999
FXUS66 KSEW 180349
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
849 PM PDT Sat Aug 17 2024


.UPDATE...Showers and thunderstorms are currently tracking
northward into Puget Sound and W WA. Current radar trends have the
activity gradually weakening but frequent lightning continues to
be observed. The previous discussion remains below as the
inherited forecast remains on track. An updated marine/aviation
section is provided also.

&&

.SYNOPSIS...A strong upper level low will deepen off the coast of
Washington this afternoon, and will swing a shortwave trough across
the region tonight. A complex of showers and thunderstorms are
expected to develop and move through Western Washington during the
evening and overnight hours. A few of the storms may have the
potential to be severe, particularly in the Southern Cascades. The
unsettled weather pattern continues into next week, with additional
chances for showers and thunderstorms possible.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY/...An interesting synoptic
weather continues to take shape over the Pacific Northwest this
afternoon. Upper level analysis shows a strong upper level low
digging off the coast of Washington and Oregon over the Pacific,
deepening as it tilts negatively with S/SE flow aloft. A jet max
on the east side of the low will continue to move up from
California into Washington ahead of an embedded shortwave trough
that will swing around the low for an interesting evening.

THIS AFTERNOON INTO TONIGHT:

Diurnal heating will continue to boost surface instability this
afternoon, mainly over the Cascades and the Olympics - which will
help prime the atmosphere for the expected thunderstorm
development. Convection is expected to develop over the Oregon
Cascades later this afternoon as mid level heights begin to fall.
With southerly flow aloft, convection will start to move northward
into western Washington by this evening, continuing into the
nighttime hours. The main time frame for thunderstorm development
and movement across our area is generally 5PM-2AM, but cannot
rule out some stray showers and thunderstorms outside of that time
frame.

Convection will likely start first in Lewis County
(5PM-7PM) and generally move northward as the evening progresses.
Latest hi-resolution guidance indicates a cluster of storms
moving right through Puget Sound between 7 to 10 PM, reaching
Whatcom County/North Cascades by midnight. Again, these time
frames demonstrate the highest probability of thunderstorm
development, but outside any of these mentioned time frames, there
is still potential to have storms in the area. Those outdoors
during this period will want to monitor the forecast. If you are
outdoors during a thunderstorm, make sure to seek shelter in a
building or car.

DYNAMICS: Mean MUCAPE values from the latest 12z HREF generally show
values across Puget Sound ranging from 500 to 1000 J/kg ahead of the
convection, with locally higher values southward in Lewis and Pierce
Counties. Along with MUCAPE, DCAPE values (downward convective
available potential energy) values ranging from 1000-1200 J/kg,
the primary hazard from these thunderstorms will be frequent
lightning and gusty erratic outflow as winds aloft mix down to
the surface. With increased surface based CAPE due to diurnal
heating, along with steep lapse rates (7.0-7.5 deg/km), this may
promote an isolated severe threat, with storms potentially having
gusts > 58mph, and small hail. This isolated severe threat will
mostly exist across the southern Cascades, with the potential to
drift towards the Cascade Foothills in Lewis & Pierce Counties.
This threat can be noted on the latest Storm Prediction Center Day
1 Outlook, where a slight risk is shown clipping the Cascades of
Lewis County. It is also worth noting that areas north/northeast
from Toledo are in a marginal risk from the SPC (Seattle, Tacoma,
Everett, Gold Bar, and North Bend).

In addition to the lightning/severe threat, there are concerns
for heavier rain rates, particularly over burn scar areas in the
Cascades. PWATs, expected to range 1.25-1.50", are well above the
90th percentile for August (taken from climo of RAOBs at KUIL).
Instantaneous rain rates of short duration may exceed 1"/hour
(perhaps up to 1.5"/hour) in locally heavier thunderstorms.

