Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA
FXUS66 KSEW 262200
Area Forecast Discussion
National Weather Service Seattle WA
300 PM PDT FRI AUG 26 2016
.SYNOPSIS...Cooler marine air will push into Western Washington
for the weekend and at times through next week. An upper level
trough will bring a chance of showers next week.
.SHORT TERM...The record warmth will end and temps will get back
down to more average readings. It may take until Sunday morning
for much marine stratus to form, there still is not much off the
coast, and the OTH-SEA pressure gradient had not yet come up much
early this afternoon--but cooler air is a sure bet for the weekend
as an upper trough replaces the upper ridge. The chance for showers
is likely to hold off til around Monday.
.LONG TERM...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 are possible at the coast late tonight and
Saturday morning, with mostly MVFR conditions inland with local IFR
possible. Cigs will lift Saturday afternoon with partial scattering.
KSEA...West to Northwest wind 5-8 kt, easing by midnight. Southerly
winds 4-5 kt after midnight, rising to 6-11 kt Saturday morning. VFR
conditions with just high clouds tonight. MVFR stratus is expected
to reach the terminal around 14z Saturday morning, then lift and
scatter in the afternoon. dtm
.MARINE...Onshore flow will develop over the waters tonight as
thermally induced low pressure shifts inland, and east of the
Cascades overnight. The onshore push will produce the highest wind
and waves in the central and eastern strait of Juan de fuca where a
small Craft Advisory is in effect for westerlies up to 30 kt
tonight, and possibly near-gale force.
Onshore gradients strengthen further on Saturday with Gale force
winds more likely in the central and east Strait during the late
afternoon and evening hours. A gale watch remain in effect from late
Saturday afternoon through Saturday night. A frontal system will
reach the area on Monday. dtm
.FIRE WEATHER...The thermally induced surface trough that was along
the WA coast this morning has shifted E of the Cascades this
afternoon. Winds have shifted onshore along the coast and will
gradually increase and spread inland tonight then continue on
Saturday. The onshore flow will eventually flood W WA with a cooler
moist marine air mass. The marine layer will be shallow tonight and
will not push completely inland by Saturday morning, but will finish
the job as it gradually deepens through Saturday. Highs on Saturday
will be significantly cooler with readings in the mid 60s through
the 70s. Minimum RH values will higher, above 50 percent for just
about everywhere except the mountains which may still have some
spots down to 30 percent.
A pattern change is currently taking place over the region. The
longwave upper level ridge that was just offshore a couple of days
ago has shifted farther W along roughly 155W. This will allow a
longwave trough to dig south over the Pacific Northwest on Saturday,
then continue through next week. Computer models show several small
upper level shortwave troughs rotating around the main longwave
trough through next week. Each of these shortwaves could bring some
showers as they cross the region. Confidence is not high regarding
the strength and timing of these features so for now the forecast
has more of a broadbrush chance of showers. Confidence is high that
conditions will be mostly cloudy and cool, with highs near or a
little below normal. Kam
WA...Heat Advisory until 8 PM PDT this evening for Bellevue and
Vicinity-Bremerton and Vicinity-East Puget Sound Lowlands-
Hood Canal Area-Lower Chehalis Valley Area-Seattle and
Vicinity-Southwest Interior-Tacoma Area.
PZ...Small Craft Advisory from 5 PM this afternoon to 5 AM PDT
Saturday for Central U.S. Waters Strait Of Juan De Fuca-
East Entrance U.S. Waters Strait Of Juan De Fuca.
You can see an illustrated version of this discussion at