Area Forecast Discussion
Issued by NWS Seattle/Tacoma, WA

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FXUS66 KSEW 152319

Area Forecast Discussion
National Weather Service Seattle WA
319 PM PST Thu Feb 15 2018

.SYNOPSIS...A weather system will arrive tonight and Friday. A much
stronger system will reach the area Friday night and Saturday. Cold
air will push into the region Sunday with the snow level falling to
sea level. It will be dry for the first part of next week.


.SHORT TERM...The chance for showers will increase overnight and
then the precip will fill in with areas of rain on Friday. The
heavier precip will be in the mountains and winter weather advisory
continues in the Cascades through the day on Friday. A more vigorous
frontal system will bring a round of rainy windy weather later
Friday night through Saturday. This system will likely warrant wind
advisories for some areas and heavy snow will fall in the mountains
on Saturday. It will not be cold yet Saturday--in fact the snow
level should rise to around 3000 feet and there is a chance that the
precip will turn over to rain for a little while on Snoqualmie Pass.
I think there is a better chance that it stays heavy wet snow
however, and certainly all the higher elevations have a good chance
of picking up two feet of snow from this weather system between late
Friday night through Saturday evening. Behind the front it will be
showery Saturday night, and the snow level will plunge at some
point, perhaps as early as later Saturday evening, but more likely
around daybreak Sunday.

.LONG TERM...Cold air aloft will arrive Sunday and Fraser outflow
will develop--pushing a modified arctic front south through Western
Washington during the day on Sunday. As mentioned above, that could
occur as early as Saturday night but will probably be Sunday morning-
-perhaps around daybreak for the Seattle metro area. There should be
some snow showers for the lowlands, especially along the arctic
boundary that pushes south but it is too far out to pin down details
like timing or accumulations. The UW WRFGFS suggests the Port
Angeles area could do quite well for snowfall Saturday night and
Sunday morning while the metro area only gets perhaps an inch--and
mainly in the south. The GEFS ensemble mean has been hovering around
two inches of snow for Sea-Tac--but is still too early to say, the
models could change quite a bit. Looking at the 500mb height change
forecast has 280m falls with the upper trough reaching Vancouver
Island Saturday morning--but height rises behind the system over
Western Canada are not strong--and the absence of strong height
rises over the Yukon and B.C. on Sunday mean that this is not the
canonical perfect storm like a November `85 type of event: The
strong shortwave trough pulls in the cold air aloft behind it--but
it isn`t forced down by a strengthening arctic high.

Looking at sea level pressure falls under the second round of more
modest height falls: On the 12z GFS there is a small area of
-2mb/3hr pressure falls along the modified arctic front which tracks
from the north Washington coast 12z Sunday and then across the south
part of the Olympic Peninsula around 15z--after that it fades over
southwest Washington around 18z. That is a pretty modest system--but
it is exactly what you want to watch--development on the modified
arctic front itself. Later shifts can start to tease out the chance
for some snow behind the main front Saturday night versus the chance
of lowland snow with the modified arctic front. Fwiw, it is
traditional to say `modified arctic front` rather than `cold winter
continental Canadian air`, but obviously this isn`t true arctic air
and lows that will probably dip into the teens by Monday night
outside of the city is hardly arctic...but it is awfully cold
compared to what we have seen so far this winter.

On Monday, gradients relax awfully fast with the Fraser outflow
piddling out quickly--but 850mb temps will remain cold around -13C
Sunday night through Monday night: The coldest min temps of the
winter sure seem like a good bet. Sunday night wont be the coldest
night--except for wind chill, with breezy cold winds, but after
the gradients relax Monday then Monday night should be the coldest
night with light winds under cold dry air aloft. If there is any
snow on the ground in the lowlands that could help get outlying
areas drop below the current forecasts Monday night. Dry northerly
flow aloft will prevail through Wednesday.


.AVIATION...Strong northwest flow aloft will push a cold front
through Western Washington on Friday. The air mass will generally
be moist and stable, though shallow surface-based instability is
possible. Warm advection in advance of the front will spread rain,
mountain snow, and lowering cigs into the area this evening.
Fropa at the coast 18z Fri, inland around 21z Fri.

KSEA...Will be a mix of scattered and broken clouds in the 030-050
range through 06z (10 pm). Cigs will come down steadily after 06z,
reaching the 010-020 range overnight as warm advection and light
rain in advance of Saturday`s front gets underway. Will be breezy
around mid-day Friday around the time of cold frontal passage.
S-SW wind gusts in excess of 20 knots are likely. Haner


.MARINE...A cold front will move southeast through the
waters on Friday, with southerly gradients and wind increasing

A strengthening low pressure center will move southeast down
Vancouver Island and across the Northwest Washington waters as a
999 mb low on Saturday afternoon, before continuing to northeast
Washington late Saturday. This will bring gales to most waters,
especially in the strong southwest gradients which will follow the
closest passage of the low center.

Strong northeast Fraser outflow wind will spread out across the
waters on Sunday and Sunday night, bringing gales to the northern
inland waters, the Strait, and possibly Admiralty Inlet. Fraser
Outflow will ease on Monday. Light gradients are expected Tuesday
while a low center drops south along 130W. Haner


.HYDROLOGY...River flooding is not expected for the next 7 days.


WA...Winter Weather Advisory from midnight tonight to 7 PM PST Friday
     for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit

     Winter Storm Watch from late Friday night through late Saturday
     night for Cascades of Pierce and Lewis Counties-Cascades of
     Snohomish and King Counties-Cascades of Whatcom and Skagit

PZ...Small Craft Advisory from 10 PM this evening to noon PST Friday
     for Central U.S. Waters Strait Of Juan De Fuca.


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