Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 012240
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
240 PM PST Thu Dec 1 2016

.SYNOPSIS...Showers will turn back to rain at times Friday
and Saturday as weather systems reach the area. A strong cold
front Saturday night will be followed by a cold upper trough
Sunday and Monday. Cold and mainly dry weather on Tuesday could
turn wetter again around the middle of next week.

&&

.SHORT TERM...There is more shower activity spreading back into the
area this afternoon and showers tonight will transition back to rain
at times for Friday as a dirty ridge sort of pattern breaks down and
weak fronts move into the area through Saturday. A strong cold front
will push through the region Saturday night. The ECMWF has a little
better development of that frontal wave and puts a 1015mb low
through NW oregon around daybreak Sunday and that moves to the
Columbia Basin by 18z Sunday at 1010mb--a few days ago the Euro
developed that low quite a bit, but that has not been the trend and
the GFS only shows an average looking front.

.LONG TERM...There has been very good agreement in the models that
it will be about as cold as it gets aloft Sunday and
Monday in an upper trough. That means at 500mb, where nobody lives,
it will be -38c. But at the surface, that means that as showers move
though the area, the snow level will fall to near sea level. Looking
at 850mb--at first wet snow showers would probably only stick to the
grass, but as you get out into Monday and Tuesday as colder air
comes down the Fraser--then colder weather and a hard freeze become
likely. Of course it often happens that the moisture is gone by the
time the coldest air arrives. For now the forecast is light on
details and light on snowfall. By the time you get out to Wednesday,
the GFS suggests an overrunning snowfall that turns to rain later in
the day, but the Euro is slower--holding that off til Thursday.

&&

.AVIATION...Weak lift and warm air advection is resulting in
light precipitation across Western Washington this afternoon. The
flow aloft is N/NW. The air mass is moist and stable with MVFR
cigs. Rain will increase on the coast tonight and in the interior
early Friday morning as a warm front moves in. Showers will
continue Friday afternoon behind a cold front. 33

KSEA...MVFR conditions will likely continue tonight with light
precip in the vicinity. South winds to 10 kt. Rain will increase
early Friday morning with a warm front. 33

&&

.MARINE...Southerly winds will increase tonight and Friday as a
warm front moves in. The cold front will flip winds to onshore
Friday afternoon...with onshore flow continuing through Friday
night. A stronger system will bring rising winds Saturday and
Sunday. Northerly flow will develop on Monday as strong high
pressure forms over interior British Columbia. Fraser river
outflow may develop on Tuesday. 33

&&

.HYDROLOGY...No river flooding is expected over the next week.

&&

.CLIMATE...With an average temperature of 50.9 degrees November
2016 in Seattle was the second warmest November on record in
Seattle. The record is 51.9 degrees in 1899. The average high
temperature for the month was 56.0 degrees which was the 4th
warmest on record. The average low was 45.7 degrees which was the
2nd warmest on record. Felton

&&

.SEW WATCHES/WARNINGS/ADVISORIES...
WA...None.
PZ...Small Craft Advisory from 4 AM to 4 PM PST Friday for Coastal
     Waters From Cape Flattery To James Island 10 To 60 Nm-
     Coastal Waters From Cape Flattery To James Island Out 10 Nm-
     Coastal Waters From James Island To Point Grenville 10 To
     60 Nm-Coastal Waters From James Island To Point Grenville
     Out 10 Nm-Coastal Waters From Point Grenville To Cape
     Shoalwater 10 To 60 Nm-Coastal Waters From Point Grenville
     To Cape Shoalwater Out 10 Nm-Puget Sound and Hood Canal-
     West Entrance U.S. Waters Strait Of Juan De Fuca.

     Small Craft Advisory for rough bar until 4 AM PST Saturday for
     Grays Harbor Bar.

     Small Craft Advisory until 4 PM PST Friday for Admiralty Inlet-
     East Entrance U.S. Waters Strait Of Juan De Fuca-Northern
     Inland Waters Including The San Juan Islands.

&&

$$

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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