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000
FXUS66 KSEW 210925
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
225 AM PDT Sun Apr 21 2024

.SYNOPSIS...Scattered post-frontal showers across western
Washington will gradually taper off to the Cascades through the
day. High pressure will build across the Pacific Northwest,
bringing warmer and drier conditions Monday and Tuesday. A weak
cold front will cross the region Tuesday night with additional
weak disturbances and upper level troughing leading to more
unsettled conditions for the second half of the week.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...Much cooler today with high
temperatures in the mid to upper 50s and scattered showers and
mountain snow showers across the region. Mesoscale guidance
continues to hone in on convergence banding, staying confined to
Whatcom, Skagit, and far northwestern Snohomish Counties this
morning, then pushing further south towards the I-90 corridor as
an afternoon push through the Strait of Juan de Fuca brings
northerly winds southward across Snohomish and northern King
Counties. Can`t rule out a rumble or two of thunder with SBCAPE
values between 100 and 150 J/kg, though greatest chances will be
across the northwestern Olympic Peninsula. Total rainfall amounts
stay less than a tenth of an inch for most locations, save for
under heavier showers and downpours that may form in convergence
banding through the afternoon.

Chilly low temperatures tonight with patchy frost possible,
especially in rural areas with low temperatures in the low 30s.
Greatest chances for sub-freezing lows through rural Thurston,
Mason, Lewis, and eastern Grays Harbor Counties (10%). Some
uncertainty in cloud cover and still rather low probabilities for
temperatures near freezing will preclude any Frost or Freeze
headlines with this forecast package.

High pressure will build onshore over the region Monday and
Tuesday with high temperatures warming into the low 60s Monday and
into the upper 60s to near 70 by Tuesday as the ridge moves
directly overhead. Offshore flow looks to develop late Monday into
Tuesday morning as surface high pressure moves further inland and
a thermal trough develops along the coast of the Pacific
Northwest. This will likely lead to another period of low
relative humidity values with a 30% chance for min RH values below
35% Monday and a 45% chance for Tuesday along and south of I-90.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...A weakening cold front
will cross the Pacific Northwest Tuesday night, quickly shifting
well inland by Wednesday. Precipitation amounts look to be on the
lighter side once again. Long range ensemble guidance continues to
deepen a longer wave trough across the western CONUS with ridging
across the Mississippi River Valley Thursday and Friday. Main
disagreement appears to be over how quickly a cutoff upper low
across Southern California and the Desert Southwest merges with
upper level energy with Wednesday`s system. This will affect how
quickly an upper-level ridge builds across the Pacific Northwest
through the weekend. The consensus Thursday through Saturday is
for near to slightly below normal temperatures with scattered
showers and mountain snow nearly each day, though likely non-
impactful amounts.

Davis

&&

.AVIATION...Currently west-southwest flow aloft before turning
northwesterly tonight as an upper-ridge looms offshore. Low-level
onshore flow will weaken as the morning progress. But scattered
showers are to remain throughout the day with VFR cigs. A
convergence zone is forecasted to form over Puget Sound in the
afternoon before gradually fizzling out overnight. Localized MVFR
cigs are possible with this feature for airfields such as KPAE.
Breezy S to SW winds again today before decreasing overnight. More
VFR is expected into Sunday.


KSEA...VFR conditions so far this morning with post-frontal showers
in the vicinity. Cigs are to remain mostly VFR throughout the TAF
period but a developing convergence zone between 20Z Sunday and 00Z
Monday could influence things a bit. S/SW surface winds between 8 to
12 kt today before veering to the N/NW later this evening and
decreasing in magnitude.

McMillian

&&

.MARINE...Small craft advisories over the coastal waters and
west strait will be allowed to expire as the morning progress.
However, surface high pressure looks to build over area waters today
keeping SCA westerlies through Strait of Juan de Fuca through
tonight. The surface ridge shifts inland on Monday as a thermal
trough expands northward along the Oregon coast. This will turn the
flow more northerly, possibly near advisory levels across southern
portions of the coastal waters. An additional front is on track to
enter the region Tuesday night into Wednesday with more activity
possible later in the week.

Seas may briefly get above 10 feet over portions of the outer
coastal waters today before subsiding below that threshold for the
remainder of the forecast period.

27

&&

.HYDROLOGY...No river flooding is expected over the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT early this morning for
     Coastal Waters From Cape Flattery To James Island 10 To 60
     Nm-Coastal Waters From Cape Flattery To James Island Out 10
     Nm-Coastal Waters From James Island To Point Grenville 10
     To 60 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
     Grenville To Cape Shoalwater Out 10 Nm-West Entrance U.S.
     Waters Strait Of Juan De Fuca.

     Small Craft Advisory until midnight PDT tonight for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$