Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 251553
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
850 AM PDT THU AUG 25 2016

.SYNOPSIS...A warm ridge of high pressure through Friday will give
way to an upper trough and much cooler onshore flow Saturday. An
upper trough will linger into next week with a chance of showers.

&&

.SHORT TERM...It will be quite warm today and again on Friday for
the interior of Western Washington. The coast will start to cool on
Friday. Much cooler air will push into Western Washington for the
weekend as an upper trough arrives. The forecast looks pretty good
and no changes to it are expected.

.LONG TERM...
Now for more on this big weather pattern change. Looking further
upstream is a shortwave moving off the east coast of Asia supported
by a strong jet stream and Typhoon Lionrock south of Japan. These
kinds of systems often foretell a change in the pattern across the
northern Pacific and that is the case here heading into next week.

The devil is in the details though. The GFS, ECMWF, and Canadian
models all agree on the large scale pattern adjustment, yet differ
on the timing and intensity of shortwaves that may offer some
precipitation. Of note, western Washington has not had any
precipitation since August 7th and significant precipitation since
July 22nd.

For now, the big story is maintaining the cooler temperatures and
cloud cover arriving Saturday into next week and introducing the
threat of showers early next week in the forecast. Once the progs
obtain a better feel for the evolving events in the western Pacific
and Bering Sea, the finer details should emerge. Buehner

&&

.AVIATION...Northerly flow aloft will continue today and tonight
with an upper ridge offshore. At the surface, offshore flow will
prevail with a thermal trough along the coast. The air mass is dry
and stable.

KSEA...Clear. Northerly wind 5-10 knots. Schneider

&&

.MARINE...A thermal trough along the coast will shift inland
Friday afternoon and evening with offshore flow switching to
onshore. This will bring small craft advisory west winds to the
Central and Eastern Strait of Juan De Fuca Friday evening. Gales
are possible with the initial push but seem more likely later
Saturday as onshore flow strengthens. Onshore flow will weaken on
Sunday. A front will reach the waters Monday. Schneider

&&

.FIRE WEATHER...Thermally induced low pressure along the coast
today will increase north to northeasterly flow over the area. This
pattern will produce warmer temperatures and low daytime humidity
through Friday. Sustained winds over the Puget Sound lowlands and
the northwest Olympic Peninsula will exceed 10-12 mph this afternoon
and evening. A Red Flag Warning remains in effect for 4 fire weather
zones where we are most likely to see 3-5 hours of moderate north-
northeast breezes combined with relative humidity below 30 percent.
On Friday, the breeze does not look strong enough to reach the
Crater for Red Flag conditions, but it will be hot and dry on Friday
nonetheless.

Any fires that get started in these conditions will have the
potential to exhibit rapid rates of spread. Mercer

&&

.SEW Watches/Warnings/Advisories...
WA...Red Flag Warning until 10 PM PDT this evening for Black Hills
     and Southwest Interior Lowlands-Central and South Puget
     Sound Lowlands-North Coastal Lowlands-Southeast Puget Sound
     Lowlands Generally Below 1500 Feet.

     Heat Advisory from noon today to 8 PM PDT Friday 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...None.

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.weather.gov/seattle/gafd/latest_webafd.html



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