Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 171732

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1032 AM MST Sat Feb 17 2018

.DISCUSSION...(Today Through Friday)
Issued at 315 AM MST Sat Feb 17 2018

Current surface analysis shows cold front across ND and MT,
moving southeastward. Upper level analysis shows northwest flow
continuing across the region, with weak shortwave noted in water
vapor imagery moving across the northern high plains. Regional
radars are showing scattered snow showers from southeast MT into
far northeast WY and northwest SD. Accumulations this morning will
generally be less than an inch across far northwest WY, the
northern Black Hills, and northwest SD before activity moves out
of the area late this morning. Skies are variably cloudy, with the
most clouds across northern and western portions of the CWA.
Temps are mostly in the mid teens to mid 20s with light west to
southwest winds.

Cold front will slip south across the area today with some cooler
air behind it. Highs will range from the 20s across northwest SD to
near 40 across parts of far southwest SD. Clouds should give way to
some sunshine late this morning and afternoon, especially east of
the Black Hills, but clouds will increase again across western areas
later in the day.

A large scale trough will move into the Pacific NW tonight and track
east-southeast across the western states and into the Rockies Sunday
and Monday before eventually weakening as it moves through the
Plains Tuesday. A stormy period can be expected for the latter part
of the weekend and through the Presidents Day holiday as a series of
disturbances eject out of the trough and move across the Northern
Plains. Models have been very consistent on general development of
the system over the next couple of days, though there are
differences on the track of these disturbances as they cross the
region. EC and NAM are farther north and a bit slower with the
trough, while GFS and to a lesser extent the Canadian have been
further south. Either way, a good portion of the forecast area would
receive several inches or more of snowfall from the system. Would
lean more toward the EC at this point, but should have a better idea
when the first disturbance moves across ND later tonight and Sunday
morning. EC keeps pcpn north of the area, while GFS has the southern
edge of the main band clipping the ND/SD border. Will keep main pops
along the ND border and a light accumulation possible. Forecast
soundings also support potential for light freezing rain there if
pcpn makes it that far south. Shortwave energy further to the south
will start to bring increasing chances of pcpn into northeast WY
late tonight/Sunday morning. Gusty southwesterly winds will develop
over parts of northeast WY and the Black Hills tonight, with gusts
of 45 to 50 mph expected over central and southern portions of
Campbell County overnight. Disturbance to the north will help to
push an arctic front south through the area on Sunday, with
temperatures falling behind it.

At this point, the main disturbance that will bring snowfall to much
of our CWA is expected to move into northeast WY during the late
morning and afternoon hours then push east-northeast into western SD
late Sunday and Sunday night. Strong jet aloft will combine with
fairly robust mid-level frontogenesis and tightening baroclinic zone
over the area to produce fairly widespread light to moderate
snowfall Sunday night Monday morning. A band of heavier snowfall
will likely develop somewhere within the main band from northeast WY
to western SD, but location very uncertain right now. Snow will
continue into Monday, but intensity should taper off some by late
morning. However, another stronger disturbance will push through the
trough and possibly bring some heavier snowfall to the south and
east 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.

Snowfall 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. Coverage of winter storm watch looks good.
The only change is to add Bennett County. Snowfall totals of
around 6 inches or more are a good possibility across the watch
area. Amounts up to a foot or so 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 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>044-072>074.

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

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



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