442
FXUS66 KSEW 301631
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
931 AM PDT Tue Apr 30 2024

.SYNOPSIS...Shower activity will continue through the evening
tonight before shifting southward. Conditions will be drier and
warmer Wednesday through Friday, with cooler and wetter weather
returning by the weekend.

&&

.UPDATE...A low pressure system will track through SW WA and NW OR
today brining with it showers and a 15-20% chance of
thunderstorms, mainly south of Seattle. Higher chances will exist
as you move closer to Portland. Any storms could produce small
hail, brief heavy rain, and lightning. The other concern we are
monitoring is the potential for areas of frost by Wednesday
morning, especially away from the metro areas and the coast where
overnight lows will fall to 33-36 degrees. A Frost Advisory has
been issued for these areas for tonight. Otherwise, the forecast
is on track and no additional updates are planned this morning.
-Wolcott-

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...A surface low will continue
to push onshore through the morning and slide eastward throughout
the day as an upper trough axis shifts over the Pacific Northwest.
Scattered showers will continue to stream eastward through the
morning hours, slowly shifting southward through the afternoon as
high pressure builds inland behind. The core of the surface low
will track onshore along the Columbia Gorge today, enhancing
instability with ensembles showing an average of 200-300 J/kg this
afternoon mainly south and west of Seattle. Lightning and graupel
cannot be ruled out of any stronger showers that develop in the
afternoon. Precipitation will taper off by the evening, with
temperatures staying on the cool side peaking in the mid 50s
across the lowlands.

Weak ridging will allow conditions to dry out late Tuesday, and
decreasing cloud cover overnight into Wednesday morning will allow
some locations to see temperatures bottom out near freezing.
Otherwise, dry conditions will prevail for much of the day
Wednesday with highs warming up to the upper 50s and lower 60s.

Another weak low pressure system will pass over the region late
Wednesday into Thursday, with some uncertainty remaining over the
exact track of the storm system. While most models bring this
system south of the area, a chance of precipitation is maintained
for much of the area through Thursday, favoring higher terrain.
Snow levels will hover near 3500-4000 ft with light precipitation
amounts, and temperatures will continue a warming trend increasing
into the low to mid 60s.

.LONG TERM /FRIDAY THROUGH MONDAY/...Weak ridging will expand
inland late Tuesday through Friday morning, with the potential for
some areas to approach 70 degrees by Friday afternoon. A low
pressure system is on track to move towards the Pacific Northwest
by next weekend, but forecast models show fairly large differences
in terms of placement and timing of any precipitation with this
system. While uncertainty remains through the weekend, ensembles
favor a cooler and wetter troughing pattern through Monday with a
return to near-normal or slightly cooler temperatures.

Lindeman

&&

.AVIATION...Cool upper level trough over the area today shifting
east tonight into Wednesday. Light flow aloft becoming northerly
overnight. In the lower levels surface low near KAST dissipating
this morning. Northerly flow developing in the lower layers this
afternoon.

MVFR ceilings this morning will slowly improve with mostly VFR
ceilings by 23z. Skies clearing from north to south this evening.
Patchy visibility 1 sm or less Northwest and Southwest Interior
12z-16z.

KSEA...MVFR ceilings lifting to VFR 21z-23z. Skies clearing after
05z. Light southerly winds switching to northerly 18z. Northerly
winds 8 to 10 knots 22z-04z. Winds switching back to southwesterly
6 knots or less after 06z. Felton

&&

.MARINE...A weak surface low will traverse the coastal waters
today and push inland along the WA/OR border by late morning/early
afternoon. Seas briefly around 10 feet over the outer coastal
waters with small craft advisory today. May see another low-end
advisory strength push of westerly winds through the Strait
tonight into early Wednesday, with probabilities highlighting a
50-60 percent chance of gusts occasionally exceeding 21 kt. Have
held off of small craft issuance for now, however this will
continue to be monitored through the day.

High pressure will build back into the area waters tonight into
Wednesday, before yet another low and its associated frontal
system move into the region Wednesday night into Thursday. The
period will remain active, with another frontal system expected to
traverse the area waters Friday night into the weekend.

Seas over the outer coastal waters look to subside below 10 ft
tonight. Seas over all the coastal waters will continue to subside
into midweek, hovering between 3 to 6 ft Wednesday through
Friday. Seas will then increase closer to 6 to 8 ft again over
the weekend. 14

&&

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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Frost Advisory from 8 PM this evening to 10 AM PDT Wednesday for
     East Puget Sound Lowlands-Everett and Vicinity-Hood Canal
     Area-Lower Chehalis Valley Area-Southwest Interior-Tacoma
     Area.

PZ...Small Craft Advisory until 5 PM PDT this afternoon for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm.

&&

$$