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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS66 KSEW 292243
AFDSEW

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

.SYNOPSIS...
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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.
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&&

.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
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&&

.AVIATION...
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Areas of mid and high level clouds continue this
afternoon, with coverage expected to increase further this evening
and into the overnight period. 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
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&&

.MARINE...
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A storm system well south of the area off the CA Coast
continues to spread cloud cover across the waters this afternoon,
with overall light winds and calm seas. A very weak weather system
will approach the area late tonight, making for a wind shift back to
onshore. Expect the strongest winds through Thursday morning to be
down the Strait of Juan de Fuca where a small craft advisory will
remain in place. Winds across the coastal waters will likely range
from 15-20kt, with isolated small craft advisory conditions
possible, but not widespread enough for an advisory.

A few showers will develop through the day tomorrow, especially near
the coast, with the next seasonally strong storm system slowly
approaching from the open Pacific. West winds look to be slow to
subside through the Strait through the day tomorrow, and it`s
possible the SCA will need to be extended beyond 18z. Elsewhere,
expect winds to be slightly breezier than previous days from the
W/SW 15-20kts.

Winds from this approaching strong storm system off the Pacific will
really begin to pick up Friday afternoon and Friday night, with
widespread SCA level criteria expecting to be met, and potentially
even gale force winds around the East Entrance/Admiralty
Inlet/Northern Inland Waters. Widespread rain will also accompany
this system Friday night into Saturday. Breezy to locally windy
conditions will continue through most of the weekend, with headlines
likely to continue and seas building towards or slightly above 10ft.

Kovacik
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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 from 6 PM this evening to 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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&&

$$