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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KSEW 300354
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
854 PM PDT Wed Mar 29 2023

.UPDATE...
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Mostly quiet conditions are expected tonight as
transient upper-level ridging passes over. Overnight lows won`t
fall too far from normal for late March. Slight adjustments have
been made to the inherited forecast. We`ll let the previous
discussion remain below with an updated marine/aviation section:
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&&

.SYNOPSIS...High clouds associated with upper level low off
Northern California coast will move out of the area later tonight.
Flow aloft becoming northwesterly Thursday with weak upper level
trough moving through late in the day. A seasonally strong frontal
system will cross the area Friday night into Saturday with lowland
rain and mountain snow. Cool upper level trough moving into the
area for the weekend into the first part of next week. Snow levels
with the trough will be very low for the first few days of April.

&&

.SHORT TERM /TONIGHT THROUGH SATURDAY/...
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Satellite imagery shows
strip of middle level clouds rotating around the upper level low
well to the south over the central portion of the area. Mostly
sunny skies over the remainder of Western Washington.
Temperatures at 3 pm/22z were in the mid 50s to lower 60s.

As the upper level low off the Northern California coast continues
to move south tonight, the mid level cloud deck over the area
will also get pulled south of the area. Flow aloft becoming
northwesterly 06z-12z. This will also help remove the cloud cover.
Lows Thursday morning in the mid 30s to lower 40s.

Weak upper level trough embedded in the northwesterly flow aloft
reaching the coast late Thursday afternoon with the trough moving
through the interior Thursday night. Main story with this feature
will be the increase in the cloud cover. Can`t totally rule out a
shower as the trough moves through so forecast will be worded
slight chance or chance of showers later Thursday into Thursday
night. The increasing clouds during the day will keep highs in the
lower to mid 50s with lows in the mid 30s to lower 40s.

Weak warm front moving into Western Washington Friday morning
bringing light rain to the area. A strong for the 1st of April
cold front approaching the coast in the afternoon. Heavier rain
out ahead of the front spreading over the area in the afternoon.
Highs will have a hard time getting out of the 40s.

Cold front moving through Western Washington Friday night. Snow
levels ahead of the front will initially be in the 2500-3000 foot
range making the precipitation at all the passes except for
Snoqualmie snow ( rain snow mix possible at Snoqualmie ) early
Friday evening. The snow levels will lower to near 2000 feet by
early Saturday morning for snow on all the passes. Breezy with
the front in the lowlands with gusts 30-40 mph. Lows in the mid
30s to lower 40s. A winter storm watch for the Cascades above 2500
feet has been issued with the watch beginning at 00z Saturday ( 5
pm Friday ).

Cool upper level trough moving over Western Washington Saturday
with snow levels continuing to slowly lower during the day.
Showers in the lowlands with snow showers in the mountains.
Increasing post frontal onshore flow will give the area some
breezy conditions. Highs will be about 10 degrees below normal, in
the upper 40s.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...A pattern that we have
seen many times before this year will continue Sunday with a broad
upper level low moving over Western Washington. 500 mb
temperatures off the 12z model runs around -35C, very cold for
this time of year. With such a cold air mass over the area any
daytime heating could trigger a thunderstorm or shower with small
hail. Winter storm watch for the Cascades coming to an end late
Sunday afternoon. Possible 1 to 2 feet of snow above 2500 feet
between late Friday afternoon and late Sunday afternoon. Highs
once again will only be in the mid to upper 40s.

Upper level low drifting off to the southeast Sunday night into
Monday morning. Snow levels at this point will be down around 500
feet. As is usually the case, the moisture is decreasing in this
time frame but not fast enough to rule out shower activity.
Surface gradients are light so at least right now it does not look
like there will be convergence to enhance the precipitation and
give Snohomish county any snow accumulations. Lows Monday morning
in the 30s with the colder locations below freezing.

Weak troughiness remaining over the area Monday keeping a chance
of showers in the forecast. Highs remaining unseasonably cool,
near 50.

Operational model solutions differ for Tuesday and Wednesday with
the GFS building a ridge over the area while the ECMWF still has
some wrap around showers from the departing low Tuesday before a
very weak ridge moves over the area Wednesday. While the ensemble
solutions on both models show a reduction in the number of wet
solutions for the period, even the GFS with the decent upper
level ridge has 20-30 percent of the ensemble solutions with
light precipitation. Will go with a broad brush slight
chance/chance of showers forecast for Tuesday and Wednesday. A
little warmer by Wednesday but highs still only in the lower to
mid 50s, below normal for the first few days of April. Felton/McMillian
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&&

.AVIATION...
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Increasing high clouds across the region this evening
and overnight. VFR ceilings are expected through at least midnight
before winds shift onshore and advect lower level cloud cover
through Thursday morning. MVFR to low-end VFR ceilings will be
possible Thursday morning before yielding a BKN VFR ceiling in the
afternoon. A few showers will also develop Thursday afternoon. Winds
this afternoon N/NW 5-10kts, shifting W/SW by mid-evening and easing
slightly before becoming more S/SW into Thursday.

KSEA...VFR ceilings through the rest of today and most of the
overnight period. Lower level clouds flirting near the VFR/MVFR
range will advect in between ~13-16z Thursday morning before lifting
back to VFR in the 4-6kft range in the afternoon. A few showers
possible in the vicinity Thursday. Winds NW 5-10kts this afternoon,
shifting W/SW around 03z easing some to 4-7kts.

Kovacik/Cullen
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&&

.MARINE...
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A weak disturbance crosses the waters tonight, bringing a
return of onshore winds. Winds will increase overnight through the
Strait of Juan de Fuca, with winds to 25 kt expected in the central
and eastern portions of the Strait. The current advisory remains
unchanged. Elsewhere, winds increase to 15-20 kt, but only
occasional gusts to 25 kt are anticipated.

A more significant frontal system approaches through the day on
Friday, with widespread advisory strength winds across the local
waters. Still some chance for gales as the southeast winds develop
through the eastern Strait and northern inland waters, but
confidence isn`t high enough to issue a gale watch yet. Expect gusty
winds to follow through the weekend.

Seas remain 3 to 5 feet across the coastal waters for the next day
or so, but will remain rather choppy as a short-period component
dominates. Seas then build closer to 8 to 10 feet over the weekend
behind the next frontal system. Cullen
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&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Winter Storm Watch from Friday afternoon through Sunday
     afternoon 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 until 11 AM PDT Thursday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.
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&&

$$