With that said, due to relatively fast storm motion, 1hr
accumulation`s are likely to be closer to 0.25"-0.60" during the
heaviest period. Despite the fast storm motion, if a thunderstorm or
multiple thunderstorms continuously entrain over an area, especially
over a burn scar, rainfall rates could be enough to trigger flash
flooding or debris flows, which we will continue to monitor.
Refer to the Hydrology section below for additional information.
Also, for fire weather concerns, refer to Fire Weather section.

Again, the primary impacts of ANY thunderstorm that develops is
frequent lightning, gusty erratic winds, small hail, and heavy
rain. Be weather aware, and have multiple ways to get warnings.
Those outdoors during this period will want to continue monitor
the forecast. If you are outdoors during a thunderstorm, make sure
to seek shelter in a building or car.

SUNDAY/MONDAY:

The low will begin to track northeast towards B.C. Sunday
into Monday. As it slides up, there continues to remain a chance
of isolated thunderstorms Sunday morning into the afternoon, with
the best chances of thunderstorms over the Olympics and western
portions of the CWA (with latest NBM probabilities showing
20-25%).

Although most areas are expected to remain dry Monday, latest
guidance indicates that there may be an uptick in precipitation
potential along the Olympic Peninsula given the proximity of the
aforementioned upper low. In addition, a few isolated
thunderstorms may be possible across the far northern Cascades.
Otherwise, similar temperatures in the 60s and 70s are expected
on Monday and Tuesday.

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...Ensemble and deterministic
models have strong agreement of troughing off the coast moving
inland Wednesday into Thursday. This will keep the pattern
unsettled through the week, with the chance of showers and
thunderstorms, primarily in the Cascades through Thursday. Cooler
and showery conditions will continue into the weekend.

Mazurkiewicz

&&

.AVIATION...Showers and thunderstorms are tracking north through
the sound from roughly between now and 07Z, diminishing
thereafter to more stratiform rain. Along with the showers there
will be a southerly windshift with gusts to 30kt possible.
Isolated thunderstorm threat continuities Sunday afternoon.

KSEA...TSRA observed at the airfield currently. Gusty, erratic
winds possible, as well as a southerly wind shift with gusts to
30kt. Thunderstorm threat eases after 07z but still expecting
showers through the night.

33/McMillian

&&

.MARINE...A storm system is tracking north through western WA
this evening with a southerly wind shift and a brief period of
gusty S winds to 30kt possible. A westerly push is also expected
through the Strait of Juan de Fuca. Winds turn more SE on Sunday
and Monday with low pressure over the outer Coastal Waters.
Diurnal western pushes may bring Small Craft Advisory winds to the
strait at times (late afternoon and evening hours) the rest of
the week ahead. 33


&&


.FIRE WEATHER...A low pressure system moving over the area will
push a fast moving disturbance over western washington today,
bringing widespread thunderstorms across the area. However, given
the relatively high fuel moisture and widespread rain expected
with the thunderstorms, it is still expected that fire starts due
to lightning will be rather limited across the area. Even if
there is plenty of lightning and fire starts, the conditions will
likely curtail fire growth as wetting rains are likely with these
storms. 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/Mazurkiewicz

&&

.HYDROLOGY...The low pressure system that moves over the area
Saturday into Sunday will bring significant moisture into the
region. Rainfall will be widespread, but total QPF is expected to
range from 0.50 to 0.75 inches (locally higher amounts are
possible in heavy thunderstorms). If a thunderstorm (or multiple
thunderstorms) pass over an area, especially a burn scar,
rainfall rates and/or amounts could be enough to trigger flash
flooding and debris flows. JBB/HPR

&&


.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flash Flood Watch until 8 AM PDT Sunday for West Slopes North
     Cascades and Passes-West Slopes North Central Cascades and
     Passes-West Slopes South Central Cascades and Passes.

PZ...Small Craft Advisory until 2 AM PDT Sunday for Central U.S.
     Waters Strait Of Juan De Fuca-East Entrance U.S. Waters
     Strait Of Juan De Fuca.

&&

$$