474
FXUS66 KSEW 032138
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
238 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/...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.
Mid to high level clouds have spread throughout most of the
interior ahead of an approaching frontal system. VFR conditions this
afternoon will likely continue throughout the majority of the
evening. Winds at the surface will remain south to southeasterly
and be gusty at times as the frontal system moves through the
area. Winds will turn more north/northeasterly into the overnight
period.

Rain will start along coastal terminals this afternoon around
21-23z, with rain slowly making its way into the interior, likely
around 03-07z. Ceilings will stay VFR into the evening before
gradually falling to MVFR/IFR, starting with coastal locations
around 05z, and interior terminals around 10-12z. A mixed bag of
MVFR/IFR conditions will persist through Saturday morning as
showers continue over the region.

KSEA...VFR this afternoon with high clouds already over the area
ahead of a front moving onshore. S/SE winds this afternoon around
8 to 12 knots will transition to more of a SW component this
evening as rain showers moves towards the terminal. Rain showers
will start to impact the terminal around 04z but VFR conditions
should continue. Winds will then turn N/NE around 08z and
increase to around 10 to 12 kts by Saturday morning. Ceilings
will gradually lower into Saturday morning, with MVFR conditions
likely around after 12z

Maz

&&

.MARINE...A frontal system will continue to move through the
coastal waters into afternoon with increased southerly winds over
the outer coastal waters. A small craft advisory for the outer
waters will continue through the evening, with southerly winds
will subside into the overnight hours. 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

&&

.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.

&&

$$