Washington State Forecast Discussion

Western Washington


195
FXUS66 KSEW 281101
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
401 AM PDT Sun Apr 28 2024

.SYNOPSIS...Unsettled weather with a series of frontal systems
crossing the region through the beginning of next week. Multiple
rounds of showers are expected, with the chance of thunderstorms
Sunday and Monday and moderate snow in the Cascades. The pattern
will dry out some mid week, with more showers possible Thursday
and Saturday. Warmer temperatures will also return second half of
this week.

&&

.SHORT TERM /TODAY THROUGH TUESDAY/...An occluded front passed
through the region yesterday, that brought showers across the
region. Post front, there are a few bands of showers still on
radar in the Cascades and along the coast this morning (via a
couple of surface troughs in these regions). Between systems (with
an approaching shortwave trough from Canada), the showers will
remain isolated through the Sunday morning hours, and begin to
increase in coverage this afternoon as an upper-level low tracks
southward from Canada by Monday. This system is unusually cold
aloft for a late April system, with 500 mb temperatures down at
-35 C, which has some implications for the impacts expected in the
next 48 hours.

The precipitation Sunday/Monday breaks down into higher
elevations,and lower elevations. For highland areas (Cascades and
Olympics), the cool air aloft is expected to work its way towards
the surface tonight/Monday morning, with morning lows expected to
be in the low 30s and upper 20s. This will drop snow levels to
just under 2,000 feet. Southwest/west flow aloft is expected to
tap moisture (in addition to a jet max driving moisture from the
Pacific via westerlies).

All things considered, snow will be the primary precipitation
falling in the mountains from Sunday through Monday. A Winter
Weather Advisory continues for the Cascades at elevations greater
than 3,500 feet for moderate snowfall amounts, though impacts may
be enhanced given the late season timing of this system. With
cold air coming from the north, the precipitation should
transition from rain to snow during the late Sunday evening/Monday
morning time frame. When all set and done, most areas will likely
see 4 to 8 inches of snow (despite the warmer surfaces that may
compact some of the snow down on the surface). There's a 20%
chance that snowfall at Snoqualmie and Stevens Pass exceeds 6
inches. There still will be some snow flakes for areas between
2,000 feet and 3,500 feet (i.e. Snoqualmie Pass), but the snow is
not expected to stick on roadways. For areas in the advisory
(including Stevens Pass), minor travel impacts are expected due to
slick roads.

For the lowlands, post frontal isolated showers are expected to
increase this afternoon as the low approaches from Canada. There's
a slight chance for thunder this afternoon, which may be limited
due to slightly warmer temperatures aloft today. With the cooler
air aloft in place on Monday (along with 200 J/kg CAPE, 0-3 km
SRH around 100 m2s2, and 40 kt of 0-6 km shear), the thunder
chances will be significantly greater. Some of the stronger storms
may be able to produce small hail and gusty winds. Again, coverage
will be scattered, with breaks in between bands/clusters of
showers.

Tuesday will start to gradually dry out as the low departs the
region to the southeast. Expect high temperatures in this period
to top out in the mid 50s in the low lands, with 40s in the
highlands. Southwest winds will remain light around 5 to 10 mph,
with higher gusts possible near waterways today, however.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...The drying trend from
Tuesday continues into Wednesday with a break in troughs. There
still some uncertainty as to if a trough will pass through on
Thursday with showers, but it appears that ensembles/deterministic
models are pointing towards Friday being dry (via a ridge), and
potentially a trough passing through Saturday with showers.
Temperatures will start to increase on Wednesday through the
remainder of the week, with highs in the low 60s for lowlands (50s
for mountains), and lows in the 40s. Winds will remain light.

HPR

&&

.AVIATION...West-southwest flow aloft as upper-level troughing
remains anchored over the northeastern Pacific. Southwest surface
flow once again today with a 70% chance for wind gusts from 20 to
25 kt at the Puget Sound terminals and HQM from 18 to 02Z.

Conditions have improved to mostly VFR this morning, with a few
locations still holding on to MVFR. Mostly VFR with CIGs from 3000-
4000 feet generally this afternoon with a gradual lowering to MVFR
levels possible (35% chance) after 06Z this evening for the Puget
Sound terminals. Scattered showers with a lightning strike or two
possible this afternoon and evening thanks to the weak instability
associated with the aforementioned troughing over the Pacific
Northwest.

KSEA...MVFR this morning, with breezy southwest winds from 10 to 15
kt, with wind gusts increasing to 20 to 25 kt from 20-02Z (25%
chance for wind gusts reaching 25 kt). Expect CIGs to gradually
rise to 3000 to 4000 feet this afternoon with southerly wind gusts
developing once again. There's currently a 70% chance for wind
gusts of at least 20 kt from 18-02Z. CIGs look to lower after 06Z
tonight with a 40% chance for MVFR conditions to return.

Davis

&&

.MARINE...Unsettled conditions continue over the area waters with
breezy to windy conditions today as another round of Small Craft
Advisories go into affect later this morning for Puget Sound with
southerly winds increase and through the central and East Strait of
Juan de Fuca as westerly flow strengthens. Steep seas from 6 to 8
feet will build to 8 to 10 feet this afternoon. Scattered showers
and thunderstorms will be possible over the area waters this
afternoon and Monday afternoon, with brief and localized wind gusts
to SCA thresholds possible, though not widespread enough to warrant
advisories at this time over the coastal waters.

Seas remain elevated through Tuesday from 8 to 10 feet followed by a
gradual decrease to 4 to 6 feet into the latter portion of the week.

Davis

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory from 11 PM this evening 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 early this morning 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 from 11 AM this morning to 5 AM PDT Monday
for Central U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory from 1 PM this afternoon to 5 AM PDT Monday
for East Entrance U.S. Waters Strait Of Juan De Fuca.

Small Craft Advisory from 8 AM this morning to 8 PM PDT this
evening for Puget Sound and Hood Canal.

&&