Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 270432
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
932 PM PDT FRI AUG 26 2016

.SYNOPSIS...High pressure aloft over the region will weaken,
allowing the tail end of a low pressure system over western Canada
to move across the area on Saturday. This combined with moderately
strong onshore flow will result in significantly cooler
temperatures on Saturday. Weak high pressure aloft will prevail
Sunday and Monday.

&&

.SHORT TERM...
The low level onshore pressure gradient continued to tighten this
evening. The KUIL (Quillayute) to KBLI (Bellingham) gradient was
+2.1 mb and the KOTH (North Bend, OR) and KSEA (Seattle) gradient
was +5.3 mb at 9 PM PDT. Expect the onshore flow to continue
strengthening overnight, resulting in stratus over some areas
Saturday morning. The satellite derived fog product showed low
stratus along and off the Oregon coast drifting north at this
time. Expect clouds to develop and/or advect over the WA waters
overnight, with low clouds advecting over parts of the interior
Saturday morning.

Expect the tail end of an upper level trough moving across western
Canada to move across the area during the day Saturday. This
combined with moderately strong onshore flow will result in
significantly cooler temperatures. Most places will be 15-20
degrees cooler or about normal for this time of the year. A weak
short wave ridge will prevail over the region Saturday night and
Sunday. Onshore flow will persist, thus expect morning clouds
across the interior. The coast will likely stay cloudy.

Another upper level trough and associated cold or occluded front
will slowly approach the Pacific Northwest Sunday and Monday. The
flow aloft will back to southwest. The onshore gradient will also
weaken due to falling pressure over the offshore waters; therefore,
there will probably be less stratus Monday morning across the
interior. The latest GFS and NAM MOS guidances showed warming on
Sunday, with temps on Monday being about the same or slightly
cooler than on Sunday.

.LONG TERM from the previous discussion...
There will be a chance of showers at times next week as the
weather pattern changes to a fairly unsettled and more active
regime--with upper level troughs moving through the region at
times, or perhaps a more stationary trough parking just off the
coast. Model solutions differ in the details a great deal but the
12z GFS ensemble members agree pretty well that Friday could be
wet. The ECMWF is different from the GFS but the strongest
shortwave trough in the 12z Euro is timed for Thursday afternoon
and evening, so there is a little agreement there. Temps will be
near average, and maybe cooling a little below average at some
point.

&&

.AVIATION...High pressure will break down tonight as an upper low
approaches from the north. The trough will brush Western Washington
on Saturday and increase the low level onshore flow. Light northerly
flow aloft becoming moderate westerly later tonight and Saturday.
Dry and stable conditions this evening, then marine stratus reaches
the coast overnight and pushes into the interior lowlands Saturday
morning. IFR cigs at the coast late tonight and Saturday morning,
with mostly MVFR conditions inland. Clouds will lift and
scatter out over most of the interior Saturday afternoon.

KSEA...Tough call on the stratus at the terminal Saturday
morning.Surface gradients have turned onshore but stratus at 04z
still only along the Oregon coast. Onshore gradients continuing
overnight. At this point will stay with the idea of a late arrival
of the stratus at Sea-tac Saturday morning...14z-15z.
Ceilings with the stratus will be near 1000 feet. Conditions improving
with the cloud deck scattering out in the afternoon. Light southerly
winds rising to 8-12 kt Saturday morning. Felton

&&

.MARINE...Onshore flow will increase overnight as the thermally
induced low pressure shifts east of the Cascades. The onshore push
will produce the highest wind and waves in the central and eastern
strait of Juan de fuca where a gale warning has been issued for
Saturday into Saturday night for westerlies 25 to 35 kt. small
craft advisory winds could develop over the Northern Inland waters on
Saturday. for the remainder of the area winds are expected to remain
20 knots or less. A frontal system will reach the area on Monday.
Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Gale Warning in effect from 5 AM Saturday until 2 AM Sunday
 for the central and eastern Strait Of Juan De Fuca.

&&

$$

www.weather.gov/seattle

An illustrated version of this forecast discussion can be seen at
www.weather.gov/seattle/gafd/latest_webafd.html


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