169
FXUS66 KSEW 171642
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
942 AM PDT Sat Aug 17 2024

.UPDATE...There are a few showers and isolated thunderstorms that
developed over south Lewis county early this morning. These
storms/showers will continue to drift into portions of the
Olympic Peninsula and dissipate later this morning. Active weather
is still expected later on this evening, with widespread
thunderstorms and showers expected to develop early this evening
through tonight.

&&

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

&&

.SHORT TERM /TODAY THROUGH MONDAY/...A strong synoptic weather
pattern is taking shape today across the region. 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 to the northwest. A jet max on the east side of the
low will move up from California into Washington today ahead of an
embedded shortwave trough that will swing around the low.
Dynamics are impressive with strong vorticity advection in the
left exit region of the jet max. The cool temperatures and
moisture advection aloft will help prime the atmosphere for active
weather later today.

Daytime heating will help destabilize the atmosphere, as
temperatures climb into upper 70s and low 80s today. Moisture
will also be present down at the surface, with dew points climbing
into the upper 50s to low and mid 60s. The 00Z HREF ensemble has
areas of 1000-2000 J/kg of CAPE in the Cascades, Olympics, and
around 500 J/kg in the Southwest Interior. Sounding profiles show
the classic "inverted v" shape, with capping in place until
Saturday afternoon/evening (after daytime heating).

The shortwave trough is expected to fire a complex of showers and
thunderstorms this afternoon over the Oregon Cascades.
Hodographs/SRH favor storm motion to the north, which will bring
the storms into Washington by 5 PM along the Washington/Oregon
border. They will reach Puget Sound between 7 and 10 PM, and the
Canadian Border around midnight. Storm mode is favored to be in
clusters/segments, spanning from the Cascades to the Washington
coastline. Additional pre-complex isolated storms are possible in
the South Cascades/Olympics mid afternoon, but confidence is low.

SPC has placed far eastern Lewis County in a slight risk for
severe thunderstorms Saturday, and a marginal risk east of a line
from Toledo to Puyallup to Gold Bar. WPC has also placed a
marginal risk for excessive rainfall for all of Western Washington
today. The strongest potentially severe storms may produce
damaging winds of up to 60 mph (based on strong DCAPE of 1200
J/kg), hail up to quarter size (particularly in Cascade areas
south of Paradise where lapse rates reach 7.7 deg/km), and heavy
rainfall (PWAT anomalies up to 1 inch). Remaining storms will
likely contain smaller hail and gusty winds. Rainfall rates may
approach a quarter of an inch per hour with the strongest storms.
If the storms entrain over same areas over any length of time
(storm totals expected to be 0.50 to 0.75 inches), it may be
enough to trigger flash flooding/debris flows in the Cascades,
especially over burn scars (see hydrology section). Persons in
Western Washington should stay tuned for possible warnings later
today, and have a plan of action if you are under one.

Sunday/Monday: Storms will exit into B.C. Canada Sunday morning.
The low will begin to track northeast towards B.C. Sunday into
Monday. As it slides up, there will remain a chance of isolated
thunderstorms Sunday morning into the afternoon (best chances in
the Olympics). Flooding/severe weather is not expected with any
new storms that develop Sunday. Some sun will make its way back
into Western Washington Sunday afternoon, with more clouds on
Monday, and a slight chance of showers for the coast/Olympics and
North Cascades. High temperatures Sunday and Monday top out in the
low to mid 70s (60s on the coast).

.LONG TERM /TUESDAY THROUGH FRIDAY/...Ensemble/deterministic
models have strong agreement of troughing off the coast moving
inland Wednesday into Thursday. This will keep the pattern
unsettled through mid week, with the chance of showers Tuesday
through Thursday. There is a slight chance of isolated
thunderstorms Tuesday afternoon for a majority of the coverage
area, and in the North Cascades/Bellingham area Wednesday
afternoon. No severe weather is expected at this time with these
storms. Friday will be the driest day in the extended, with partly
sunny skies. High temperatures will remain in the low to mid 70s
through this period (60s along the coast).

HPR

&&

.AVIATION...Low pressure remains centered offshore, which maintains
generally southwest flow aloft. Generally clear skies over the area
with passing high clouds, but low stratus remains just offshore and
likely returns to the coastal terminals for a few hours around 12z.
Otherwise, attention turns to the next disturbance that will lift
north this evening and overnight, providing the trigger for
scattered thunderstorms to move from south to north. Strong storms
will feature abundant lightning, gusty outflow winds, and low
ceilings/visibility in heavy rain. Expect the prime threat starting
by 00z in the south and ending early Sunday morning by 12z, with
some lingering isolated showers into Sunday. In addition to the
direct impacts from thunderstorms with gusty and erratic winds,
expect a general increase in gusty southerly winds this evening in
association with this disturbance lifting north.

KSEA...VFR conditions through the morning. Surface winds north 7
to 10 kt though early afternoon, switching to southerly this
evening. Expect scattered thunderstorms to develop after 00z and
move north across the terminal around 03-09z tonight. Abundant
lightning, heavy downpours bringing IFR ceilings/vis, and gusty
and erratic winds are the predominant hazards, though some hail is
also possible. Strong thunderstorms taper overnight with some
lingering showers into Sunday. Cullen

&&

.MARINE...Low pressure center well offshore with a high over the
far northeastern Pacific through the weekend. A passing disturbance
will produce southerly winds over most waters and west winds through
the Strait of Juan de Fuca through the weekend.

Winds generally 15 to 20 kt through early next week, but will need
to closely monitor the potential for a push of gusty southerly winds
through much of the interior waters starting in Puget Sound and
extending through the Strait of Georgia in association with a
passing disturbance late Saturday evening and night. This appears
like a short duration burst of wind, so will closely monitor the
evolution and likely handle this with MWS/SMW hazards rather than a
broad SCA given the shorter duration impact. Additionally, scattered
thunderstorms will bring gusty winds and frequent lightning, and
perhaps some hail as the move from south to north.

Seas will hold around 4 to 6 feet over the coastal waters through
the early portion of next week and remain dominated by short period
waves. Cullen

&&

.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. Even if there is plenty of
lightning and multiple fire starts, the conditions will likely
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/Mazurkiewicz/HPR

&&

.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 from 5 PM PDT this afternoon through Sunday
     morning 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 from 5 PM this afternoon to 11 PM PDT this
     evening for Central U.S. Waters Strait Of Juan De Fuca-East
     Entrance U.S. Waters Strait Of Juan De Fuca.

&&

$$