930
FXUS66 KSEW 170351
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
851 PM PDT Fri Aug 16 2024

.SYNOPSIS...An upper low will deepen offshore over the
northeastern Pacific on Saturday. A shortwave will move northward
across the area Saturday night into Sunday bringing an increased
threat for thunderstorm and widespread rain. Unsettled conditions
then continue into next week with the upper low offshore before it
slides over the area midweek.

&&

.SHORT TERM /TONIGHT THROUGH MONDAY/...Troughing will linger
across the Pacific Northwest tonight. Radar shows showers along
the Cascade crest continuing to diminish this evening. The
forecast remains on track, so no updates are needed. The previous
discussion follows below. 14

Upper troughing will then deepen offshore of Washington/Oregon on
Saturday, acting as the catalyst for an active weather period,
more info below:

Saturday Afternoon-Saturday Night: A shortwave impulse located
within the aforementioned deepening trough will move inland over
Oregon Saturday and slide northward across Washington Saturday
night. This shortwave will become more negatively-tilted for
increasing S/SE flow aloft. Diurnal heating will boost surface
instability, mainly over the Cascades and Olympics, Saturday
afternoon. Precipitation has also slowed on model guidance over
the last 12-24 hours, allowing for additional surface heating on
Saturday, and warmer highs into the 80s. Convection is expected to
develop over the Oregon Cascades mid Saturday afternoon
associated with mid-level height falls. Southerly flow will then
allow for convection to move northward into Western Washington by
Saturday evening and continuing through Saturday night. Mean
MUCAPE values from 12z HREF across Puget Sound will range 700-1200
J/kg ahead of the convection Sat PM. In combination with dCAPE
(downward convective available potential energy) values near 1000
J/kg, the primary hazards from thunderstorms will be frequent
lightning and gusty outflow as winds aloft mix to the surface.
Furthermore, increased sfc based CAPE due to the diurnal heating,
along with steep lapse rates, may promote isolated severe storms
(gusts> 58 mph, <1" hail), mostly across the southern Cascades,
but also perhaps drifting towards the Cascade Foothills in
adjacent areas of Lewis & Pierce Counties. This is noted on the
latest Storm Prediction Center outlook introducing a Slight Risk
for the Cascades of Lewis County (last slight risk in SEW CWA was
5/30/2020). Thunderstorms will likely begin to slowly weaken
Saturday night across northern Washington, with HREF calibrated
thunder probs ranging from 80-90% from Seattle southward, and near
50-70% over Whatcom County.

In regards to timing, particularly given likely outdoor ongoing
weekend events, peak concern for thunderstorms is from 5PM-2AM.
HREF probs for 0.01" of precip initially begins in Lewis County
generally around 5-7PM, with convection moving northwards into
central Puget Sound (including Seattle) between 9PM-11PM, and
further north into Whatcom County from 12-2AM. With that said,
cannot rule out an isolated shower or thunderstorm developing in
the afternoon mainly over the southern Cascades or Olympics. 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.

In addition to the lightning/severe threat, concerns also exists
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 (from climo of RAOBs at KUIL).
Instantaneous rain rates (short duration) are likely to exceed
1"/hour (perhaps up to 1.5"/hour) in heavier thunderstorms. With
that said, due to relatively fast storm, 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 move over an area, especially a burn scar, rainfall
rates could be enough to trigger flash flooding or debris flows.
Refer to the .Hydrology section below for additional information.
Also, for fire weather concerns, refer to .Fire Weather section.

All in all, despite some uncertainty existing in the exact
locations of thunderstorms, in addition to the timing of storms
weakening, this looks to be an impressive event for Western
Washington standards for convection, particularly for areas
Snohomish County southward.

Sunday-Monday: HREF/guidance suggests convection continues to
slide northward early Sunday morning across the BC border. With
that said, there also remains a chance of thunderstorms on Sunday.
However, probabilities are lower around 10-15% on the NBM. Clouds
will likely linger into Sunday morning before some improvement in
the afternoon. Temperatures will also be cooler with highs in the
mid 60s along the coast to mid 70s in the interior. The upper low
continues to linger offshore of Vancouver Island on Monday,
allowing for continued onshore flow. Although most areas are
expected to remain dry Monday, there may be an uptick in
precipitation potential along the Olympic Peninsula given the
proximity of the 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.

.LONG TERM /TUESDAY THROUGH FRIDAY/...Ensembles are generally
consistent that the troughing offshore begins to slide over
Western Washington Tuesday and Wednesday. Precipitation potential
increases during this period, along with cooler temperatures. NBM
thunderstorm probabilities also increase a bit during this period
(mainly 10-20%) though uncertainty exists on the extent of
instability. Ensembles are then in agreement in the reinforcement
of troughing offshore over Oregon/California Thursday and Friday.
Cooler than normal temperatures are likely to continue for this
reason with continued onshore flow across the area. JD

&&

.AVIATION...An upper-low remains offshore bringing southwesterly
flow aloft. At the surface, mostly VFR conditions are being met area-
wide as marine stratus is gradually lifting and scattering. The
trend is set to continue tonight but, can`t rule out lingering
patches of stratus promoting IFR/LIFR conditions, particularly for
coastal terminals (KHQM, KCLM, etc.) and isolated inland locations
(KPWT) into early Saturday morning. Thunderstorms today are to
remain in the North Cascades however, they`re expected to become
more widespread Saturday evening. A few strong storms may be capable
of producing frequent lightning, gusty winds, small hail, and heavy
downpours.

KSEA...OVC skies with VFR conditions being reported. Skies are
slated to trend mostly clear tonight with VFR continuing overnight.
SW winds 5 to 10 kt turning NNE at 4 to 8 kt after 01z Saturday
before increasing out of the NW between 8 to 12 kt Saturday morning.
A 30% chance of thunderstorms are in the forecast Saturday evening.

McMillian

&&

.MARINE...1035mb surface high is well offshore but maintains onshore
flow here across area waters. Westerlies will persist through the
Strait of Juan de Fuca but no genuine threat of SCA are currently
expected. Seas will remain at 3 to 5 feet through next week. Marine
layer activity may limit visibility at times throughout the next
several days with the coast seeing the better chance. Also, a potent
disturbance may bring thunderstorms Saturday evening, particularly
over the inner waters. A few strong storms may be capable of
producing frequent lightning, gusty winds, small hail, and heavy
downpours.

McMillian

&&

.FIRE WEATHER...A low pressure system moving over the area will
push a fast moving disturbance over western washington on Saturday,
which looks to bring 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

&&

.HYDROLOGY...Impulses coming out of the low pressure trough aloft
will bring very slight thunderstorm chances to the northern Cascades
near the crest this afternoon and evening. 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 as stated above. Rainfall will
be widespread, but total QPF has 90% of 0 to 0.25in., ranging to a
10% chance of 1in. to 3in.. It will be fairly unstable with
thunderstorm chances having increased up to 75%. 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

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Flash Flood Watch from Saturday 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...None.
&&

$$