754
FXUS66 KSEW 222312
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
312 PM PST Wed Jan 22 2025

.SYNOPSIS...The upper level ridge currently in place over the
Pacific Northwest will shift west Thursday afternoon. A weak weather
system will move through the area Thursday night into Friday. Upper
level ridging will amplify offshore on Friday and move eastward by
Monday. Ridging begins to weaken Tuesday into Wednesday.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...A beautiful day across
Western Washington with mostly clear skies and a few thin high
clouds passing by. Expect another round of patchy freezing fog
Thursday morning mainly south of the Puget Sound. Upper level
ridging will continue to dominate the upper level flow over the
area until Thursday night. The ridge will shift westward and allow
a weak shortwave trough to dig southward into the area that will
bring light snow to the Cascades. The highest probabilities
(40%-55%) still remain over the Northern Cascades for Thursday
night. On Friday, there is a slightly lower chance of
precipitation (20%-30% probability) over the central and southern
Cascades. Less than an inch of snow with this system. No
significant impacts expected. As for the lowlands and coastal
locations, expect increased cloud cover with this system and even
a brief drizzle. High temperatures on Friday will be in the low
to mid 40s; low temperatures will be in the low 30s.

As the shortwave digs southward into Western Washington on Friday,
the upper level ridge will amplify over the Pacific Ocean.
The ridge will gradually begin to move east toward British
Columbia on Saturday, bringing drier conditions to Western
Washington. Mostly clear skies on Saturday. High temperatures will
be in the 40s, and low temperatures will be in the mid to upper
20s.

Cold overnight temperatures are expected to continue throughout
the short term period. With clear skies and calm winds, patchy
morning freezing fog possible this weekend in the Southwest
Interior.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...There is good agreement
between ensemble clusters that the upper level ridge will continue
to dominate the upper level flow through Monday. On Tuesday, the
cluster solutions show the ridge beginning to weaken slightly as the
ridge axis begins to approach Washington`s coastline. On Wednesday
cluster solutions begin to differ. By Wednesday, cluster solutions
favor moderate broad ridging over the Pacific Northwest, although
there is some disagreement in ridge axis location and exact strength
of the ridge. A small amount of ensemble members (12%) favor zonal
flow over Western Washington, as an upper level trough moves over
British Columbia.

Mostly clear skies and calm winds expected through Tuesday. High
temperatures will be in the low to mid 40s Sunday and Monday. A
slight warm up starting on Tuesday, with high temperatures in the
mid to upper 40s. Low temperatures will be in the mid to upper 20s
and warm up slightly to the upper 20s to low 30s by Wednesday.
Multiple rounds of patchy freezing fog possible while under the
influence of the upper level ridge and temperatures below freezing.

MGF

&&

.AVIATION...Northwest flow aloft continues tonight as an upper level
ridge remains centered offshore. The ridge will weaken overnight and
increasing westerly flow aloft will develop on Thursday as a weak
upper trough digs southward over British Columbia. VFR conditions
prevail this afternoon across the region. Patchy LIFR/IFR fog and
low clouds will redevelop overnight across the Southwest Interior
and portions of central/south Puget Sound. Light surface gradients
developing overnight are somewhat favorable for greater areal fog
coverage 12Z-18Z Thursday morning.

KSEA...VFR with north winds through this evening. Guidance points
toward a higher likelihood of some patchy fog in the vicinity of the
terminal on Thursday morning. For now, will likely retain the
mention of fog in the vicinity in the TAF and amend as trends
dictate. Surface winds northerly generally 8 knots or less through
the evening becoming light northeasterly overnight then
light/variable much of Thursday. 27

&&

.MARINE...Surface ridging centered over the coastal and offshore
waters will weaken on Thursday as a weak trough and associated front
slide southward from British Columbia. Surface ridging rebuilds over
the interior of British Columbia on Friday and remains in place over
the weekend with broad troughing well to the south of the area for
northerly flow. The flow turns more offshore early next week as a
weak trough expands northward along the coast and high pressure
remains in place east of the Cascades.

Seas will remain around 4 to 6 ft through early Thursday. Seas will
then slowly build to 8 to 12 feet across the coastal waters Thursday
night into Friday before subsiding once again this weekend.

27

&&

.HYDROLOGY...No river flooding the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$