into
Tuesday. Another round of moderate to heavy rainfall will return
to the region ahead of yet another front, expected to pass across
the region throughout the day on Tuesday. PWATs are lower with
this second surge as are rainfall amounts. Rainfall will gradually
subside in the transition from stratiform rain to showers Tuesday
afternoon with lighter shower activity lingering overnight.
Showers look to linger through much of the day Wednesday post-
front, with the 500 mb flow pattern favoring Puget Sound
Convergence Zone formation across northern portions of the sound
from Whidbey Island into northwestern Snohomish and Skagit
Counties.

Total rainfall amounts between Sunday morning through Wednesday
morning will likely range from 1 to 2 inches across the lowlands
and 2 to 5 inches across the mountains, though some uncertainty
still remains in how strong this atmospheric river will be.
Ensembles show a high (70%) chance for at least 4 inches across
the southwest Olympics and Cascades in King, Snohomish, and Skagit
Counties. Localized amounts of up to 6 inches also cannot be
ruled out across central Snohomish County according to the NBM
(40% chance), which is where the highest event total liquid
precipitation totals are likely to occur. A Flood Watch has been
issued for many of the area rivers for local rises that may exceed
minor flood stage.

A few periods of breezy southwest winds are likely ahead of each
of the aforementioned frontal systems. Winds gusts between 20 and
25 mph are expected on Monday and Tuesday, with a 20-30% chance
for gusts up to 30 mph. Strongest wind gust signal is across
Whidbey Island and northward through the San Juan Islands and into
the North Interior.

High temperatures heading into early next week will continue to
cool under the deep troughing pattern. Upper 50s to mid 60s are
expected for Saturday while temps cool even further with both
Sunday and Monday only seeing highs ranging in the mid 50s to
around 60. The increased cloud cover from these systems will
hamper the diurnal spread as overnight lows throughout the near
term ranging in the upper 40s to lower 50s.

Davis/Lindeman

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Upper-level ridging
will slowly build across the Western US Wednesday and through the
remainder of the week. Showers will gradually dissipate through
the day Wednesday as drier air moves in, with potential for
lingering light shower activity into early Thursday over the
mountains. A warm up then looks to commence as high temperatures
climb into the 70s and near 80 by Friday. The 00Z run of the GFS
and several additional GEFS members now bring a frontal system
across the region Friday through Sunday, though the EC and ENS
members largely keep the ridge in place. This can be seen in the
long range ensemble cluster analysis, with the GEFS favoring a
weaker ridge and the EC favoring ridge city. Definitely something
to watch in the coming days. Didn`t deviate from the NBM Thursday
and Friday given this uncertainty, which keeps highs above normal,
though mostly in the upper 70s rather than in the 80s as EC
ensemble is predicting. For now, HeatRisk values remain the
yellow/minor risk category Friday.

18/Davis

&&

.AVIATION...A weak upper ridge will continue eastward across
Western Washington tonight with northwest flow aloft becoming west
to southwesterly. VFR conditions will prevail this evening before
ceilings lower later tonight ahead of an approaching weak frontal
system. MVFR conditions in light rain will spread inland from the
coast early Saturday morning.

KSEA...VFR into tonight. Ceilings lower overnight under increasing
high and mid level clouds. MVFR in light rain expected to develop
12Z-15Z Saturday morning. Surface winds northerly with occasional
shifts to the northeast or northwest 4 to 8 knots this evening
before shifting to light southerly by 12Z Saturday. 27/18/McMillian

&&

.MARINE...Surface ridging over the coastal and inland waters will
weaken this evening ahead of a frontal system that will dissipate as
it moves onshore Saturday morning. A stronger front will approach
the waters Sunday night then sweep onshore early Monday morning.
This system is likely to produce headlines for the coastal waters on
Sunday. Unseasonably strong post-frontal onshore flow will likely
generate headlines for both coastal and much of the inland waters
late Sunday night into Monday. Another front is expected to arrive
on Tuesday with elevated winds and choppy seas requiring additional
headlines. 27/18

&&

.HYDROLOGY...A late season atmospheric river pattern is expected
to develop Sunday through Tuesday, bringing ample rainfall to
western Washington. Rainfall totals of 1 to 2 inches across the
lowlands and 2 to 4 inches, with a 40% chance for totals of up to
6 inches in central Snohomish County. With snow levels forecast to
remain mostly above 5000 feet, runoff will be increased with
river likely to run unusually high for this amount of rainfall
after a relatively benign past month or two. Currently, the
Snohomish, Skykomish, White, Skagit, and Skokomish Rivers are
forecast to approach or enter Action Stage, with the Snoqualmie
River forecast to near Minor Flood Stage at Carnation by Monday
afternoon. This may impact use of the river flood planes that are
normally dry this time of year. A Flood Watch remains in effect
for much of western Washington. Area rivers are expected to crest
between Sunday evening and Tuesday afternoon.

Davis

&&

.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.
&&

$$