170
FXUS66 KSEW 301617
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
917 AM PDT Thu May 30 2024

.SYNOPSIS...High pressure will briefly build across the Pacific
Northwest with slight warmer temperatures and drier conditions. A
progressive weather pattern takes shape this weekend and into the
first half of next week with a parade of frontal systems crossing
the region Friday night, Monday, and Wednesday, bringing
additional periods of rainfall and cooler temperatures.

&&

.UPDATE...There remains a couple isolated showers on radar
(primarily around Friday Harbor and Bellingham), but otherwise
conditions are drying out this morning. Partial clearing of the
cloud cover has taken place, and will continue to burn off some
throughout the day. Please see details below about the upcoming
atmospheric river event early next week, along with a refreshed
aviation/marine sections.

HPR

&&

.SHORT TERM /TODAY THROUGH SATURDAY/...Only a few showers and
downpours lingering on radar this morning between Everett and
Seattle, extending eastward along I-90 towards Snoqualmie Pass.
These will slowly dissipate through the morning as upper-level
flow weakens and diminishes Puget Sound Convergence Zone forcing.
Hi-res guidance winds this activity down by 7 AM.

Shortwave ridging will build across the Pacific Northwest later
today into Friday in the wake of yesterday`s departing shortwave
trough. While temperatures will warm, 500 mb heights only climb
slightly above normal for the end of May, with the ridge axis
moving overhead Friday. With clear skies today and and high
pressure moving eastward, highs climb by 5 degrees or so into the
low to mid 60s. By Friday, high clouds will begin spilling
eastward ahead of the next frontal system, which will hinder more
widespread highs in the 70s, though most locations south of I-90
will have the best shot at jumping into the 70s (80-90% chance).

A weak frontal system will cross the region Saturday bringing
chances for light rain, though measurable precipitation looks to
remain mostly isolated to the Cascades and Olympics with totals
generally around a tenth of an inch to perhaps a quarter of an
inch at best over the Mount Baker area. Highs drop a few degrees
into the mid 60s as the ridge flattens across the Pacific
Northwest.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...An active pattern
continues through the extended forecast into next week as an
atmospheric river pattern with embedded frontal systems cross the
region bringing wet weather, high snow levels, and seasonably
cool temperatures.

A brief lull in precipitation early Sunday morning but rain looks
to quickly fill back in ahead of a much stronger system Monday.
Long range ensembles continue to track a robust cold front across
the region Monday with strong winds and moderate to heavy
rainfall. The heaviest rainfall totals look to be in the Sunday
night through Monday timeframe, where the probabilities for total
rainfall amounts in excess of 3 inches from Sunday morning through
Tuesday morning top out around 80% across the southwestern
Olympics and Cascades in King and Snohomish Counties. The lowlands
don`t miss out either, with increasing likelihood for at least an
inch of rain during the Sunday morning through Tuesday morning
timeframe as well (50-60% chance). In fact, the NBM deterministic
output brings event total rainfall amounts between one and a half
and two inches from Sunday through Tuesday night.

Will need to continue to monitor area rivers as the Skokomish,
Snohomish, and Skykomish Rivers are forecast to rise near to in
Action Stage Monday and Tuesday. With snow levels forecast to
remain above 5000 feet, much of the precipitation from this
prolonged atmospheric river pattern will fall as rain.

Also of note, Monday and Tuesday look to breezy across much of
western Washington, with a 70-80% chance for wind gusts of 30 mph
or greater Monday and 50-60% chance on Tuesday. This may lead to
potential power outages, especially as soils saturate. Be sure to
secure loose outdoor items this weekend.

High temperatures will trend on the cool side in the upper 50s to
low 60s, which is 5-10 degrees below normal for the beginning of
June. Good news for those eagerly awaiting warmer weather, though,
as we head into the end of next week and into early next week.
Confidence continues to grow in support for above normal
temperatures during this period as ensemble clusters support the
development of a strong ridge across the western US, with
subsequent long-range ensemble forecasts shifting this ridge
slightly westward. As mentioned in the previous discussion, the
Climate Prediction Center has much of the area highlighted with a
50-60% chance of seeing above normal temperatures within the 8-14
day outlook (June 6-12), which appears to be supported with the
NBM output this morning. This is something that will be closely
watched and at this time, no projections on exact temperatures can
be made, but rather something to think about if you still need to
get air conditioning units put back in windows, etc.

Davis

&&

.AVIATION...A weak upper ridge will begin to build into Western
Washington today with northwest flow aloft. The air mass will
gradually dry through the day as residual convergence zone
cloudiness over Puget Sound continues to dissipate. A mix of
VFR/MVFR this morning will lift to VFR areawide by early this
afternoon. Low level flow will transition to northerly through Puget
Sound after 18Z. Elsewhere, westerly to southwesterly surface flow
can be expected.

KSEA...Remnant convergence zone clouds will continue to dissipate
this morning with borderline MVFR conditions becoming VFR by 18-19Z
VFR conditions are expected to persist for the remainder of the TAF
period. Northeasterly surface winds running 5-10 kts turning more
northerly/northwesterly by 18Z and remaining there through this
evening. After 03Z, will shift more north/northeasterly with speeds
remaining generally the same.  27/18

&&

.MARINE...A broad surface ridge over the area will maintain
light onshore flow today into Friday. Borderline SCA winds are
possible in the Central and Eastern Strait...however, forecast data
suggests speeds should remain below thresholds. Will monitor in case
a short-fused headline is needed. A weak front will move onshore
early Saturday with little to no impact. However, a secondary front
on Sunday will be relatively strong for early June and will likely
require headlines for portions of the the coastal waters. Post-
frontal onshore flow on Monday could be strong enough to require
headlines for much of the inland waters in addition to the coastal
waters if model trends remain as they are. 27/18

&&

.HYDROLOGY...Late-season atmospheric river activity will bring a
prolonged period of rainfall and high snow levels above 5000 feet
to western Washington Sunday through the first half of next week
with total forecast rainfall amounts of 1-2 inches across the
lowlands and 3-5 inches over the mountains. While no river
flooding is currently forecast, several river gages are forecast
to rise near to just above Action Stage Monday and Tuesday along
the Skokomish, Snoqualmie, Skykomish, and Snohomish Rivers.

Davis

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$