Washington State Forecast Discussion

Western Washington


138
FXUS66 KSEW 280350
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
847 PM PDT Sat Apr 27 2024

.SYNOPSIS...The active weather pattern will continue into early next
week as a series of fronts pass through the region. Snow is expected
in the passes Sunday night into Monday. A trend toward temperatures
closer to normal and somewhat drier conditions is expected during
the later half of the coming week.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...Latest satellite imagery
shows W WA socked in with clouds as a weak front continues to slog
through the area. Current radar does a good job showing the porous
nature of this feature as echoes certainly represent a system
passing through, even though activity is essentially on the
scattered side. Models continue to support what radar loop is
showing with the front essentially stalling out over the area for
just long enough for the associated upper level trough to sweep into
the area Sunday, keeping the forecast wet. Inherited forecast
continues to handle this well and as such will not do a full rehash
here. For additional forecast details, please refer to the Previous
Discussion section below.

18

From Previous Discussion...Light precip continues to spread across
the area this afternoon ahead of an occluded front that will
dissipate as it moves onshore this evening. A few showers will
linger over Western Washington overnight behind the front. An upper
trough west of Haida Gwaii will dig southward into the region later
Sunday into Monday for cool, showery conditions. The cooler
temperatures aloft will destabilize the atmosphere enough for a
slight chance of thunder as well as lower snow levels to most of the
passes by Sunday night into Monday. Relatively warm ground
temperatures may limit accumulations at Snoqualmie Pass.
Nonetheless, higher Steven Pass may accumulate somewhere in the
range of 5 to 7 inches over a 12 hour period (06Z-18Z Monday).
Another shortwave arrives Monday night into Tuesday. The focus with
the precip with that system may be a little further south...perhaps
mainly the southern half of the CWA.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Deterministic model runs
have shown little to no run-to-run consistency for the long term
forecast over the past couple of days. And this meshes well with the
high degree of uncertainty expressed by the NBM during that time
period. There's increasing confidence that we'll see a short break
in the weather on Wednesday, but the forecast picture becomes
increasingly murky Thursday through Saturday. Despite the fact that
deterministic runs of the GFS/Euro have flipped from earlier
solutions, some of the ensemble means (the Euro in particular)
maintain some positive height anomalies late next week. Current
forecasts for late next week look awfully close to climatological
norms...and that's probably not a bad approximation at this time.

27

&&

.AVIATION...West-southwest flow aloft continues this evening with a
mixed bag of flight conditions as the front continues to advance
inland across Western Washington. Expect gusty surface winds and
lower ceilings across most of the area as the front approaches, with
a shift to west/southwest winds behind the front. Lower ceilings and
rain remain the story overnight, with some improvement in conditions
through the day Sunday as the rain transitions to showers in the
post-frontal air mass. Can't rule out an isolated thunderstorm or
two late in the period, after 00z Mon, but confidence too low for
inclusion at this time.

KSEA...Steady rain and gusty winds ahead of the approaching front
through the evening. Expect steady rain to transition to showers by
daybreak Sunday behind the front, with ceilings gradually trending
lifting through the day. However, surface winds likely to remain
gusty through the period. Isolated thunderstorm possible near
terminal after 00z Mon, but confidence remains low.

&&

.MARINE...The strongest winds over the coastal waters have eased
this evening as the front is pushing onshore. However, the seas
remain steep and as a result have extended the small craft
advisories for the coastal waters into the overnight period. Expect
seas to gradually ease and dominant period to again lengthen as the
locally generated short-period waves fade away. Elsewhere, will
maintain the advisories with southerlies ahead of the front over
parts of the interior, followed by a strong west push through the
Strait of Juan de Fuca.

Marginal wind gusts to SCA thresholds possible Sunday afternoon (35%
chance) over the offshore waters with another round of Small Craft
Advisories likely through the Strait with another afternoon push and
possibly through the beginning of next week. Expect combined seas to
build to near 10 feet Sunday night and Monday, before subsiding
through the early part of next week.

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 11 PM Sunday to 11 AM PDT Monday
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 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-Coastal
Waters From Point Grenville To Cape Shoalwater 10 To 60 Nm-
Coastal Waters From Point Grenville To Cape Shoalwater Out
10 Nm-East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory until 11 PM PDT this evening for Admiralty
Inlet-Northern Inland Waters Including The San Juan Islands.

&&