433
FXUS66 KSEW 311023
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
323 AM PDT Fri May 31 2024

.SYNOPSIS...High pressure will maintain mild and dry conditions
today. 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 /TODAY THROUGH SUNDAY/...

...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.

High pressure will move across northern Oregon today bringing mild
and dry conditions today with partly cloudy skies as high clouds
filter overhead in advance of the next frontal system set to cross
the region Saturday. High temperatures today will be pleasant in
the upper 60s and low 70s. A few locations may reach into the mid
70s across the Southwest Interior.

Light rain makes a return Saturday as a weak shortwave trough
moves across southern British Columbia, driving a weakening cold
front across western Washington. Rainfall amounts will light with
only a few hundredths of an inch across the lowlands, mostly
along and north of I-90. Slightly higher amounts in the Olympics
and Cascades with a 55% chance for a quarter of an inch.
Otherwise, mostly cloudy conditions in the morning, with a few
breaks of sun possible in the afternoon as a weak mid-level ridge
attempts to build across the region ahead of the next and much
stronger system. Cooler as highs drop back below normal into the
low to mid 60s.

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).

.LONG TERM /MONDAY THROUGH THURSDAY/...The atmospheric river
pattern remains in place through Monday and Tuesday, though peak
rainfall should wind down through Monday morning.

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. Highs stay on the cool
side in the upper 50s to around 60. 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.

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. 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, 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 both
Monday and Tuesday afternoons ahead of each of the fronts. Winds
gusts between 20 and 25 mph are expected, with a 20-30% chance for
gusts up to 30 mph, especially Monday afternoon. 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.

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
today with the exception of some patchy IFR/low MVFR in stratus near
the coast and Strait of Juan de Fuca this morning. High level
moisture will increase later today with ceilings lowering tonight
ahead of an approaching frontal system. MVFR conditions in light
rain will spread inland from the coast early Saturday morning.

KSEA...VFR today. Ceilings lower tonight under increasing high and
mid level clouds. MVFR in light rain expected to develop 12Z-15Z
Saturday morning. Surface winds light northeasterly this morning
becoming northwesterly 4 to 8 knots this afternoon. Winds will back
to light southerly by 12Z Saturday.

27

&&

.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

&&

.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...None.
PZ...None.
&&

$$