Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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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.

&&

$$

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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