333
FXUS66 KSEW 040309
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
809 PM PDT Fri May 3 2024

.SYNOPSIS...A shift back to cool, wet unsettled conditions will
prevail across Western Washington through early next week. A
significant change in the weather pattern is expected for the
second half of next week as high pressure aloft leads to warmer
and drier conditions.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...Light rain ahead of
incoming front is making slow progress into the interior as
expected. Rain chances around much of Puget Sound will increase on
Saturday as weakening front pushes across the region, but the bulk
of the precip with this initial system looks to fall south and west
of Seattle. No changes anticipated this evening to existing
forecast. Previous discussion follows with updates to marine and
aviation portions.   27

Rain had yet to reach coastal areas as of 130 PDT (2030Z) as an
occluded front ahead of a broad upper trough offshore gradually
advances toward the region. Rain will spread across the Olympic
Peninsula by early evening, but will be slow to get into Puget Sound
eastward initially. As the upper trough axis shifts into the Oregon
coastal waters early Saturday, a weakening frontal boundary will
slowly lift northeastward across Western Washington. Low level
easterly flow will shift onshore and increase behind it. This will
allow the air mass to moisten up for increasing precip coverage by
Saturday afternoon. Overall QPF will be heavily focused on the
southwest half of the CWA.

The upper trough is expected to shift into the Great Basin by
early Sunday with wrap-around precip continuing to fall across
parts of Western Washington. Onshore flow will ramp up through the
day on Sunday as another upper trough moves toward the area.
Increasing orographic lift will lead to a fairly wet Sunday
afternoon and evening in the mountains, but much of the Puget
Sound area looks to be rain shadowed.

An upper trough axis will swing onshore on Monday. With colder
air aloft, this will destabilize things enough to warrant the
mention of a slight chance of thunderstorms. Snow levels will dip
to some of the higher passes, but amounts look to be light. A
convergence zone looks likely to form on Monday as well.
Temperatures throughout the short term forecast period will be
several degrees below normal, but changes are afoot.


.LONG TERM /TUESDAY THROUGH FRIDAY/...Cool, showery conditions
will persist into Tuesday as the upper trough axis slowly shifts
east of the Cascades. For Wednesday and beyond, ensembles are
nearly unanimous that upper ridging will build over the northeast
Pacific as downstream a broad upper trough takes up residence
over the eastern two thirds of the lower 48. There is still
considerable uncertainty with regards to how warm it will get.
Current forecasts reflect the NBM 50th percentile forecast...which
brings mid 70s to Seattle by next Friday. A noteworthy number of
ensemble members (particularly the GFS) have temperatures teasing
80 degrees by late next week for some areas. Rough winds do shake
the darling buds of May, but we might compare thee to a summers
day by next weekend. 27

&&

.AVIATION...An upper level trough will move through western
Washington this evening with south to southwesterly flow aloft. A
frontal system is beginning to approach the area, with light rain
falling throughout the Pacific Coast and the Southwest Interior.
Conditions remain VFR this evening with MVFR ceilings moving onshore
as the front approaches. Rain will struggle to become widespread in
the interior north of Seattle tomorrow. Scattered showers move in
overnight becoming more widespread later tomorrow morning.
VFR ceilings continue through tomorrow morning through the interior,
falling to MVFR tomorrow afternoon. MVFR along the coast tonight and
tomorrow and around OLM, with isolated IFR conditions possible.

Winds at the surface will remain west to southwesterly through most
of the TAF period. Winds may briefly turn light northerly overnight,
but will turn back to southwesterly tomorrow. Winds may be gusty
tomorrow with the frontal passage.

KSEA...VFR this evening with ceilings gently lowering as a front
moving onshore. Rain to the south and west of the terminal, likely
not fully reaching the terminal until tomorrow. Ceilings lowering to
low end VFR, with MVFR conditions possible as early as 15Z Saturday
but not expected to be longer lasting until tomorrow evening. Winds
WSW and decreasing overnight. Winds will likely turn northerly
overnight before spinning back around to southwesterly tomorrow
morning.

LH

&&

.MARINE...A frontal system is currently traversing the coastal
waters and will continue to work its way onshore tonight into
tomorrow. Winds will decrease tonight as the front moves through the
area. As such, the Small Craft Advisory has been allowed to expire.
Heading into the weekend, guidance suggests several westerly pushes
through the Strait of Juan De Fuca, likely yielding additional
headlines. Another system looks to track into the area waters
through the first half of next week.

Combined seas this afternoon around 3 to 5 feet will gradually
increase to around 6 to 8 feet by Sunday. Confidence is increasing
that seas look to hover around 9 to 11 feet Monday night into
tuesday, with the higher heights mostly in the outer water zones.

Maz/LH

&&

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


&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 7 PM PDT this evening for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

&&

$$

NWS SEW Office Area Forecast Discussion