Washington State Forecast Discussion

Western Washington


188
FXUS66 KSEW 041800
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
1000 AM PST Wed Dec 4 2024

.SYNOPSIS...Morning fog and low stratus will persist today under
high pressure. The upper level pattern will begin to shift
Thursday into Friday as the next storm system approaches, bringing
widespread precipitation and breezy winds across the region
through the weekend.

&&

.SHORT TERM /TODAY THROUGH FRIDAY/...The persistent upper level
ridge will remain in place today, resulting in another round of
dry and stable conditions. Fog and freezing fog is beginning to
slowly rise and scatter across the southern Puget Sound and
Southwest Interior this morning, allowing the Dense Fog advisory
to expire this morning. Localized dense fog and dense freezing fog
will persist in some locations through the late morning into the
early afternoon. Fog will give way to mostly sunny skies this
afternoon. Highs today will peak a few degrees below normal, with
highs in the upper 40s to low 50s across the lowlands.

Forecast models continue to highlight a pattern change Thursday
and beyond as troughing develops over the Pacific Northwest. A
weak trough will overrun the upper level ridge throughout the day
on Thursday, bringing in a quick round of light shower and virga
activity while introducing enough mixing in the boundary layer to
limit fog development Thursday morning. Temperatures will warm up
a degree or two on Thursday, with highs once again in the upper
40s to low 50s across the lowlands and Cascade valleys and lows
several degrees above freezing.

Weak transitory ridging will build overhead Thursday night into
Friday morning, but winds will likely be enough to inhibit another
round of widespread morning fog. A warm front will lift across
western Washington on Friday, bringing in breezy winds and more
widespread precipitation with snow levels rising as high as
7000-9000 ft. Temperatures will continue a warming trend into
Friday, with most lowland areas seeing highs in the 50s and
overnight lows well into the 40s.

.LONG TERM /SATURDAY THROUGH TUESDAY/...Warm frontal precipitation
will continue into early Saturday morning ahead of a trailing cold
front that will cross western Washington on Saturday. Winds will
become breezy as precipitation intensifies, with the bulk of the
moisture set to fall over the Olympic Peninsula and northern
Cascades. The upper level trough will shift inland late Saturday
as snow levels lower to 3000-3500 ft, with continued precipitation
on Sunday with orographic enhancement over the Cascades. Ensembles
continue to highlight the potential for 3 inches or more of
liquid over the weekend over portions of the Olympics and North
Cascades, trending towards higher percentile precipitation after
a period of dry weather. The lowlands will also see around an inch
of rainfall over the course of the weekend.

Heading into next week, ensembles and medium range models remain
in good agreement over ridging developing overhead and northerly
flow aloft keeping conditions dry and cool.

Lindeman/62

&&

.AVIATION...High pressure remains in the place across the
region with light surface winds and widespread fog/freezing fog
across the interior. Meanwhile, winds remain breezier near the
waters. A few spots across southern Puget Sound may only improve
towards MVFR briefly this afternoon should fog continue to linger,
but elsewhere a return to VFR conditions is expected. Another round
of fog will be possible overnight into Thursday, however do not
expect the extent of fog to be as widespread as this morning as more
mid to high level clouds start to stream in across western WA.

KSEA...LIFR conditions in freezing fog. Expect LIFR/IFR conditions
to persist through the morning hours and expect gradual improvement
towards VFR around 21Z. Another round of fog will be possible
tonight into Thursday, with guidance suggesting a around a 20%
chance again tonight through early Thursday morning. Winds remain
generally southeast/east this morning, becoming variable and light
later in the day.

&&

.MARINE...Winds will generally remain light and offshore today as
high pressure remains situated across the area waters. Another
round of widespread fog has developed and may bring visibilities
down to 1NM for portions of the interior waters through the
morning hours today. Seas across the coastal waters will
generally hover between 5-8 ft today.

A weak frontal system will approach the coastal waters on
Thursday and will dissipate as it moves inland. This may bring
gusty small craft southerlies to portions of the coastal waters
through Thursday afternoon. Another, stronger frontal system will
then approach the area waters on Friday and move inland over the
weekend. This system will bring a round of gusty southerly winds for
the coastal waters, but also for portions of the interior waters
like the eastern Strait of Juan de Fuca. These southerly winds will
allow for steep seas to develop across the coastal waters on Friday.
Winds will switch to westerly in the wake of the frontal system on
Saturday, but will remain breezy across the coastal waters and along
the Strait of Juan de Fuca as the associated low pressure system
pushes inland over BC/WA. Seas will build towards 15-18 ft across
the coastal waters on Sunday as a new swell train arrives.

High pressure then looks to build back into the coastal waters by
late Sunday/early Monday, promoting lighter winds and a switch
back to offshore flow. Seas hovering around 12-14 ft will gradually
subside through the day Monday back towards 8-10 ft.

14

&&

.HYDROLOGY...An upper ridge will remain in place across western
Washington today before a frontal system moves through on
Thursday for lighter QPF amounts. Another weather system Friday
into Saturday is forecast to bring heavier QPF amounts. In
addition, snow levels are expected to rise later Friday into early
Saturday to 7000 to 9000 feet. This, combined with rainfall, will
result in rises on rivers across western Washington. At this
time, the Skokomish River is the main river of concern for
potential river flooding during this period. There is a fair
amount of uncertainty as to the highest amounts of rainfall that
are possible with Saturday's storm, with some models producing
over 4 inches over the Olympics and Central Cascade north. This
could drive some rivers to close to or over minor flood. Due to
this and the higher snow levels Friday into Saturday, other area
rivers will need to be monitored for flood potential as well.

JBB/JD

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Dense Fog Advisory until 10 AM PST this morning for Bellevue and
Vicinity-Bremerton and Vicinity-Everett and Vicinity-Hood
Canal Area-Seattle and Vicinity-Southwest Interior-Tacoma
Area.

PZ...None.
&&