318
FXUS66 KSEW 060318
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
818 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/...An occluded front
continues to barrel towards the coast this evening, just now
moving into the offshore waters and will move onshore later this
evening into the early overnight. We`ve seen some clearing just
before sunset across the Puget Sound region as stronger 850 mb
winds move into the region ahead of the incoming upper-level
trough. Expect a gradual transition to more showery conditions
into tonight as the front moves across the region and low-level
flow turns more to the west. Surface temperatures remain above
freezing at the passes this evening 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. 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...Flow aloft west/southwesterly as an upper level trough
digs in from the northwest. A brief moment of continued improvement
to MVFR to VFR for some with some breaks in the low-level cloud deck
as the next frontal system moves onshore. Ceilings will lower back
down to widespread MVFR with localized IFR conditions with scattered
rain showers tonight as the front moves through. Behind the front,
cloud cover will break up and a return to low-end VFR conditions as
early as tomorrow morning while showers become more convectively
driven. Expect mostly VFR conditions continuing into tomorrow. Post-
frontal instability will increase tomorrow afternoon, so a few
lightning strikes nearby cannot be ruled out. Thunder activity will
be very isolated in nature, so not including in the TAF.

Breezy S/SW winds this afternoon around 10 to 15 kts with gusts up
to 25 kt will continue into the evening, especially around the time
of frontal passage. Winds will start to ease by late Monday
afternoon.

KSEA...A brief break to VFR early this evening as the next frontal
system approaches. MVFR conditions return by 05-06Z Monday as
scattered showers move through the region. VFR conditions return
perhaps as early as 12Z Monday, but more likely later towards 15-18Z
Monday as the cloud cover breaks up behind the frontal passage. VFR
through most of tomorrow with isolated showers. A few isolated
lightning strikes are possible tomorrow evening. Breezy S/SW winds
this evening 10 to 15 knots with potential gusts up to 20 to 25 kts
into the night. Winds will start to ease by late Monday afternoon.

LH

&&

.MARINE...A frontal system located just offshore will continue to
work its way through the coastal waters tonight into Monday, with
increasing onshore flow along with it. Small Craft Advisories
continue for the Central and East Entrance to the Strait of Juan de
Fuca through tonight. Winds will drop off quickly as the front
passes through the Strait, but will begin to ramp back up to SCA
criteria tomorrow morning through the Central Strait, then into the
East Strait again tomorrow evening. Winds increase into Tuesday up
to 25 to 30 kt as high pressure begins to develop to the west over
the open ocean. Gusty winds on Tuesday may bleed into Admiralty
Inlet and into the southern side of the San Juan Islands.

Breezy southwesterly winds continue tonight through the Puget Sound,
in particular for areas around and north of Seattle up to Whidbey
Island, lasting into tomorrow as well as the flow turns more due
southerly following the frontal passage tonight. A Small Craft
Advisory remains in effect through tomorrow afternoon as winds will
begin to taper off then.

Breezy southwesterly winds will also continue over the coastal
waters tonight as the front progresses through. A few gusts to 25 to
30 kt are possible near and south of Grays Harbor, but will diminish
later tonight following the frontal passage. Winds will turn
westerly behind the front then northwesterly and slowly decrease as
high pressure moves into the region for the middle and end of the
week.

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

LH

&&

.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.

&&

$$