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FXUS66 KSEW 201018
AFDSEW

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

.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...NW flow aloft will turn more from the SW this
afternoon as an upper-ridge shift east of the Cascades. High-level
moisture will increase as the day progresses due in part to an
incoming front. As precipitation spreads onshore from the
aforementioned front, MVFR cigs are possible for KHQM. However,
mostly VFR should remain throughout the TAF period for most
airfields. Low level easterly flow will quickly snap onshore with
the arrival of the front. Gusty post-frontal W/SW surface winds are
expected across much of the area Saturday afternoon and evening
before decreasing overnight. More VFR conditions are expected for
Sunday as high pressure builds.


KSEA...VFR observed over the terminal currently. That trend should
last throughout much of the upcoming TAF period. A front is expected
to reach the coast this afternoon with areas of light precip
arriving at the terminal after 00Z, but ceilings likely remaining
above MVFR thresholds. E surface winds will reverse to S/SW around
20Z on Saturday and remain gusty well into Saturday evening. Gusts
up to 25-30 kt are possible before decreasing overnight.

McMillian

&&

.MARINE...High pressure continues across much of PNW. However, the
next weather maker looms offshore around 130 W as of this morning.
Offshore flow at the surface currently but as a front enters, we`ll
see 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. 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

&&

.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.

&&

$$

NWS SEW Office Area Forecast Discussion