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FXUS66 KSEW 111004
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
304 AM PDT Fri Apr 11 2025

.SYNOPSIS...A few showers will continue across western Washington
Friday into Saturday as a trough exits the region. Cooler
temperatures will settle across the region this weekend behind the
cold front, with the coldest temperatures expected Sunday morning.
This increases the chance of locations seeing frost over the
weekend. High pressure will build in early next week, with highs
increasing back into the 60s by Monday.

&&

.SHORT TERM /TODAY THROUGH SUNDAY/...Western WA this morning
currently sits post frontal (with yesterday`s cold front bringing
the showers across the region - now east of the region). There
remains some shower activity north of Seattle this morning as a
convergence zone continues for the northern counties. A few
lighter showers will also trickle in across the Olympics into the
rest of western WA today. The upper level trough associated with
the cooler and wet weather will start to slide east inland on
Saturday. The shower activity will continue Saturday as the
trough swings through. Snow levels will drop to around 2,000 ft.
Light snow accumulations in the Cascades of 1-2 inches in the
passes are possible during the day Saturday. A few more inches
will be possible over the peaks of the Cascades. Little or no snow
will is expected in the Olympics.

The other story this weekend is cool temperatures behind the cold
front. Temperatures Friday morning are currently in the mid 40s in
most lowland areas (thanks to some cloud coverage and patchy fog
in a couple isolated areas) - but highs Friday will be noticeably
cooler in the low to mid 50s. The gradients will weaken somewhat
between the trough and the approaching ridge arriving by Sunday -
this will lead to light winds down at the surface through the
weekend. As the trough moves inland, a nose jet with an associated
ridge will funnel cooler air from Canada down into western WA.

The morning lows Saturday morning will dip into the low 40s.
Depending on how much clearing takes place behind showers on
Friday night, a few areas may dip into the upper 30s Saturday
morning (particularly in the South Sound/Chehalis Valley regions).
Confidence in the clearing Saturday morning was not high enough
to issue any frost advisories for the possible cold spots (but
can`t rule out patchy frost for areas that get down into the mid
30s Saturday morning).

Cloud coverage is expected to decrease further Saturday evening
into Sunday as the trough exits the region. Areas east of the
Olympics/coast will see the clearest skies, with little or no
wind. This will lead to pockets of frost Sunday morning. The
highest confidence of widespread patchy frost is in the Chehalis
Valley region, where guidance has remained consistent and has
given a 70-80% chance of temperatures dipping below 36 degrees
(frost advisory was issued Saturday night through Sunday morning).
A few valley/foothill areas of the Cascades may also see frost
formation, but confidence in the morning lows Sunday were a few
degrees warmer (and confidence remained too low to issue a frost
advisory for these areas for this morning`s forecast package).

.LONG TERM /MONDAY THROUGH THURSDAY/...The models have solid
agreement on ridging building across the region Monday into
Tuesday. This will help improve the high temperatures back into
the 60s for the lowlands, and lows improving into the 40s. It
remains split in the ensembles for how long the ridging will last
past Tuesday into the end of next week. A few models suggest that
a weak trough may pass through midweek, with limited precipitation
chances for the mountains. The overall pattern for next week is
very dry, with a possible return to showers towards the end of
next week. The forecast keeps the 60s in through the end of next
week.

HPR

&&

.AVIATION...West to southwesterly flow will persist aloft as
another upper level trough swings through the region today.
Conditions are primarily VFR across the area terminals early this
morning, except for KPAE within the vicinity of showers across
the central and north Sound. Southerly surface winds have started
to ease overnight and will continue to do so over the next few
hours, generally becoming light outside of shower activity by
12Z. Expect conditions at most terminals to remain low-end VFR
this morning, with cigs generally between 3000-5000 ft. However,
lingering shower activity (especially around the Sound) could
briefly bring cigs and vis down to MVFR throughout the morning
hours. Winds will shift to the north this afternoon, likely
between 21-00Z for central Sound terminals and persist around 5-10
kt into this evening before becoming light and shifting back to
the south by early Saturday. Lingering showers look to taper
tonight into Saturday.

KSEA...VFR conditions early this morning, with SW winds persisting
at 5-10 kt. Winds will continue to ease over the next few hours,
but will increase again and shift to the north this afternoon,
likely between 21-23Z. N winds will persist at 7-10 kt at the
terminal through the evening, before becoming light and shifting
back to the S early Saturday. Conditions expected to stay mostly low-
end VFR today, though could see brief reductions to MVFR cigs and
vis in any shower activity that makes it into the terminal.
Probabilistic guidance hinting at a 25-35 percent chance of MVFR
cigs developing by early Saturday.

14

&&

.MARINE...High pressure will build over the northeastern Pacific
today, generally promoting calmer conditions across the area
waters. A weak disturbance riding the periphery of the ridge will
push into the region tonight into Saturday, promoting onshore
flow and another push of westerly winds through the Strait of Juan
de Fuca, with gusts to around 25 kt likely into early Saturday.
Thus, have issued another round of small craft advisories for the
central and eastern Strait tonight into Saturday morning. Small
craft advisories will also remain in effect for the outer coastal
waters into this afternoon with seas hovering around 10 ft. Seas
will gradually subside towards 6-8 ft by tonight.

High pressure will then persist over the coastal waters through
much of the weekend, with seas primarily ranging between 4-6 ft.
A thermal trough then looks to extend northward along the Washington
coast late Sunday into Monday and could allow for winds to briefly
turn offshore early Monday. A weakening frontal system may approach
the area waters early next week, but does not look to bring much
impact to the area at this time outside of a slight uptick in
winds and seas building back towards 6-8 ft. High pressure then
looks to build back into the region behind the system and persist
into midweek. Seas look to approach 8-10 ft Tuesday night into
Wednesday.

14

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Frost Advisory from midnight Saturday night to 8 AM PDT Sunday
     for Foothills and Valleys of Thurston and Lewis Counties-
     Lowlands of Lewis and Southern Thurston Counties-Middle
     Chehalis River Valley-Willapa and Black Hills.

PZ...Small Craft Advisory until 3 AM PDT early this morning for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
     Saturday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 2 PM PDT this afternoon 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.

&&

$$