158
FXUS66 KSEW 052131
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
231 PM PDT Sun May 5 2024

.SYNOPSIS...Cool and unsettled conditions through Tuesday with a
frontal system set to slowly move across western Washington
tonight into Monday morning. A pattern shift still on as we head
into the second half of the week with strong high pressure
building across the Pacific Northwest, bringing steadily warming
and dry conditions through next weekend.

&&

.SHORT TERM /TONIGHT THROUGH WEDNESDAY/...Damp conditions continue
this afternoon with a band of more noticeable rainfall associated
with weak Puget Sound Convergence Zone enhancement across
southern Snohomish into central King County and the Cascades.
Surface temperatures remain above freezing at the passes this
afternoon with just wet roadways as per traffic cams. Current
temperatures in the upper 40s to lower 50s with little variation
across much of western Washington. Breezy south-southwesterly
winds through tonight ahead of an incoming cold front.

This front will move onshore late tonight, with precipitation
coverage increasing along with southwest winds this evening. Lows
not falling a whole lot once again tonight into the low to mid
40s. Snow levels gradually begin to drop from around 5000 feet
closer to 4000 feet into Monday morning.

An upper-level trough will dig southward across the Pacific
Northwest Monday in the wake of the passing surface low and weak
cold front. This will bring the chance for convective showers and
thunderstorms thanks to steep low-level lapse rates as anomalously
cold air aloft moves just overhead and 500 mb heights approach
-30C. Brief gusty winds and small hail along with isolated
lightning will all be possible, especially with the development of
a Puget Sound Convergence Zone across Snohomish into central
Skagit County by late morning, slowly drifting southward into King
County by the evening. With fairly deep moisture, expect isolated
lightning and small hail to be the primary convective hazards.
Cannot rule out another round of showers and perhaps a lightning
strike or two on Tuesday, though less instability as the main
upper-level trough shifts off to our east. High temperatures
nearly 10 degrees below normal Monday in the low to mid 50s,
rising into the upper 50s Tuesday.

Light to moderate snowfall is expected across the Cascades as snow
levels drop through the day Monday to below 3000 feet by early
Tuesday morning, perhaps down to 2700 feet, which will allow for
light snowfall accumulations late Monday through Tuesday and
Tuesday night. Snowfall ratios will stay mostly below 10:1 Monday,
largely mitigating snowfall accumulation at the Cascade passes,
but greatest concern will be for convergence zone enhancement of
snowfall rates into Monday night, where snowfall would be the most
likely to accumulate on roadways above 2700 feet. At this time,
total snowfall Monday through Tuesday looks to remain below
advisory thresholds, with a 60% chance for at least 4 inches of
snow through 5 pm Tuesday at Stevens Pass and 20% at Snoqualmie
Pass. Chances for at least 6 inches drops much more to around 25%
and 5%, respectively. Though this will be a late-season snow,
Winter Storm Severity Index forecasts generally less than a 50%
chance for only minor travel disruptions, furthering the decision
to hold off on any Winter Weather Advisories at this time. Will
need to monitor exactly how convergence banding sets up and for
how long across the passes late Monday into Tuesday, however.

Heading into Wednesday, a pattern change begins with upper-level
ridging and surface high pressure building across the Pacific
Northwest. High temperatures nudge upward into the upper 50s to
lower 60s with partly cloudy skies.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Little change in the
forecast into the second half of the week as long range ensemble
guidance develops a Rex Block across the West with strong ridging
across the Pacific Northwest Thursday through Saturday north of an
upper low that cuts off across California and Nevada. This low
and block begin to disintegrate by Sunday with the ridge
deamplifying. A thermal trough also looks to develop along the
coast Thursday and Friday, helping to promote offshore flow and
weak downslope warming. Ensemble guidance continues to hone in on
Saturday for the warmest high temperatures, though the NBM
indicates a nearly 60% chance for highs above 80 on both Friday
and Saturday for most of the lowlands aside from areas along
waterfronts. The warmest temperatures appear most likely across
the Southwest Interior with over a 90% chance for highs over 80
Friday and Saturday. Though the high temperature forecast is
likely to fluctuate to some extent in the coming days, this looks
to be are first run at 80 degrees for many. Keep in mind that
local waters remain very cold, with hypothermia risk possible even
with these warm air temperatures. Adhere to safety practices if
planning time outdoors near any bodies of water late this week and
weekend.

Davis

&&

.AVIATION...Light flow aloft becoming westerly as a frontal
system moves down from the Gulf of Alaska. Conditions have
improved this afternoon to mostly MVFR throughout most interior
terminals, and VFR at coastal terminals (KCLM and KBLI). MVFR
conditions will most likely continue into the evening and
overnight for all terminals as shower coverage increases into
Monday morning. As shower coverage increases in the interior,
there is a 20% chance of brief lower visibility`s and ceilings
down to IFR into Monday 12z. Expect conditions to gradually
improve by 20z Monday to mostly VFR through interior terminals.

Breezy S/SW winds this afternoon around 12 to 17 kts will
continue into the evening.. with gusts possible up to 25 kts.
Winds will start to ease by late Monday afternoon.

Mazurkiewicz

KSEA...MVFR conditions this afternoon will continue into the
overnight as showers move through the area. Breezy S/SW winds this
evening around 13 to 16 knots with potential gusts up to 20 to 25
kts into the night. MVFR restrictions will start to lift up to a
VFR potentially by 19-22z Monday. Winds will start to ease by
late Monday afternoon.

Mazurkiewicz

&&

.MARINE...A frontal system will move through the area waters late
tonight into Monday, with increasing onshore flow along with it.
Small craft advisory strength southwesterly winds will continue
for the Central and Eastern Entrance of the Strait through
tonight. Winds in the Eastern Strait will subside tonight but will
increase again up to small craft strength by Monday afternoon.

Small craft southerlies will also continue through the Puget
Sound and Hood Canal areas until Monday afternoon. At this time,
winds in the Northern Inland Waters look to remain below any
thresholds, staying around 10 to 20 kts - along with winds on the
coastal waters, although cannot rule out a gust or two of small
craft advisory strength.

High pressure looks to build offshore beginning Tuesday night into
Wednesday for more calmer and benign marine conditions.

Combined seas around 6 to 8 feet will slowly build up to near 10
feet by Monday night and will remain 10 feet into Tuesday. Seas
will subside to around 5 to 7 feet into Wednesday.

Mazurkiewicz

&&

.HYDROLOGY...No river flooding is expected through the next seven days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Tuesday for Central U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 2 AM PDT Monday for East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 6 PM PDT Monday for Puget Sound and
     Hood Canal.

&&

$$