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000
FXUS66 KSEW 310321
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
821 PM PDT Thu Mar 30 2023

.UPDATE...
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Forecast generally on track this evening, just some minor
modifications in the sky cover. Radar trends indicate continuing
spotty light shower activity across the area which should
continue. Temperatures will range from the upper 30s to lower 40s
for lows overnight, which is very close to normal for the time of
year, and warmer than lows will be over the next several nights.
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&&

.AVIATION...
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Light northwesterly flow tonight will transition
southwesterly on Friday as an upper trough moves towards the area.
VFR mid level clouds across most of the region this evening will
prevail, with pockets of lower ceilings along the coast associated
with increasing moisture and isolated showers.  Expect ceilings to
lower with mix of MVFR/VFR overnight most terminals. Some brief
improvement midday Friday, but increasing rain and lowering (to
MVFR) ceilings likely by late Friday as gusty winds to 20-30
kt arrive ahead of the front.

KSEA...VFR ceilings around 5000 ft this evening, with likely (75%
chance) lowering to around 2500 feet after 09z. Ceilings likely
settle in MVFR range through much of the  period with leading rain
arriving after 15z. Gusty south surface winds to 25 kt expected in
the afternoon/evening. Steadier rain and lowest visibilities likely
after 00z Sat through evening.   Cullen
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&&

.MARINE...
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Gusty west winds through the central Strait will ease
later this evening and while observations have been marginal over
the last hour, will elect the maintain the advisory through the
planned 11 PM expiration time.

Elsewhere, strengthening winds expected Friday ahead of the
approaching frontal system. Current small craft advisories for this
round remain on track, but did upgrade the gale watch to a warning
for the eastern Strait of Juan de Fuca and Admiralty Inlet as
confidence has increased in potential for both 35 kt sustained winds
and particularly for 40 kt gusts late Friday. Stronger onshore flow
will maintain advisory strength winds through portions of the
interior waters (both Puget Sound and parts of the Strait of Juan de
Fuca).

Seas over the coastal waters generally hold around 3 to 5 ft through
early Friday, but still remain somewhat choppy as a short period
wave component dominates. Expect seas to build initially with the
front to 6 to 9 feet and largely short period. Seas will then
remain elevated Saturday with a mixed swell of south and west
components, before a return to a dominant west swell for the early
portion of next week.    Cullen
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&&

.PREV DISCUSSION... 
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/issued 352 PM PDT Thu Mar 30 2023/

SYNOPSIS...Weak upper level trough moving through Western
Washington tonight. A strong for the 1st of April front reaching
the coast late Friday night moving inland Saturday morning with
lowland rain, mountain snow and breezy conditions. Very cool and
unstable air mass settling in over Western Washington for the
remainder of the weekend through Monday. The trough will move out
of the area Tuesday. Small break in the weather possible in the
middle of next week.

SHORT TERM /TONIGHT THROUGH SUNDAY/...Satellite imagery shows
mostly cloudy skies over most of the area with less cloud cover
over Whatcom county and the Kitsap peninsula. Temperatures at 3
pm/22z were in the mid 40s to lower 50s.

Weak upper level trough moving through tonight could kick off a
couple of showers. Main message for this evening is just mostly
cloudy skies with southerly winds gusting to 25 mph.

Next system to reach the area currently just east of 140W moving
southeast. Weak warm front like feature out ahead of the cold
front will move over the area Friday morning. This combined with
the moisture already in place will produce some very light rain.
Rain out ahead of the cold front will reach the coast midday and
spread inland during the afternoon. Highs will only be near 50.

Front reaching the coast late Friday evening moving inland
Saturday morning. Heaviest precipitation will be just ahead of the
front. Snow levels in the Cascades will be below all the passes.
Have upgraded the winter storm watch to a winter storm warning
beginning at 5 pm Friday afternoon for locations above 2500 feet.
Lows near 40.

Behind the front another very cool upper level trough will move
down from the northwest over Western Washington. By late Saturday
afternoon 850 mb temperatures down of around -5C and 500 mb
temperatures near -35C. With this steep lapse rate can`t rule out
some thunderstorm activity late in the afternoon and even if we
don`t get any thunder we could certainly get some small hail out
of the stronger showers. Snow continuing in the Cascades all day. Highs
once again near 50.

Little change in the pattern Saturday night and Sunday with the
cool upper level trough remaining over the area keeping showers in
the forecast. If anything temperatures aloft are a little colder
Sunday afternoon versus Saturday afternoon keeping the threat of
thunder and/or small hail in the forecast for the afternoon hours.
Snow rates in the Cascades decreasing Saturday night with only
another 2 to 4 inches expected Sunday. The winter storm warning
ends at 06 Sunday/11 pm Saturday. Looking for a total of 12 to 18
inches of snow in the warning window. Lows in the mid to upper 30s
Sunday morning with highs only near 50 Sunday afternoon.

LONG TERM /MONDAY THROUGH THURSDAY/...Cool upper level trough
remaining over Western Washington into Tuesday. Moisture with the
feature starting to decrease. On the flip side snow levels will be
as low as 500 feet Monday morning for a chance of rain/snow mixed
showers in the lowlands. No significant accumulations expected at
this time. Trough finally moves southeast out of the region
Tuesday night.

Operational runs of the GFS and ECMWF show some ridging for
Wednesday. Ensembles not as strong with this feature so will keep
a slight chance of showers in the forecast. ECMWF operational run
keeps the ridge over Western Washington Thursday while the
operational GFS as a front approaching Western Washington.
Ensembles showing about 20 percent of solutions being wet so will
go with a slight chance of rain Thursday until there is more
consensus late in the period. Felton

HYDROLOGY...No river flooding expected in the next 7 days.
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&&

.SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Winter Storm Warning from 5 PM Friday to 11 PM PDT Saturday for
     West Slopes North Cascades and Passes-West Slopes North
     Central Cascades and Passes-West Slopes South Central
     Cascades and Passes.

PZ...Small Craft Advisory from 11 AM to 8 PM PDT Friday for West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory until 11 PM PDT this evening for Central
     U.S. Waters Strait Of Juan De Fuca.

     Gale Warning from 8 AM to 9 PM PDT Friday for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 8 AM to 11 PM PDT Friday for Northern
     Inland Waters Including The San Juan Islands.

     Small Craft Advisory from 5 AM Friday to 2 AM PDT Saturday for
     Puget Sound and Hood Canal.

     Small Craft Advisory from 5 AM to 11 PM PDT Friday 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.
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&&

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