070
FXUS66 KSEW 152205
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
305 PM PDT Thu Aug 15 2024

.SYNOPSIS...Weak troughing over the region will maintain cooler
and partly cloudy conditions through the rest of the week. A low
will deepen along the coast through the weekend, increasing the
threat of thunderstorms across western Washington on Saturday and
Sunday. Unsettled conditions look to continue into next week with
cooler temperatures and chances of showers.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY/...Recent satellite imagery
shows partly cloudy skies across western Washington this
afternoon, with a weak low pressure system lifting northeastward
through southern Oregon/eastern Washington. As this low continues
to move through the evening, there is a chance that wrap around
moisture generates some convective showers in the North Cascades.
Confidence remains quite low, but cannot rule out a lightning
strike or two especially along the North Cascade crest through
this evening.

Elsewhere, temperatures this afternoon will be below average,
in the low to mid 70s for the interior and mid 60s for areas along
the water and the coast.

The aforementioned low exiting our area will continue to maintain
a varying degree of onshore flow on Friday, with temperatures
remaining in the low 70s. There is continued chance of
thunderstorms and showers, primarily in the North Cascades through
the evening as well.

Our attention focuses to Saturday as an upper level low drops
down from British Columbia and deepens as it spins offshore, with
widespread showers expected across W WA. With S/SE flow aloft,
latest guidance continues to depict a thunderstorm threat across
the lowlands and the Cascades. Elevated instability will increase
starting Saturday afternoon, with latest probabilistic guidance
indicating a 20-35% of thunderstorms across the Cascades and
Puget Sound. Areas along the coast have at least a 10-15% chance,
with the primary areas of thunderstorm development throughout the
lowlands and the Cascades. Probabilities peak especially Saturday
evening into the overnight period, with nocturnal thunder
possible at this time.

Along with thunderstorms and rain showers, we will continue to
monitor the potential for slides, especially in the burn scar
areas in the Cascades. See the Hydrology section for more
details.

Ensemble guidance continues to fluctuate on the exact position of
the upper level low offshore, which could lead to differences in
rainfall totals, timing, and thunderstorm chances for the area.
Confidence continues to remain low on coverage of thunderstorms
and showers, but can expect locally heavier precip in any heavier
showers or thunderstorms that do develop. The main threat with any
storms that do develop will be lightning, gusty and erratic winds,
small hail, and heavy rainfall.

The threat for thunderstorms continues through Sunday morning but
will start to diminish by Sunday afternoon, with still a 10-25%
chance of thunderstorms. The highest probabilities will be focused
in the Cascades (25%) with lower probabilities in the lowlands.

.LONG TERM /MONDAY THROUGH THURSDAY/...Troughing looks to be
favored in the Pacific Northwest throughout much of the long term
period, maintaining cooler temperatures across the region
and general chances of showers. CPC and clusters generally agree
with this output, as do most of the ensembles at this time.

Mazurkiewicz

&&

.AVIATION...Southerly flow will increase aloft into Friday as upper
troughing develops offshore. VFR mid level cigs have continued into
this afternoon across central and south Puget Sound, otherwise,
clearer skies are observed generally across northern Puget Sound and
coast. SCT/BKN cigs will likely linger into this evening for
southern Puget Sound. MVFR/IFR cigs will develop along the coast
this evening and slide inland into southern and central Puget Sound
Friday morning. Stratus inland will slowly improve to VFR into the
afternoon, with lingering VFR/MVFR cigs along the coast. W/SW winds
will continue through this evening before becoming lighter tonight.

KSEA...VFR with lingering mid-level BKN cigs this afternoon. SCT/BKN
cigs likely to continue into this evening. Stratus will move into
Puget Sound and near the terminal early Friday morning. MVFR cigs
are possible Friday morning, generally 13-18z, with probabilities
around 35% of MVFR cigs. Improvement to VFR expected into Friday
afternoon. S/SW continuing into Friday. JD

&&

.MARINE...An onshore flow pattern will generally continue through
the weekend for area waters. A westerly push through the Strait of
Juan de Fuca will result in SCA winds and wind gusts this evening
into tonight, with gusts generally ranging 20 to 30 kts. Onshore
flow will become a bit lighter later Friday into Saturday for a
weaker push through the Strait of Juan de Fuca. Northwest winds will
continue for the Coastal Waters through Saturday before becoming
southerly Sunday into early next week.

In addition, the threat of thunderstorms will increase later
Saturday afternoon and continue into Sunday for all area waters,
with the threat peaking Saturday evening into Saturday night.
Thunderstorms could produce brief gusty winds as well as frequent
lightning during this period.

Seas will generally remain 3 to 5 feet into the weekend. JD

&&

.HYDROLOGY...Impulses coming out of the low pressure trough aloft
will bring very slight thunderstorm chances to the northern Cascades
near the crest this evening and Friday afternoon. There is a very
low probability of flash flooding for that area if a slow moving
thunderstorms sets up over the area Friday. The bigger threat comes
over the weekend.

The low pressure system that moves over the area Saturday into
Sunday will bring plenty of moisture. Rainfall will be widespread,
but total QPF has 90% of 0" to 0.25", ranging to a 10% chance of 1"
to 3.7". It will be fairly unstable with thunderstorm chances
having increased up to 35%. If a thunderstorm sets up over an
area, especially a burn scar, rainfall rates and/or amounts could
be enough to trigger flash flooding and debris flows. JBB

&&

.FIRE WEATHER...The low pressure system moving over the area this
weekend could bring widespread thunderstorms. However, the
conditions going into this event - fuel moistures relatively high,
showers preceding and with the thunderstorms - fire starts due to
lightning will be somewhat limited. But even if there is plenty
of lightning and multiple fire starts, the conditions will curtail
fire growth. With continued cool, cloudy, and high humidity
conditions well into next week, holdover fires may only smolder
for quite sometime until we get our next hot, dry, or windy
conditions. JBB

&&

$$

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Friday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$