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000
FXUS66 KSEW 201531
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
831 AM PDT Sat Apr 20 2024

.UPDATE...A clear and calm morning will allow for some warming
ahead of an incoming frontal system. Clouds will continue to move
inland throughout the morning, which may impact how warm
temperatures get today. A band of lowland rain and mountain snow
is on track to cross western Washington later this morning and
through the afternoon, bringing gusts to 30 mph to most places as
the front moves through and post-frontal rapid cooling across the
region. Accumulations will be light, with convective activity
behind the front and potential convergence zone showers on Sunday.
The chance for lightning is low, but a strike or two cannot be
ruled out in stronger showers that develop. No updates to the
forecast.

&&

.SYNOPSIS...After a warm start to the day, a cold front will
quickly traverse western Washington, bringing light rain and
mountain snow along with breezy conditions. Scattered showers and
light mountain snow continues into Sunday followed by drier
weather into Monday and Tuesday as high pressure builds across the
Pacific Northwest. Another quick-moving cold front looks to cross
the region Wednesday followed by a more unsettled pattern into
the latter portion of the week.

&&

.SHORT TERM /TODAY THROUGH MONDAY/...Main story in the short term
forecast period will be the roller coaster temperature swing after
another warm day today with highs nearing 70 to frost potential
by Sunday night.

A cool morning across western Washington but temperatures will
quickly warm this morning with highs inching towards 70 once again
across the lowlands (50% chance in Seattle, lower elsewhere). A
cold front will quickly cross the region late this afternoon and
evening, bringing a rapid drop in temperatures from the upper 60s
into the lower 50s and, eventually, the upper 40s by this evening.
Light rain and mountain snow will accompany this front, though it
will encounter the very dry airmass that is over the region,
resulting in only light precipitation totals. The quick 2-3 hour
bout of stratiform rain will give way to scattered showers this
evening behind the front, which will linger through much of the
day Sunday. Rainfall totals through Sunday will only measure to a
few hundredths of an inch over most of the lowlands, with the
highest totals of up to half an inch over western Whatcom and
Skagit Counties. Snow levels drop to around 2500 feet tonight,
but snowfall totals remain low with this moisture-starved, quick-
hitting system. Probabilities for at least 2 inches of snow
through Sunday night are only 20-30% at Stevens and Snoqualmie
Passes. Highest snowfall totals will stay confined to the North
Cascades with totals nearing a foot above 6000 feet in these
areas.

Breezy conditions will develop later this morning into this
afternoon as southwesterly flow overtakes the region along and
ahead of the incoming cold front. Occasional wind gusts over 30
mph will be possible in the 2-9 pm timeframe with at least 60%
confidence in gusts of at least 30 mph over much of the lowlands.

Mesoscale models are continuing to suggest convergence banding
will form late Sunday into Sunday night across King and Snohomish
Counties, which could increase snowfall totals, though confidence
on where these form and for how long remain unclear at this time.
Confidence is higher for additional light accumulations Sunday
afternoon and evening at Stevens Pass. Otherwise, much cooler
Sunday with highs in the upper 50s inland and low 50s along the
coast.

We continue to monitor the potential for frost formation into
Monday morning as skies clear and winds ease into Sunday night.
Greatest threat looks to be across the Southwest Interior and
across the East Puget Sound Lowlands. High temperatures begin to
rebound, however, as surface high pressure and mid-level ridging
quickly build eastward across the region. Highs creep back closer
to 60, with mid 60s across the Southwest Interior.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Ensemble solutions appear
supportive of a progressive weather pattern through Wednesday.
Tuesday continues to look dry and mild as high pressure builds
across Washington with highs in the upper 60s. Our next frontal
system looks to quickly move onshore by Tuesday evening with
chances for light showers and mountain snow. Long range ensembles
deepen longwave troughing across the Pacific Northwest Thursday
into Friday, though to varying degrees of amplitude, leading to
modest uncertainty in forecast high temperatures Wednesday through
Friday. Overall consensus with this broad troughing suggests cooler
and wetter conditions, with highs near to slightly below normal
for late April.

Davis

&&

.AVIATION...Combination of an upper level ridge moving onshore today
ahead of the next shortwave trough will switch flow aloft from the
northwest to the southwest. A surface cold front will swing through
with this system. Current observations show the front well off the
coast with a few high cirro-stratus clouds beginning to arrive ahead
of this system. As the front moves onshore this afternoon, a line of
rain showers will span along the front, and move east through the
entire coverage area through this afternoon/evening. A few
convergence showers may linger post-front over Snohomish County
through Sunday morning. Otherwise the showers decrease significantly
in coverage through Sunday. Ceilings will drop with the front to low-
end VFR (better chance of MVFR along the coast) through at least the
first part of Sunday. Winds remain gusty out of the southwest
through the day (up to 25 kt), but will turn to the west/northwest
later part of the day Sunday.

KSEA...Clear/VFR ahead of the front. Expect ceilings to fill in and
drop to around 4,500 ft by mid-afternoon as a cold front swings
through with showers/light rain from 22Z-04Z Sunday. Depending on if
a convergence zone sets up over Snohomish County, a small chance
that a few MVFR CIGS sneak into the terminal Sunday morning (chances
remains low at this time). Gusty southeast winds up to 25 kt will
become southwesterly rest of the day and into Sunday, with future
shifts to west/northwest expected late Sunday.

HPR

&&

.MARINE...High pressure continues across much of PNW. However, the
next weather maker looms offshore around 130 W as of this morning.
Surface flow is offshore currently, but as a front enters, expect a
sharp snap back to onshore flow across area waters. Because of this,
Small Craft Advisories will begin later this morning for the coastal
waters before spreading to inland water zones (central and east
strait, Puget Sound) as the day progresses before winds weaken
overnight. The strongest winds will be concentrated in the central
Strait of Juan de Fuca, with gusts up to 30 kt possible. High
pressure will then return Sunday afternoon into the beginning of
next week, with a few pushes of westerly winds down the Strait of
Juan de Fuca which will likely require additional headlines.

Seas currently 2 to 4 feet but will rise to 8 to 12 feet this
evening through Sunday morning before decreasing to 6 to 8 feet by
Sunday evening.


McMillian/HPR

&&

.HYDROLOGY...No river flooding is expected over the next 7 days.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Sunday for Central U.S.
     Waters Strait Of Juan De Fuca-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-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 5 PM this afternoon to 11 PM PDT this
     evening for East Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Small Craft Advisory from 2 PM this afternoon to 11 PM PDT this
     evening for Puget Sound and Hood Canal.

     Small Craft Advisory from 11 AM this morning to 11 PM PDT this
     evening for Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm.

&&

$$