Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 251702
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
900 AM PST Sat Feb 25 2017

.SYNOPSIS...An upper trough will south into western Washington on
Sunday morning. This will bring a chance for lowland snow over the
interior, especially in areas north and east of Seattle. A chance of
showers will continue through much of next week, with snow levels
rising after Tuesday.

&&

.SHORT TERM...Satellite imagery shows mostly sunny conditions on the
coast and over the south, with fog in the Chehalis valley. Mostly
cloudy conditions prevail over the northwest interior from about
Seattle north. The forecast is a bit optimistic for northern areas
today, but the clouds show a few signs of breaking up. Will wait a
bit before making any modifications to the forecast today. Highs
will be in the mid 40s.

The interesting part of the forecast is whether it will snow tonight
and Sunday morning. A deep upper low is forecast to move south along
the coast tonight, then southeast into Oregon on Sunday. A quick
glance at the latest models show most have snow levels no lower than
1000 feet, with a few dipping to 800 feet or so. The showery pattern
that will be associated with this upper low will likely bring the
actual snow level a bit lower. QPF in the models is 0.3 to 0.6 inch.
Around 12Z Sunday, the 1000-850 thicknesses are above 1300m on the
coast and just below 1300m inland. During most of the showery period
there will be southerly flow aloft and at the surface, which gives a
mild warming influence. The upshot is this looks like a marginal
event, with mixed rain and snow showers, and some light snow
accumulations possible on higher hills and also away from Puget
Sound.

Conditions warm a bit Sunday afternoon and highs are forecast to be
in the low to mid 40s. Most precipitation Sunday afternoon should be
in the form of rain showers. Another weak disturbance moves through
the northerly flow aloft on Monday morning. This will also probably
be mixed rain and snow showers, but QPF is notably lighter. Monday
may yet turn out to be the coldest day of this string, with highs in
the upper 30s to low 40s. Burke

.LONG TERM...Previous discussion...Flow aloft will become more
westerly starting Tue night, and upper heights will start to rise as
a flat ridge builds along the U.S. West Coast. So from Wednesday
through Thursday night, the weather will be characterized by
temperatures and snow levels rising back to near normal, with period
of light to moderate rain. Looks like a return to northwest flow
aloft next Friday could drive a strong, progressive cold front
through western Washington. Haner

&&

.AVIATION...At 9am it is sunny on the coast and over some areas
south of Seattle--except for some low clouds and fog along I-5 from
the Olympia area south and in river valleys. It is cloudy with cigs
around 5 or 6kft north of Seattle and into the Mountains and there
are clouds and flurries obscuring a good part of the north and
central Cascades. The volcanos are sticking up above the clouds.

Clouds will increase quickly this evening with showers developing as
onshore flow develops. This is due to a low pressure system that
will drop down from the B.C. coast this evening and over Western
Washington Sunday morning--brining rain and snow with a low snow
level.

KSEA...Showers will develop this evening and increase overnight,
with a mix of rain and snow. The precip may turn over to all snow by
daybreak Sunday--but the south wind will help keep the temp just
above freezing and the afternoon high temp should be up around 40
degrees. So, snow accumulation will most likely be confined to the
grass where a couple of inches is possible.

&&

.MARINE...A low will move over Vancouver Island this evening and
then drop over Western Washington Sunday morning. Low  pressure will
linger over the region through Monday. Stronger southwest flow will
develop Tue and Wed ahead of a stronger frontal system that will
likely reach the area around Thursday.

&&

.HYDROLOGY...No flooding is expected during the next week.

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small craft advisory central and eastern Strait of Juan de Fuca
and for the coastal waters.

&&

$$

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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