536
FXUS66 KSEW 210340
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
840 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 evening. Satellite
tonight shows some cumulus along the Cascade crest with radar
showing an isolated thunderstorm along the crest of the southern
Cascades - to the east of Mount Rainier National Park this
evening. With the upper level trough lifting to the northeast,
overall instability has been weaker than the past few days - which
has resulted in less thunderstorm activity in general, and with
most activity being confined to the east of the crest. Expect
thunderstorm activity to wane within the next hour as the sun
sets. The overall forecast remains on track. The remainder of the
previous discussion follows below, with updates made to the
aviation section. 14

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 tonight with rising
heights over western Washington, with light onshore flow in the
lower levels. VFR conditions continue to dominate over the region
this evening, 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. Northwesterly winds 8-12
knots this evening will shift to the north/northeast overnight
and decrease to around 4-8 knots by 09Z. Winds will shift back to
the northwest Friday, increasing to 5-10 knots during the
afternoon hours.

Maz/14

&&

.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...Small Craft Advisory from 5 PM Friday to 5 AM PDT Saturday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

&&

$$