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276
FXUS66 KSEW 121017
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
317 AM PDT Thu Sep 12 2024

.SYNOPSIS...
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Weak upper ridging will produce drier conditions
across the area today into Friday before another upper level
trough arrives early in the weekend with additional chances for
some precipitation. Drier conditions are expected early in the
coming week before potentially unsettled conditions return.
Temperatures will remain slightly below normal.
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.SHORT TERM /TODAY THROUGH SATURDAY/...
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Weak upper ridging is
beginning to nudge into Western Washington early this morning as
the upper trough that brought widespread shower activity on
Wednesday continues to progress eastward into the Northern
Rockies. With light surface winds and lots of low level moisture
hanging around, we can expect some pretty murky conditions around
much of Western Washington through this morning. We`ll see some
sun breaks toward afternoon, but that is likely to be the extent
of things. Upper ridging over the area is already beginning to
flatten by early Friday as another upper trough and associated
front slips southward down the British Columbia coast. Some rain
will reach portions of the coast by late Friday afternoon then
spread to the interior Friday night into Saturday morning. A
convergence zone looks likely to form behind the front Saturday
afternoon into Saturday evening. Temperatures throughout the short
term forecast period will average a few degrees below normal.

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...An upper level trough will
settle over the area on Sunday, but shouldn`t produce much more
than a few showers. A glance at the ensembles indicates that the
flow will become more amplified heading into early next week as
broad upper ridging takes up residence well offshore while a mean
trough position develops over the western third of the lower 48.
In this scenario, the flow over Western Washington becomes
somewhat split. Weak upper ridging will produce drier conditions
Monday, but confidence in the forecast quickly drops off Tuesday
into Wednesday. Both deterministic and individual ensemble members
show a considerable spread in the track of a system diving
southward near or to the west of the area before it eventually
cuts off. Current NBM forecasts have some fairly high PoPs for
Western Washington Tuesday into Wednesday, but it wouldn`t take
much of a shift in the system track to scale those back. Either
way, the signs of autumn are well underway and temperatures are
set to remain a little below normal for mid-September. 27
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.AVIATION...
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Northwesterly flow aloft as an upper-trough resides
downstream in concert with ridging upstream. Onshore flow resides in
the lower levels. Mostly VFR conditions being observed at this time.
However, light surface flow combined with ample low level moisture
will allow MVFR to IFR ceilings to develop as the morning progress.
Ceilings will lift towards VFR around 18-20z this afternoon. An
additional round of low stratus is at play overnight into Friday
morning before again lifting in the afternoon.

KSEA...VFR conditions in place currently. But, MVFR to high-end IFR
ceilings will redevelop between 11-13z this before lifting by 18-
20z. Light NW winds will transition to SW later this morning. SW
winds will increase to 4-8 kt between 19-21z before again becoming
lighter overnight along with the redevelopment of low stratus.
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&&

.MARINE...
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Surface ridging will build across the coastal
waters today followed by transient, weak surface troughing into the
weekend. Onshore flow looks to persist throughout the upcoming week.
Westerly winds through the Strait of Juan de Fuca between 10-20 kt
Thursday night into Friday. It is a borderline situation with Hi-Res
guidance indicating a 50-60% chance of SCA gusts up to 25 kt are
possible within the Central/Eastern strait for a few hours this
evening before diminishing overnight into early Friday morning.

Seas 6 to 8 ft over the outer coastal waters this morning before
decreasing to 4 to 6 ft by the evening. Seas will continue to
gradually subside again towards 3-5 ft again over the weekend.

McMillian
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&&

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

&&

$$