South Prairie Washington Weather Forecast Discussion

917
FXUS66 KSEW 120455
AFDSEW
Area Forecast Discussion
National Weather Service Seattle WA
955 PM PDT Sat Oct 11 2025
.UPDATE...Convergence zone showers continue to fall across Snohomish
and King Counties this evening. Additional showers also continue in
the north interior east of Bellingham, but are diminishing on radar.
The convergence zone is expected to continue through at least
midnight, although additional precipitation is on tap for tomorrow.
See discussion below for additional details on more lowland showers
and snow showers in the mountains along with updated aviation
section.
HPR
&&
.SYNOPSIS...Cool and unsettled conditions continue across
Western Washington this weekend. A system moving southward from
British Columbia Sunday will bring the first noteworthy high
elevation snow of the season. Drier conditions will return early
next week with offshore flow. Typical autumnal conditions return
later next week.
&&
.SHORT TERM /TONIGHT THROUGH TUESDAY/...An upper level trough is
passing east through the region today with showers and cooler
conditions across the westside. Temperatures are in the low to mid
50s which is around 5 to 10 degrees cooler than average. Showers
will mainly impact the interior and Cascades this afternoon and
evening then taper down overnight. Expect overnight lows in the
40s.
A second, cooler, system will drop down from B.C. on Sunday for
more showers and also mountain snow. The highest snow totals will
be in the Cascades and above 4,000 ft where 4 to 10 inches of snow
will fall through early Monday morning. Higher totals are
possible at Mount Baker and Paradise on Mount Rainier. This will
be the first notable snowfall in the mountains this season -
hikers and travelers should expect wintry conditions.
Drier weather is slated for Monday and Tuesday as the upper low
digs farther south toward central CA. Across western WA, the flow
will turn offshore and we could see breezy winds in the north
interior with Fraser River outflow. The air mass will be dry and
cool Tuesday morning with temperatures in the mid to upper 30s
(frost potential). Expect highs around 60 with mostly clear
skies. 33
.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Onshore flow resumes on
Wednesday as the upper low moves inland and over the Intermountain
West. Weak high pressure follows on Thursday but some light warm
frontal precip may clip the area for a chance of rain. Rain
chances increase Thursday night and Friday as a weak frontal
system moves inland. Wet weather continues over the weekend as a
stronger system moves in. 33
&&
.AVIATION...Upper level trough to the east with another trough
digging south out of British Columbia moving over the area Sunday.
Onshore flow in the lower levels becoming light tonight. Fraser
river outflow winds developing near Bellingham Sunday night.
Convergence zone over Central Puget Sound between KBFI and KPAE
dissipating after 12z.
MVFR ceilings in the Central Puget Sound with VFR elsewhere.
Patchy fog over the Northwest and Southwest interior dropping
visibilities to 1-3 sm at time through 16z. VFR ceilings all areas
Sunday afternoon.
KSEA...Convergence zone staying north of the terminal. MVFR
ceilings through Sunday morning with VFR ceilings Sunday afternoon
and evening. Southerly winds 8 to 12 knots. Felton
&&
.MARINE...Onshore flow will continue tonight. A low pressure
system will develop along the southern coast of Vancouver Island
late Sunday and move southwards along the Coastal Waters Monday.
Gale Warnings continue for the central and northern outer Coastal
Waters due to increased northwest winds through Sunday morning.
Small Craft Advisory winds exist elsewhere for the Coastal Waters.
Have issued a Small Craft Advisory for the central and eastern
Strait of Juan de Fuca this evening with increased westerly winds.
Probabilities of wind gusts of 25 kt peak between 40 to 60
percent, with higher likelihood of gusts exceeding SCA threshold
for the Strait this evening.
Winds will then transition more northeast Sunday night as the
aforementioned weather system slides southwards along the
coast. A Gale Watch has been issued for the northern inland
waters Sunday night through Monday night for these northeast
winds. SCA winds are also likely over portions of the Strait of
Juan de Fuca and the Coastal Waters. Furthermore, brief SCA wind
gusts will need to be monitored for Puget Sound on Monday.
Winds will then ease Monday night into Tuesday as high pressure
builds back into Western Washington.
Seas will build to 11 to 15 ft tonight. Seas will be steep with
a dominant period of 10 seconds. Seas will gradually subside on
Sunday. Seas will fall to 5 to 8 feet by late Monday. JD
&&
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 5 AM Sunday to 5 AM PDT Monday for
     Cascades of Snohomish and Northern King Counties-Cascades
     of Whatcom and Skagit Counties.
Winter Weather Advisory from 8 AM Sunday to 11 AM PDT Monday for
     Cascades of Pierce and Lewis Counties.
PZ...Small Craft Advisory until 5 PM PDT Sunday for Coastal Waters
     From Cape Flattery To James Island Out 10 Nm-Coastal Waters
     From James Island To Point Grenville Out 10 Nm-Coastal
     Waters From Point Grenville To Cape Shoalwater Out 10 Nm-
     Grays Harbor Bar.
Small Craft Advisory until midnight PDT tonight for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.
Gale Watch from Sunday evening through late Monday night for
     Northern Inland Waters Including The San Juan Islands.
Gale Warning until 8 AM PDT Sunday 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.
Small Craft Advisory from 8 AM to 8 PM PDT Sunday 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.
Small Craft Advisory until 8 PM PDT Sunday for Coastal Waters
     From Point Grenville To Cape Shoalwater 10 To 60 Nm.
&&
$$

NWS SEW Office Area Forecast Discussion