Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 172024

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
124 PM MST Sat Feb 17 2018

.DISCUSSION...(This Evening Through Saturday)
Issued at 124 PM MST Sat Feb 17 2018

The latest water vapor images and upper-air analysis showed a
long-wave trough covering most of the CONUS, with a short-wave
trough moving from the northeastern Pacific Ocean into the
northwestern CONUS. This trough will move southeast and deepen
over the western CONUS as a series of short-wave troughs ejects
northeastward across the CWA. The result will be widespread light
to moderate snow later Sunday through Monday, with another shot of
Arctic air moving into the region for the first half of the week.

For tonight, a lead short-wave trough will move to our north,
keeping most of the precipitation north of the CWA. Forecast
soundings support the potential for light freezing rain over the far
north, if precipitation makes it there. Otherwise, low-level warm
advection further to the south will bring increasing chances of
rain/snow into northeastern Wyoming late tonight/Sunday morning. In
addition, gusty southwesterly winds will develop over parts of
northeastern Wyoming and the Black Hills tonight, with gusts of 45
to 50 mph expected over central and southern portions of Campbell
County overnight. The trough passing to the north tonight also will
help push an Arctic front south through the area on Sunday, with
gusty northwest winds and falling temperatures behind it.

Models have been very consistent on the general development of the
system over the next couple of days, coming into a little better
agreement on timing and positioning with today`s 12z runs. The
main difference in the models is the position of the low- to
midlevel baroclinic zone, which ultimately will dictate where the
heaviest band of snow sets up. At this point, it appears this will
be just to the south of Rapid City (with the NAM being the
farthest north model). Either way, a good portion of the forecast
area should receive several inches of snow from the storm.

At this point, the main short-wave trough that will bring snow to
much of our CWA is expected to move into northeastern Wyoming
during the day Sunday, then push east-northeast into western South
Dakota late Sunday and Sunday night. A strong upper jet will
promote low-level isentropic lift and midlevel frontogenesis,
resulting in widespread light to moderate snow Sunday night into
Monday morning. A band of heavier snow likely will develop within
the main band, perhaps extending from south of Newcastle east-
northeastward to Philip, where the dendritic growth zone is
deepest and small elevated CAPE is progged to exist.

Snow will continue through Monday, but the intensity should taper
off by late morning. However, another strong short-wave trough will
move through the area and possibly bring some moderate/heavy snow to
the southeast of the Black Hills late Monday into Monday night.
Winds with the system look on the lighter side Sunday night and
Monday, but strong enough for at least patchy blowing and drifting
of snow, especially with the fluffy nature of the snow.

Snow should slowly taper off and end from west to east Monday night
into early Tuesday as the trough moves through the plains and
weakens with time. Based on a slight southward shift of the snow,
have added Mellette and Todd Counties to the winter storm watch,
plus have upgraded southern Campbell County to a warning. Snow
totals of around 6 inches or more are a good possibility across
the watch area, and amounts up to a foot certainly look possible
where banding may set up.

The rest of the forecast period through the end of the week
continues to look dry. Bitter cold air will remain in place early in
the week, with highs only in the single digits Monday and Tuesday,
with lows below zero and wind chills well below zero. Will start to
see some milder air by mid to late week, but likely remaining below
average levels.


.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1023 AM MST Sat Feb 17 2018

VFR conditions will continue over most of the area through
tonight. The exception will be over far northwestern South Dakota
where MVFR CIGS will develop after 06Z tonight. MVFR CIGS will
also develop over northeastern Wyoming around 14Z on Sunday
morning. LLWS will develop over northeastern Wyoming, the Black
Hills and much of western South Dakota after 05Z this evening and
will continue through the rest of the night. There is a chance of
snow, and possibly a period of light freezing rain, over
northwestern South Dakota and northeastern Wyoming tonight.


SD...Winter Storm Watch from Sunday morning through Monday afternoon
     for SDZ024-028-029.

     Winter Storm Watch from Sunday afternoon through Monday evening
     for SDZ012>014-025>027-030>032-041>044-046-047-072>074.

WY...Winter Storm Watch from Sunday morning through Monday afternoon
     for WYZ054-056>058.

     Winter Storm Watch from Sunday afternoon through Monday evening
     for WYZ071.

     Winter Storm Warning from 11 AM Sunday to 5 PM MST Monday for



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