437
FXUS66 KSEW 211604
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
904 AM PDT Tue May 21 2024

.SYNOPSIS...A frontal system and trailing upper trough will lead
to cooler and wet conditions across Western Washington today into
Wednesday. Weak ridging aloft will produce drier and somewhat
warmer weather on Thursday before another trough arrives Friday
into Saturday for a return of cooler, unsettled conditions. A
trend toward drier weather and more seasonable temperatures is
possible early next week.

&&

.UPDATE...Rain continues to fall across the coast this morning. It
is expected to continue and spread further inland into Puget
Sound/Cascades this afternoon and evening. Inherited forecast is
on track, no major changes to the morning forecast (other than a
refreshed aviation/marine discussion).

HPR

&&

.SHORT TERM /TODAY THROUGH THURSDAY/...Clouds are increasing
across the CWA early this morning ahead of a frontal system
pushing southeastward down the British Columbia coast. Rain will
reach coastal areas before sunrise then spread across most of the
interior by late this morning. A cold front will sweep onshore
late this afternoon with rain turning to showers this evening. QPF
remains fairly high for late May with the much of the lowlands
expected to pick up a half inch to an inch of rainfall. Post-
frontal onshore flow ramps up this evening and this should get a
convergence zone going for a few hours over Snohomish/King
Counties.

The trailing upper trough shifts southeastward across the area
during the day on Wednesday. This will keep showers and cool
temperatures in the forecast across interior areas, though the
focus for the precip should be mainly in the Cascades by Wednesday
afternoon. Snow levels will dip to 3500 to 4000 feet in the
Cascades, but impactful snowfall is not expected in the higher
passes. Much like yesterday, upper ridging centered well offshore
briefly noses in the area for drier and slightly warmer conditions
on Thursday.

LONG TERM /FRIDAY THROUGH MONDAY/...Another trough is slated to
drop into the region Friday into Saturday for more showers and
cool temperatures. There are signs of a shift in the synoptic pattern
thereafter, but how this ultimately will play out is uncertain.
Broad upper troughing over the western third of the country is
expected to be replaced by a strengthening ridge early next week
while a large trough takes up residence south of the Aleutians.
Of course, Western Washington is caught between the two. So, it`s
a question of which one will have the greater influence. The
latest suite of global ensembles have weakly positive height
anomalies over Western Washington Sunday into Memorial Day. At a
minimum, this suggests that temperatures will return to near
seasonal normals by the end of the holiday weekend with best
precip chances retreating to the Olympic Peninsula. If football is
a tale of two halves, the weekend weather may be much the same.
27

&&

.AVIATION...A mixed bag of MVFR/IFR with light rain. Conditions will
improve by the evening with post-frontal convergence zone showers
possible late Tuesday evening that could impact KPAE/KBFI/KSEA.
Lighter onshore flow will continue through the morning with stronger
southerly flow 5-15 kt developing by the afternoon. West to
southwest winds increasing to 10-20 kt by Tuesday evening with gusts
to 25 kt mainly for KHQM/KCLM.

KSEA...IFR conditions this morning with light rain, rain is expected
becoming heavier early this afternoon then ease by 22/04z.
Convergence zone is expected to stay north of KSEA/KBFI. S/SW flow
will develop through the morning with winds 10-15 kt through the
afternoon with gusts to 20 kt. Southwest winds are favored to stay
elevated into Wed morning with lower/MVFR ceilings persisting.

AL/KC

&&

.MARINE...A strong frontal system will cross area waters
today, boosting winds to SCA both offshore and through the Strait of
Juan de Fuca through the evening and into Wednesday. Seas will rise
above 10 ft across the offshore waters by Tuesday evening and
continue rise to as high as 16 ft by Wednesday especially over the
northern coastal zones. Waves will be steep with short period waves
dominating. Another frontal system may pass through area waters on
Friday, bringing elevated seas and breezy winds.

AL/KC

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 8 PM this evening to 5 AM PDT
     Wednesday for Central U.S. Waters Strait Of Juan De Fuca-
     East Entrance U.S. Waters Strait Of Juan De Fuca-West
     Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory from 8 PM this evening to 5 PM PDT
     Wednesday for Coastal Waters From Cape Flattery To James
     Island Out 10 Nm-Coastal Waters From James Island To Point
     Grenville Out 10 Nm-Coastal Waters From Point Grenville To
     Cape Shoalwater Out 10 Nm.

     Small Craft Advisory from 5 PM this afternoon to 8 PM PDT
     Wednesday 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.

&&

$$