Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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000
FXUS66 KSEW 152158
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
258 PM PDT Fri Sep 15 2017

.SYNOPSIS...An upper ridge will move inland tonight and Saturday
for dry weather with increasing mid and high clouds. Rain and
cooler weather will return Sunday and Monday as a low pressure
system moves into the Pacific Northwest. A lingering trough will
keep a chance of showers in the forecast most of next week.

&&

.SHORT TERM...An upper ridge over the area will shift inland with
a weak upper trough moving over the area on Saturday. The upper
trough looks dry and the main change will be for increasing mid
and high clouds. Highs will be in the 70s again Saturday - about
the same as today - and probably the last 70 degree day for a
while. Some smoke from fires over the Central Cascades near the
crest could drift into the area later tonight or Saturday but this
is uncertain for now.

A frontal system will bring a good shot of rain Sunday. Rain is
expected to spread onto the coast in the morning and into the
interior in the afternoon. Rain will change to showers Sunday
evening behind the front. Shower activity will be somewhat limited
behind the front Sunday night but should increase Monday as the
main upper trough swings through. Snow levels will drop to between
5000 and 5500 feet early Monday. Schneider

.LONG TERM...Western Washington will be on the backside of an
upper trough Tuesday and Wednesday for continued cool weather with
a chance of showers. An upper short wave could cause and uptick in
showers either Tuesday night or Wednesday. Models are not in good
agreement with the details and confidence is low.

Some additional drying is likely Thursday and Friday as northerly
flow aloft develops. There is too much uncertainty to go with a
completely dry forecast for now but this seems to be the way the
models are leaning. Schneider

&&

.AVIATION...An upper ridge is over the area. Northerly flow
aloft. The air is dry and stable. Cirrus will increase through
Saturday.


KSEA...Clear skies, some cirrus tonight and Saturday, light
northerly breeze this evening will become light and variable
tonight.

&&

.MARINE...Pressure gradients will be light through Saturday and
then onshore flow will increase Saturday night. A cold front will
arrive Sunday afternoon. Onshore flow will persist behind the
front Sunday night and through Monday and Tuesday as a trough
moves through the region.

&&

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

&&

$$

www.weather.gov/seattle

You can see an illustrated version of this discussion at
www.wrh.noaa.gov/sew/gafd/latest_webafd.html



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