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000 
FXUS66 KSEW 252246
AFDSEW

Area Forecast Discussion
National Weather Service Seattle WA
246 PM PST Sat Feb 25 2017

.SYNOPSIS...An upper trough will move into the region on Sunday 
morning. A cold showery pattern will bring lowland snow, with one
to three inches forecast above an elevation of about 300 feet.
Most snow will melt Sunday afternoon. A chance of showers will 
continue through much of next week, with snow levels rising after 
Tuesday.

.SHORT TERM...A weather system easily visible on satellite 
imagery is barreling into the area from the north. The northern 
half of the state is already overcast and clouds are increasing. 
An upper level low will form off the north coast late tonight, 
move south along the coast Monday morning, then move southeast 
into Oregon Monday afternoon. There will be showers associated 
with this system, and many of the showers will have snow.

The latest models have snow levels no lower than 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. Overnight low temperatures will be in
the low to mid 30s. 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 snow accumulations possible on higher hills and 
also away from Puget Sound. Have issued a winter weather advisory 
calling for 1-3 inches of snow above about 300 feet for the entire
interior lowlands, from Whatcom County to Lewis County. On the
coast, and near the strait of Juan de Fuca, temperatures will be 
somewhat warmer, so have not issued an advisory there. 

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, and most snow on the ground
will melt. 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. Tuesday will be a little warmer, but there
will still be a chance of mixed rain and snow showers in the
morning. Burke

.LONG TERM...Temperatures will be closer to normal Wednesday
through Friday. On Wednesday and Thursday, zonal flow will bring
rain to the coast and snow to the mountains, with the Seattle
metro area in something of a rain shadow. On Friday a front is
forecast to move through the area, with rain likely everywhere.
Saturday will have post frontal showers with lower temperatures.
The GFS and Euro are in broad agreement about this progression.
Burke

&&

.AVIATION...Scattered showers are increasing this afternoon and will
a become a more steady precip overnight with rain and snow showers 
changing over to snow for many areas. Airports on hills have the 
best chance of accumulation. As the precip increases overnight cigs 
and vsby will drop, and then as the low shifts south Sunday the 
precip will taper off with partial clearing from the north in the 
afternoon.

KSEA...The latest TAF shows a mix of rain and snow at first, and 
then just snow falling at Sea-Tac from 11z to 19z. A winter weather 
advisory is in effect for 1-3" of snow, and hilltop areas like Sea-
Tac have a better chance of accumulation than down by the water.
Temps will be a little on the mild side so slush is a much better 
bet than any ice.

&&

.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...Winter Weather Advisory from 10 PM this evening to 10 AM PST 
     Sunday for Admiralty Inlet Area-Bellevue and Vicinity-
     Bremerton and Vicinity-East Puget Sound Lowlands-Everett 
     and Vicinity-Hood Canal Area-San Juan County-Seattle and 
     Vicinity-Southwest Interior-Tacoma Area-Western Skagit 
     County-Western Whatcom County.

     Winter Weather Advisory from 6 PM this evening to 10 PM PST 
     Sunday for Cascades of Pierce and Lewis Counties-Cascades 
     of Snohomish and King Counties-Olympics.

PZ...Small Craft Advisory until 7 PM PST Sunday 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.

     Small Craft Advisory until 9 PM PST this evening for Central 
     U.S. Waters Strait Of Juan De Fuca-East Entrance U.S. 
     Waters Strait Of Juan De Fuca.

&&

$$

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