Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 190508
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1008 PM MST Sun Feb 18 2018

.DISCUSSION...(This Evening Through Sunday)
Issued at 206 PM MST Sun Feb 18 2018

Upper level analysis has large trof and associated low digging
into the Pacific Northwest, with fast west southwest flow across
the Northern Plains. Skies are cloudy across the area with a
moderate band of snow across the Black Hills area and just east.
At the surface, arctic front has pushed south of the area, and
strong pressure gradient is producing very gusty northerly winds
across the South Dakota plains. Temperatures continue to tumble as
arctic air moves in.

Models remain in decent agreement on the winter storm and
potential snowfall across the area. Still just some minor
differences on track and overall timing of disturbances passing
through the region over the next couple of days. Confidence is
high enough to issue a winter storm warning for south central SD
late tomorrow into early Tuesday morning as the secondary
shortwave lift through that area.

As the upper trof to the west digs further south and east across
the western states tonight and Monday, two main shortwaves will
push through the area, bringing the bulk of snowfall. The first
one is moving across northeast WY and far western SD now, with a
tightening baroclinic zone and strong mid-level frontogenesis
extending from southern parts of northeast WY to west central SD.
This first wave will exit late tonight/early Monday, with lighter
snowfall lingering across the area Monday. The second wave is
forecast to lift northeastward out of southeast WY and western NE
Monday afternoon and move across much of southwest and south
central SD late Monday and Monday night. The large scale upper
trof will then speed up and push east across the area later Monday
night and Tuesday, with snow tapering off and ending from west to
east Monday night into early Tuesday. Winds will not be too
strong with this storm tonight through Monday night, mostly 10 to
20 mph with some higher gusts at times on the plains. However,
the fluffy nature of the snow should produce some areas of blowing
and especially drifting snow.

Projected snowfall totals still show 6 to 12 inches across much
of the warning area, with locally higher amounts likely from
southern parts of northeast WY to the Black Hills area and into
the west central SD plains. 3 to 6 inches are likely across far
northwest SD, with 4 to 7 inches or more across south central SD,
most of that with the second wave later Monday and Monday night.
The other concern besides the snowfall will be the bitter cold and
very low wind chills from tonight through the first half of the
week. Wind chills will be below zero through the period, as low as
25 below to 35 below zero, especially later Monday night through
Wednesday morning.

The rest of the week looks mainly dry with some moderation of
temperatures toward the end of the week. Another large trof is
progged to dig south across the western states late in the week,
but should stay far enough south and west to limit impacts for
most of our area. However, a disturbance in southwest flow aloft
could bring a chance for snow to the south and east of the Black
Hills Thursday night into Friday. Low chances for snow are also
possible by the weekend, especially over northeast WY and the
Black Hills, as remnants of weakening western trof swing through
the Plains.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Monday Night)
Issued At 1008 PM MST Sun Feb 18 2018

Snowfall will result in widespread IFR/LIFR conditions through
tonight into Monday afternoon.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...Winter Storm Warning until 11 PM MST Monday for SDZ012-024>029-
     072-074.

     Winter Storm Warning until 5 AM MST Tuesday for SDZ013-014-
     030>032-041>044-073.

     Winter Storm Warning from 6 PM Monday to 9 AM CST Tuesday for
     SDZ046-047-049.

     Winter Weather Advisory until 11 PM MST Monday for SDZ001-002.

WY...Winter Storm Warning until 11 PM MST Monday for WYZ054>058-071.

&&

$$

DISCUSSION...15
AVIATION...7



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