Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 201716

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1016 AM MST Sat Jan 20 2018

.DISCUSSION...(Today Through Friday)
Issued at 320 AM MST Sat Jan 20 2018

Water vapor currently shows upper trof moving across the western
CONUS, with strong 125kt jet extending from Utah through the
Northern Plains into the Great Lakes. Frontogenitical forcing
with low level baroclinic zone resulting in band of snow from
central Wyoming through northeast Wyoming into western South
Dakota. An inch or two of snow has fallen overnight from the
Gillette area to the northern Black Hills. High resolution short
term models have a decent handle on current area of snow...and
show a weakening trend in lift through early this morning. As
upper trof moves towards the Rockies today, strong jet lifts
through the area, with right entrance region across western South
Dakota this evening. Frontogenesis shifts into east central
Wyoming and southwest South Dakota this afternoon and remains
across southwest/south central South Dakota tonight. Overall,
little change in location of heaviest snow and snowfall amounts
from previous forecast. SREF and GEFS still show wide spread in
snowfall would be expected with narrow axis of
best lift. Any shift in area of snow will have big impact on
accumulations. Have updated watch area to Winter Storm Warning and
issued Winter Weather Advisory for portions of northeast Wyoming
and west central South Dakota. As upper low closes off tonight and
shifts into the central plains Sunday, snow will shift southeast.
South central South Dakota will be near northwestern edge of
TROWAL Sunday night.

Dry and cool conditions then will prevail through much of the
week. Next upper trof will cross the Plains Friday-Saturday and
may bring widespread precipitation.


.AVIATION...(For the 18Z TAFS Through 18Z Sunday)
Issued At 1013 AM MST Sat Jan 20 2018

(For the 12Z TAFS Through 12Z Sunday Morning)
Issued At 438 AM MST Sat Jan 20 2018

Areas of MVFR conditions with light snow or mixed pcpn will
persist over much of northeast WY and parts of western SD this
afternoon. Snow in IFR conditions with the main storm system
will expand across northeast WY in the afternoon and then across
the Black Hills and southern SD late this afternoon and tonight.
MVFR/IFR conditions can be expected across these areas with the
snowfall, along with some areas of LIFR conditions, especially
across the Black Hills and far southern SD. Mostly VFR conditions
are expected across far northeast WY into northwest SD this
afternoon and tonight with little or no snowfall.


SD...Winter Storm Warning from 5 PM this afternoon to 2 AM MST Monday
     for SDZ042>044.

     Winter Storm Warning from midnight tonight to 6 AM CST Monday
     for SDZ046-047-049.

     Winter Storm Warning from 2 PM this afternoon to 5 PM MST Sunday
     for SDZ027-029-041.

     Winter Weather Advisory from 5 PM this afternoon to noon MST
     Sunday for SDZ026-028-030-031-074.

WY...Winter Weather Advisory from 2 PM this afternoon to noon MST
     Sunday for WYZ055-058.



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