136
FXUS66 KSEW 291048
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
348 AM PDT Mon Apr 29 2024

.SYNOPSIS...A cool low pressure system from Canada will continue
to track south into Washington today. Showers with the chance of
thunderstorms will be possible across western Washington today.
Moderate mountain snow will continue in the Cascades, with
accumulations greatest at elevations greater than 3,500 feet.
There will be a break in activity late Tuesday into early
Wednesday before another disturbance passes through. Temperatures
will increase from the 50s to the 60s by the end of the week.

&&

.SHORT TERM /TODAY THROUGH WEDNESDAY/...Complex weather picture
next 36 hours with a system passing through. An upper level low is
tracking southward from B.C. Canada southward into Washington
today. This is driving a jet streak with upper level
divergence/low level convergence over the state, which combined
with a 500 mb vorticity max. provides a lifting mechanism for
precipitation. The temperatures with this low at the mid levels
are unusually cool for this time of year, with 500 mb temperatures
at -30 to -35 C. Satellite also shows a surface low over Haida
Gwaii, with an associated surface trough boundary tracking
southeastward with the previously mentioned upper-level features.

From this morning, convergence zone showers over Skagit/Whatcom
counties dissipated. Radar shows an apparent meso-low rotating
convective showers inland from Clearwater to Chehalis. Hi-res
model guidance shows this feature tracking into Puget Sound during
the early morning hours, which will cause a wet commute for most
areas in the sound. Coverage of showers will increase this morning
into the afternoon with multiple rounds making their way through
the coverage area.

SPC has all of western Washington in a general thunder risk for
Monday (as well as Tuesday in the Southwest Interior). With the
cool air aloft and upper air dynamics, this will cause the
atmosphere to become unstable throughout the day. Given CAPE
values up to 200 J/kg (with little CIN), 0-6 km shear 35-40 kt,
and SRH 80-100 m2/s2, a couple organized thunderstorm cells are
possible with the shower activity that develops. Exact location of
the thunder activity is hard to pinpoint at this time, but Hi-res
guidance suggests a convergence zone developing Monday afternoon
over Island/Snohomish/King counties, with some additional cells
forming in south Puget Sound, and the coast/Southwest Interior.
The strongest cells may be capable of producing small hail and
gusty winds, as well as lightning. Activity is expected to last
through Monday night/Tuesday morning.

For the mountains, moderate snow remains on track to fall in the
Cascades today. A couple of rounds of snow are expected in the
morning and afternoon/evening. Based on snow levels dropping
quickly to around 2,000-2,500 ft, and above freezing temperatures
from these snow levels to around 3,500 ft, snow is not expected to
be impactful at these levels (including Snoqualmie Pass). Below
freezing surface temperatures at 3,500 ft and higher will cause
snow to stick to surfaces. Snowfall rates are expected to range
from a quarter of an inch to half an inch an hour (with any
convective banding capable of brining rates to 1 inch an hour).
With confidence that snowfall will last through Monday evening,
the winter weather advisory was extended to 2 AM PDT (for
elevations greater than 3,500 ft - including Snoqualmie Pass).
Snow totals are expected to range from 4 to 8 inches, with heavier
amounts possible with any convective snow banding that
materializes.

Shower activity will continue through first part of Tuesday. As
the low departs, a weak upper level ridge will dry the region out
late Tuesday/early Wednesday. A weakening surface system/upper
level trough will return the chance of showers Wednesday
afternoon. High temperatures will range from the 50s in the
lowlands, to the 30s and 40s in the mountains. Lows will dip down
into the upper 30s and 40s in the lowlands, and the 20s/30s in
the mountains. Winds will remain light around 5 to 10 mph.

.LONG TERM /THURSDAY THROUGH SUNDAY/...Still a lot of disagreement
between ensemble/deterministic models with the long-term pattern.
With most models showing troughing in the extended, a chance of
showers will be possible Thursday into the weekend. Heavy
precipitation in the Southwest Olympic region is possible Thursday
and Sunday, but confidence is low due to variations in the models.
Temperatures return into the mid 60s by Friday into the weekend.
Winds are expected to be light.

HPR

&&

.AVIATION...Broad upper-level troughing remains anchored across the
Pacific Northwest with a pair of shortwave impulses pivoting through
the main trough axis-the first this morning and the second Tuesday
morning. Each of these are driving weak surface low pressure
centers, traveling across southern Washington and northern Oregon.

A variety of flight restrictions this morning, though conditions
have largely improved as more organized showers have moved onshore.
A mix of MVFR and low-end VFR through 18Z before CIGs lift into more
solid VFR levels. An organized complex of showers will move across
the southern Puget Sound terminals this morning through 12Z, with
more VCSH through the remainder of the day. Once again, cannot rule
out a lightning strike or two given modest instability and
anomalously cold air aloft, though confidence is too low to include
mention in any TAFs at this time.

Winds have shifted more to the south-southeast early this morning,
but are expected to shift back to the south-southwest by 12Z with
westerly winds at CLM and HQM. Mesoscale guidance has been pushing
the convergence zone southward through PAE around 16Z, though how
far south this area of convergence shifts remains uncertain. Winds
should largely favor the southwest after 03Z through the overnight.

KSEA...VFR conditions this morning as an more organized area of
light rain moves over the region through 13Z. A 60% chance for MVFR
CIGs by 12Z, which have been observed in this area of rain to our
west. A return to VFR is expected after 14Z through the evening with
MVFR CIGs developing after 10Z Monday night. Scattered showers with
a lightning strike possible this afternoon. Winds have shifted to
the southeast ahead of this area of showers, but should back to the
southwest later this morning from 8 to 12 kt, where they are
expected to hold through early tonight. The convergence zone looks
to shift south towards BFI after 16Z, but only a 25% chance for
winds to shift to briefly shift to the north after 20Z.

Davis

&&

.MARINE...Active and unsettled weather continues as we move into the
new workweek. Showers, some convective in nature with isolated
lightning strikes, will continue to skirt across the area waters
today into tonight as a pair of mid-level disturbances pivot
southeast beneath a longwave trough, driving a pair of weak surface
lows across the southern Washington offshore zones this morning and
Tuesday.

Small Craft Advisories continue over the coastal waters for steep
seas around 8 to 10 feet with a dominant period around 9 to 10
seconds through 5 AM. Breezy winds will continue today, though opted
to issue Small Craft Advisories through the central and eastern
Strait of Juan de Fuca, where confidence is highest for wind gusts
over 21 kt (80%). Brief and more isolated advisory-level wind gusts
cannot be ruled out elsewhere with the convective showers today,
though confidence is too low for additional advisories.

Seas from 8 to 10 feet this morning will drop closer to 7 to 9 feet
later today, then rise back to 8 to 10 feet into Tuesday as the next
weak surface low traverses the area waters. Seas then look to
subside to 4 to 7 feet Wednesday into the latter half of the week.

Davis

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Winter Weather Advisory until 2 AM PDT Tuesday 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 11 AM this morning to 10 PM PDT this
     evening for Central U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 1 PM this afternoon to 10 PM PDT this
     evening for East Entrance U.S. Waters Strait Of Juan De
     Fuca.

     Small Craft Advisory until 5 AM PDT early this morning for
     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.

&&

$$