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FXUS66 KSEW 121516
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
816 AM PDT Sat Oct 12 2024

.SYNOPSIS...An upper level ridge will produce dry and mild
conditions across Western Washington this weekend before a series
of frontal systems bring rain, high mountain snow, and locally
gusty winds to the region throughout much of next week.

&&

.UPDATE...There continues to be isolated areas of patchy to dense
fog across the region this morning. Motorists should be prepared
to encounter these this morning through the mid and late morning
hours by leaving extra space and using low-beam headlights (especially
if traveling on freeways where traffic is moving at higher
speeds). Once the fog clears this morning, a beautiful Saturday is
on tap with mostly clear skies, and highs around 70.

HPR

&&

.SHORT TERM /TODAY THROUGH MONDAY/...Skies have cleared across
much of Western Washington overnight as an upper level ridge
rebuilds into the area. Along with clearing skies, patchy dense
fog has formed across much of the Southwest Interior, south Puget
Sound, and outlying river valleys. The stratus/fog will burn off most
areas by late this morning for a mostly sunny and mild afternoon
with light low level offshore flow. Low cloud coverage should be
more extensive on Sunday morning as the low level flow turns
onshore, but will burn off again most areas by midday for a dry
and mild afternoon. We`ll likely start to see some high clouds
begin to filter overhead by later Sunday afternoon as the upper
ridge axis shifts well east of the area ahead of an approaching
frontal system.

Clouds will increase Sunday night and light rain will likely reach
much of the coast before dawn breaks on Monday. Some light rain
will spread to interior areas by Monday afternoon, but this
initial frontal system will be pretty weak and QPF will be light.
This will be just the beginning of a substantial shift in the
pattern toward an extended period of wet and cooler weather
through much of the upcoming week.

.LONG TERM /TUESDAY THROUGH FRIDAY/...An upper trough digging
southward out of the Gulf of Alaska will send a more potent
frontal system across the area later Tuesday into early Wednesday.
Models have a 140+ knot jet streak offshore sending yet another
front quickly onshore later Wednesday. Periods of rain and locally
gusty winds can be expected with snow levels falling to the higher
passes by Wednesday night or Thursday as a trailing upper trough
axis swings onshore and 500 millibar heights fall below 550 dam
for the first time since last spring. Model consistency and timing
(both deterministic and ensembles) falls off by late next week,
but the overarching trend toward unsettled conditions looks to
remain in place well into the long term forecast period and
beyond.

27

&&

.AVIATION...South to southeasterly flow today as an upper ridge
remains over the area and an upper low moves across Oregon.
Generally clear skies early this morning with areas of fog
developing, particularly in areas of Puget Sound at KOLM and PWT,
but also near KBLI. Shallow fog expected to continue to become more
widespread into this morning with mostly clear skies and light
winds. LIFR conditions expected in fog through the morning, with
improvements between 17-19z towards VFR for the afternoon with just
a few high clouds. Onshore flow will then increase tonight into
Sunday morning for increasing stratus, initially along the Olympic
Peninsula, and moving into the interior into the morning. This will
result in additional lower cigs on Sunday morning. Light winds
today, increasing a bit more S/SE tonight into Sunday.

KSEA...LIFR at the terminal with fog and half mile visibilities.
Light flow and a moist airmass will keep fog present at the terminal
until around 17-18Z before scattering out. VFR conditions are
expected thereafter. Another round of low stratus is expected early
Sunday morning with increasing onshore flow. JD/Kristell

&&

.MARINE...High pressure over Western Washington will result in
generally light southeasterly/offshore flow today. Light winds are
expected for the interior waters with south to southeast winds over
the Coastal Waters into Sunday. Southerly winds will increase,
especially for the Coastal Waters, Sunday afternoon into Monday
morning as a front moves across Western Washington. At this time,
there is approximately a 60-80% chance of Small Craft Advisory winds
for the majority of the Coastal Waters, with the highest
probabilities along the outer Coastal Waters. South winds will also
increase for portions of the interior waters Monday morning,
particularly Admiralty Inlet and eastern Strait of Juan de Fuca, but
SCA winds are not expected at this time. Otherwise, a potentially
stronger front looks to move through Tuesday night into Wednesday
for another period of breezy winds. In addition, a post-frontal
westerly push is expected through the Strait of Juan de Fuca later
on Wednesday.

Seas of 4 to 6 feet this morning will build to 6 to 8 feet this
afternoon into Sunday. Seas will then build further to 11 to 13 feet
Sunday night into Monday. Seas may briefly subside below 10 feet on
Tuesday, before seas may build above 10 feet Wednesday and into
Thursday. JD

&&

.HYDROLOGY...Signals for two atmospheric rivers early next week are
appearing in the forecast. The first will arrive late Sunday night
into Monday, and a second following in short succession Tuesday into
Wednesday. The second one looks to be somewhat stronger in terms of
integrated water vapor transport and forecast precipitation amounts
into western Washington, though there is considerable uncertainty
and model run-to-run variability. With these two systems in quick
succession next week, significant rises in area rivers will be
expected. Currently, no rivers are forecast to rise into action or
minor flood stage at this time, but the potential will continue to
be monitored and evaluated.

Kristell/LH

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...None.
&&

$$

NWS SEW Office Area Forecast Discussion