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752
FXUS66 KSEW 210957
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
257 AM PDT Sun Jul 21 2024

.SYNOPSIS...
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A weak shortwave will produce spotty showers to the
Olympic and Cascade mountains through this afternoon and evening
along with the potential for elevated thunderstorms. An upper
level trough moving into British Columbia into the first half of
the week will cool temperatures closer to seasonal norms and
increase night and morning cloudiness.
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.SHORT TERM /TODAY THROUGH TUESDAY/...
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Upper level ridging will
shift eastward later this morning allowing for a broad upper level
trough offshore to nudge closer to western Washington. This will
allow for stratus to spread inland this morning, clipping Puget
Sound. Stratus will burn back by the afternoon, but will allow
temperatures to be cooler today, with locations in the interior
generally in the low to mid 80s this afternoon, and coastal
locations in the mid 60s.

The aforementioned upper level trough will eject a weak shortwave
through our area later this morning into the afternoon. Hi-
resolution guidance suggest activity starting early this
afternoon along the coast and the Olympics, then moving eastward
into the Cascades. Model guidance lands mid- level lapse rates in
the 7-8 C/km range, which would support elevated thunderstorms in
the mountains. Moisture is very limited, but with southerly flow,
these factors can definitely help kickstart a thunderstorm,
especially with diurnal heating. The main concern with any
thunderstorm that does develop is the risk for lightning and
erratic winds, for fuels remain very dry in both the Olympics and
the Cascades. See the fire weather section below for more
information regarding fire weather.

Any thunderstorm potential will end by tonight as instability
values vanish and diurnal heating ends. Low level onshore flow
will increase into Monday and Tuesday as broad upper level
troughing takes over. Normal temperatures will return over western
Washington, with highs in the interior in the mid to upper 70s,
and 60s for coastal locations. Morning cloudiness will be likely
each day with periods of sun in the afternoon.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Broad upper level
troughing remains situated over the Pacific Northwest through the
long term, with varying degrees of onshore flow, morning stratus,
and temperatures hovering around normal. As of right now, no
significant weather is expected through the long term (and no
precipitation).

Mazurkiewicz
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.AVIATION...
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An upper ridge remains centered over the Intermountain
West with a broad upper level trough offshore for southerly flow
aloft over Western Washington. The air mass will become somewhat
unstable as an upper level disturbance embedded in the southerly
flow lifts northward across the area. Low level onshore flow will
increase today. LIFR/IFR stratus will spread inland to just west of
Puget Sound before burning back to near the coast by around 19Z.
The aforementioned upper level disturbance will produce isolated
thunderstorms across the coastal waters and Olympic Peninsula
through 18Z. Isolated thunderstorm potential continues over the
Cascades through 00Z.  The air mass will stabilize tonight with
increasing onshore flow spreading IFR or low MVFR ceilings in
stratus into the Puget Sound terminals Monday A.M..

KSEA...VFR conditions with mid and high level moisture associated
with elevated instability spreading overhead this morning. Current
thinking is that thunderstorm potential will remain west of the
terminal this morning. Stratus will spread inland tonight and likely
reach the airport 12Z-16Z on Monday morning with IFR or low MVFR
ceilings. Surface winds north to northwesterly 5 to 10 knots backing
to southwesterly after around 09Z Monday.  27
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.MARINE...
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Broad surface ridging will remain situated over the
offshore waters with lower pressure east of the Cascades through
much of the week ahead. A series of weak upper troughs moving into
British Columbia will increase the onshore flow at times.
Strengthening onshore flow today ahead of the first of these is
expected to produce westerly gales in the central/east strait by
this evening. Additionally, northwesterlies will increase across the
coastal waters. A small craft advisory has been posted for
the southern portion of the outer coastal waters for a combination
of choppy seas and wind. This could be expanded to a larger portion
of the coastal waters if conditions warrant. Persistent onshore flow
will likely necessitate additional headlines for the strait and
portions of the coastal waters in the days ahead. 27
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.FIRE WEATHER...
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A Red Flag Warning will continue for zones 652,
661, 658, and 659 through Monday morning. A weak disturbance will
bring the potential for scattered elevated thunderstorms over
receptive fuels, particularly this afternoon into the evening
hours. All it takes is one lightning strike to ignite a fire on
such dry fuels. Convective activity should end by late tonight
possibly into early Monday morning. Will continue to monitor the
situation as the conditions evolve. A cooler and more humid
pattern develops Monday as an upper level trough settles over the
region.
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.SEW WATCHES/WARNINGS/ADVISORIES...
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WA...Red Flag Warning from 5 AM early this morning to 6 PM PDT this
     evening for East Portion of the Olympic Mountains-West
     Portion of the Olympic Mountains.

     Red Flag Warning from 8 AM this morning to 5 AM PDT Monday for
     West Slopes of the Central Cascades Generally above 1500
     Feet-West Slopes of the North Cascades Generally above 1500
     Feet.

PZ...Gale Warning from 5 PM this afternoon to 3 AM PDT Monday for
     Central U.S. Waters Strait Of Juan De Fuca-East Entrance
     U.S. Waters Strait Of Juan De Fuca.

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