649
FXUS66 KSEW 312253
AFDSEW

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

.SYNOPSIS...High pressure will maintain mild and dry conditions
into this evening. A weak cold front will brush the region Saturday
bringing light rain. Two stronger frontal systems will cross the
region Monday and Wednesday bringing atmospheric river moisture and
heavy rainfall to much of western Washington Sunday through Tuesday.
High pressure looks to build across much of the western US for the
second half of next week brining much warmer and drier conditions.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...Upper level ridging continues
to shift east with the ridge axis approximately over the Cascades at
the time of this writing. Mostly clear skies over W WA as of 330 PM
PDT this afternoon with some slight infiltration of high level
cirrus. These high level clouds will only increase as the next weak
frontal system approaches. Overnight will see mid and low level
clouds filter in as the front draws closer. This front is expected
to fall apart as it passes through the area Saturday. Best chances
for rainfall appear to be on the Olympic Peninsula and the norther
third of of the CWA, including portions of Whatcom and Skagit
counties. The remainder of the area could still see some precip, but
PoPs only enter into the chance category at best.

Moving into Sunday, W WA enters into an unseasonably active weather
pattern. Key messages are as follows.

...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 mountains during
  this timeframe. 55% chance for 3-day totals to exceed 3 inches
  in 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.

Sunday, the next much stronger frontal system will begin moving
towards the Pacific Northwest as a shortwave trough ejects from a
strong 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, 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 will occur Sunday night into early Monday
morning, making for soggy and slow Monday morning commute. 1-day
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). This
atmospheric river will continue to bring very wet conditions
throughout the day Monday...however it is worth noting that the peak
rainfall should wrap up during the morning hours. QPF amounts, while
still elevated, do begin to ease some by Monday afternoon.

Though rainfall rates will subside through the day Monday, breezy
and wet conditions will linger through the day as additional rounds
of rain move across western Washington. Can`t rule out a few
lightning strikes Monday afternoon across the entire area as weak
instability develops thanks to cold air aloft associated with the
strong shortwave trough moving overhead. Currently there is a 20%
chance for thunder Monday afternoon. Additional 1-day 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 1-2 inches over the Cascades.


High temps for the period will continue to cool from the peak temps
of today. 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.

18/Kristell/Davis

.LONG TERM /TUESDAY THROUGH FRIDAY/...The atmospheric river
pattern remains in place for Tuesday.

Ensemble guidance brings a brief lull in rainfall intensity late
Monday into Monday night, before another round of moderate to
heavy rainfall moves back into the region ahead of yet another
front, expected to pass through on Wednesday. PWATs are lower with
this second surge as are rainfall amounts. Rainfall will gradually
subside in the transition from stratiform rain to showers through
Tuesday afternoon and 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 spread in possible rainfall amounts during the Sunday
morning through Wednesday morning timeframe range from 1 to 3
inches across the lowlands to 2 to 6 inches across the mountains,
though the most likely amounts range from 1 to 2 over the
lowlands to 2 to 5 inches over the mountains. Probability for
3-day (5 am Sunday - 5 am Wednesday) rainfall amounts of at least
2 inches across the lowlands is around 20% and 70% for at least 4
inches across the southwest Olympics and Cascades in King,
Snohomish, and Skagit Counties. Localized amounts of up to 6
inches 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 few periods of breezy to gusty southwest winds are likely again
Tuesday afternoon ahead of each of the aforementioned secondary
front. Winds gusts between 20 and 25 mph are expected, 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.

Upper-level ridging will slowly build across the West Wednesday
through the remainder of the week. Showers will gradually
dissipate through the day Wednesday, lingering 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 slide eastward across Western
Washington today with northwest flow aloft becoming west to
southwesterly tonight. VFR conditions will prevail across the region
into this evening before high level moisture will increase and
lowering ceilings 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 tonight 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

&&

.MARINE...Surface ridging over the coastal and inland waters will
weaken today 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. 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.
&&

$$