296
FXUS66 KSEW 211556
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
856 AM PDT Fri Mar 21 2025

.UPDATE...Another day of rain showers at times for the lowlands,
with continued mountain snow due to onshore flow. A convergence
zone may develop later this evening into tonight, resulting in
periods of heavier snow possible near Stevens Pass. Otherwise,
thunder probabilities this afternoon peak around 10-15%, as such a
brief thunderstorm is possible with the convective showers. No
major forecast updates this morning.

&&

.SYNOPSIS...Lowland rain and mountain snow will continue through
the day as a frontal system makes its way across the region.
Showers will become more confined to the mountains on Saturday as
weak ridging moves into the area. An atmospheric river will likely
move into the region Sunday into Monday. Ridging will then
amplify over the region for warmer and drier conditions on Tuesday
before another system looks to move into the region midweek.

&&

.SHORT TERM /TODAY THROUGH SUNDAY/...Lowland rain and mountain
snow will continue across western Washington today as a frontal
system continues to make its way across the region. Shower
activity will continue through much of the day, though some of the
latest guidance does indicate another swath of more widespread
precipitation moving up from the south and pushing inland across
the southern Cascades this afternoon. Isolated thunderstorms will
be possible in the shower activity this afternoon and evening,
with probabilistic guidance generally showing a 15-20 percent
chance of development for areas across the interior. Snow levels
hovering between 3500-4000 feet will gradually lower throughout
the day towards 2500-3000 feet tonight. Expect to see an
additional 1-2 feet of snow for higher Cascade passes like Stevens
and around a foot for Snoqualmie Pass through Saturday. With snow
levels expected to hover above 3000 feet through the day,
however, expect Snoqualmie Pass to see periods of rain, before the
rain transitions back to snow tonight. Have maintained the
current Winter Storm Warning for the North Cascades and the Winter
Weather Advisory for areas from Snoqualmie southward. Hurricane
Ridge in the Olympics will also see an additional 4-6 inches of
snow accumulate throughout the day, so have maintained the Winter
Weather Advisory there as well. With onshore flow increasing
tonight, expect a convergence zone to develop across the north-
central Sound this evening, which will bring a period of steadier
lowland rain and heavier mountain snow for areas underneath it.

A weak upper level ridge will then move into the area on Saturday,
allowing for lingering precipitation to become more confined to
the mountains through the day. Drier conditions are expected for
the lowlands.

An atmospheric river then looks to take aim at the region on
Sunday. While there`s still a decent spread in QPF across the
ensembles, can expect Sunday to be a wet day with the next round
of more widespread precipitation moving into western Washington.
Snow levels look to climb to 6000-7000 feet in the warm air mass
and would promote rain falling on recent snow in the mountains,
likely elevating the potential for flooding into early next week.
See the hydrology section for more details.

Not much change in temperatures expected throughout the short-
term, with afternoon highs generally trending in the 50s and
morning lows trending in the upper 30s to low 40s.

.LONG TERM /MONDAY THROUGH THURSDAY/...High snow levels and
showers will continue into Monday, though the precipitation will
gradually shift more northward throughout the day as the upper
ridge over the region starts to amplify. The ridge looks to
further amplify into Tuesday, pushing the swath of moisture into
British Columbia and promoting drier conditions across western
Washington. Temperatures also look to climb to above normal on
Tuesday, with afternoon highs looking to climb into the 60s. A few
spots may potentially even approach 70 across the southern
interior of Lewis county. Mild conditions look to continue into
Wednesday, however the upper ridge will start to push inland,
allowing for the next frontal system to make its way into western
Washington. With high snow levels between 7000-8000 feet, expect
another round of widespread rain for the area. Wet conditions will
linger into Thursday as an upper trough sets up offshore. 14

&&

.AVIATION...Several rounds of precipitation are expected through
early next week, which will bring rises to the area rivers. The
Skokomish River continues to rise this morning in response to
recent rains and will need to be monitored over the next several
hours.

There is still potential for flooding on the Skokomish River late
in the weekend, with the river running higher from recent rains
and with abundant precipitation moving into the region on Sunday
with significantly higher snow levels. Ensemble guidance continues
to show a large range of solutions for precipitation amounts
Sunday and Monday so confidence in this scenario remains low at
this time. 14

KSEA...Breezy SW winds of around 10-15 kt (gusts to 25 kt). Expect a
decrease of winds by midday. Rain showers will continue throughout
the day with ceilings staying around MVFR and/or low-VFR.
Predictability of thunder is too low to include a PROB30 or tempo
group at this time (best chances are north of the terminal, but
cannot rule out a cell near the terminal this afternoon).
Improvements to mostly VFR conditions are expected early Saturday.

HPR/McMillian

&&

.MARINE...A small trough will drive a surface system over the waters
today. Gusty winds ahead of the warm front continue this morning
over the inland waters, with a couple isolated gale gusts
(particularly around Smith Island). With the warm front moving quick
to the east this morning, the southwest winds will decrease over the
inland waters. A cold front will follow the warm front going into
the afternoon. Unstable air in between fronts will introduce a
chance of thunder this afternoon. The best chance for seeing thunder
will be for inland waters, as a convergence zone is expected to
develop going into the evening post-front. Main threats are
lightning, downpours, and potential for erratic wind gusts.
Otherwise, seas will continue to remain around 8 to 10 feet in the
coastal waters today into Saturday morning (small craft advisory
remains). A push of westerlies is expected later this evening into
Saturday morning in the central/east Strait of Juan de Fuca (issued
a new small craft advisory). Finally, the seas at buoy 211 remain at
11 to 14 feet this morning just outside of Grays Harbor (will
continue the advisory for rough seas through this afternoon).

Going into the weekend, another disturbance will likely bring breezy
20-25 kt southerly winds over the coastal waters (a few interior
waters may see winds up to 20 kt as well). The seas will remain at 8
to 10 feet through the weekend (higher seas further out). Expect a
drop below 10 feet going into the work week before another potential
system Wednesday/Thursday brings seas and winds up over the coastal
waters.

HPR

&&

.HYDROLOGY...Several rounds of precipitation are expected through
early next week, which will bring rises to the area rivers. The
Skokomish River continues to rise this morning in response to
recent rains and will need to be monitored over the next several
hours, although expected to remain below minor flood stage.

There is still potential for flooding on the Skokomish River late
in the weekend, with the river running higher from recent rains
and with abundant precipitation moving into the region on Sunday
with significantly higher snow levels. Ensemble guidance continues
to show a large range of solutions for precipitation amounts
Sunday and Monday so confidence in this scenario remains low at
this time. We continue to monitor rises on other area rivers as
well early next week. JD/14

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Storm Warning until 5 AM PDT Saturday for Cascades of
     Snohomish and Northern King Counties-Cascades of Whatcom
     and Skagit Counties.

     Winter Weather Advisory until 5 AM PDT Saturday for Cascades of
     Pierce and Lewis Counties-Cascades of Southern King County-
     Olympics.

PZ...Small Craft Advisory until 5 PM PDT this afternoon for Grays
     Harbor Bar.

     Small Craft Advisory until 11 PM PDT this evening for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm-West 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.

&&

$$