462
FXUS66 KSEW 202217
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
317 PM PDT Thu Jun 20 2024

.SYNOPSIS...A upper level ridge offshore will continue to build
into the region through Friday. The ridge will shift east of the
area on Saturday. An upper level trough moving into British
Columbia on Sunday will produce cloudier and cooler conditions along
with a chance of showers. Weak upper level ridging will bring a
modest warming trend and generally dry conditions for much of the
area early next week, followed by another trough and cooler
conditions late week.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...Mostly clear skies continue
to prevail across western Washington this afternoon. A few
building cumulus clouds are evident over the northern half of the
Cascades. With the upper level trough lifting to the northeast,
instability will be weaker than the past few days. However, the
potential still exists for a few isolated thunderstorms along and
east of the Cascade crest this afternoon. Most activity this
afternoon and evening should be confined to east of the crest.

Temperatures across the area are mostly in the mid 70s. Onshore
flow is helping to keep the immediate west-facing coasts
comparatively cool, in the upper 50s to low 60s. High
temperatures today should reach the upper 70s to low 80s, up to
the mid 80s in the Southwest Interior. Clear skies and light winds
tonight will allow temperatures to cool efficiently, likely
reaching the low 50s to upper 40s in some spots. Another marine
push overnight will bring some low stratus and patchy fog to the
coast that should dissipate in the early morning hours tomorrow.

An upper level ridge building offshore will quickly pass over the
area tomorrow, bringing the warmest day of the week to most. High
temperatures across most of the interior in the mid 80s, though
onshore flow will keep the Pacific Coast and North Interior
cooler.

No major heat impacts are expected today or tomorrow, with only a
few pockets of Moderate HeatRisk in the warmest areas and urban
cores, affecting the most vulnerable populations.

A large closed low from the Gulf of Alaska will being to move its
way southeastward towards the region on Saturday, quickly pushing
the ridge off to the east. Highs across the interior will be a
few degrees cooler on Saturday, but remain above average.
Increasing clouds Saturday evening with a deepening marine layer
along the coast as onshore flow ramps up ahead of a weak frontal
system, with some light drizzle possible along the Pacific Coast.
The upper-level low will move across the region on Sunday,
bringing additional showers, mostly for the higher elevations and
northern portions of the region. Temperatures will be much cooler,
with highs in the mid to upper 60s.

.LONG TERM /MONDAY THROUGH THURSDAY/...Ensemble and deterministic
models are in agreement that the upper-level low will be quickly
ejected to the east on Monday. A weak ridge will move through in
its wake, amplifying as it moves east out of the region on
Tuesday. If this remains consistent, Tuesday will be a bit warmer
and drier than the rest of next week, as another large trough
looks to move through the region late week, keeping the pattern
cooler and moist.

LH

&&

.AVIATION...Light westerly/northwesterly flow this afternoon with
rising heights over western Washington, with light onshore flow in
the lower levels. VFR continues to dominate over the region this
afternoon, with mostly clear skies across the area. Low clouds and
fog will once again develop along the coastline around 06z-09z
tonight and continue into early Friday morning, mainly affecting
KHQM with IFR/LIFR conditions. Elsewhere, VFR will continue
into Friday.

KSEA...VFR with clear skies into Friday. North/northeasterly winds
8 to 12 knots this afternoon will continue through the evening,
with a potential for a gust or two around 15 knots. Winds will
start to decrease in speeds after 09z to around 4 to 8 knots.

Maz

&&

.MARINE...Broad high pressure continues to be situated over the
coastal waters with lower pressure inland. Onshore flow will start
to increase later on Friday as an incoming frontal system
approaches the area waters. Latest guidance shows small craft
advisory strength winds through the Central and Eastern Strait of
Juan De Fuca, so went ahead and issued a Small Craft Advisory
through early Saturday morning. There may be a gust or two that
nears gale strength, but mostly winds will remain at small craft
strength.

Another push down the Strait Saturday night will likely lead to
another round of headlines, with this push looking to be stronger,
with a 40% chance of gale strength force winds at this time.

Weak high pressure will begin to rebuild through the water waters
into the early part of next week, with onshore flow weakening.

Combined seas 4 to 6 feet will remain through the rest of the
week, rising around 6 to 8 feet beginning on Sunday.

Maz

&&

.FIRE WEATHER....High pressure continues with warmer and dry
conditions through Friday with daytime humidities dipping into the
20-30% range in the driest spots as light offshore flow develops. In
addition, mid level Haines values of 6 is likely through Friday. The
return of cooler air and increased moisture looks to arrive by later
Saturday into early next week.

JD

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$