Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 142126

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
326 PM MDT Wed Mar 14 2018

.DISCUSSION...(This Evening Through Wednesday)
Issued at 255 PM MDT Wed Mar 14 2018

The expected track of the winter storm and the associated
temperature profiles, continue to change enough that determining
the precipitation types, amounts, duration and location remains
difficult. Will continue the Winter Storm Watch for Thursday night
and Friday.

Current upper analysis shows a ridge over the forecast area with a
deepening upper trough over the west coast. The upper ridge will
begin to be shunted off to the east tonight as the upper trough
moves farther onshore over the west coast. WAA aloft may produce
some very light rain or freezing rain over parts of northeastern
Wyoming later tonight. The surface warm front will stall it`s
northward progress and will return as a back door cold front in
response to pressure falls as an upper shortwave swings into MT
and Wyoming. The southeasterly upslope low level flow will lead to
patchy fog and areas of stratus later tonight especially over the
lingering snow pack over parts of northwestern South Dakota.

On Thursday, the colder air will sink southwestward across western
South Dakota probably advancing as far as the edge of the nwrn SD
snow line. As the upper trough continues to swing eastward, a lee
side trough will develop into a closed surface low over eastern

Thursday night into Friday, models remain in agreement in showing
a significant late winter storm that will affect the forecast area.
Main shortwave trough ejects into the central/northern plains
with relatively strong developing aloft, over the forecast area.
The system will be moisture laden as surface low moves into
eastern NE by 00z Saturday and inverted trough moves eastward
across the forecast area. Strong synoptic scale lift will link
with tight baroclinic zone and subsequent low level frontogenesis.
Thermal profiles suggest mainly wet heavy snow over parts of far
northeastern Wyoming, the Black Hills area and southwestern into
south central South Dakota. Some measurable ice accumulations due
to a period of freezing rain are also possible, generally on the
plain east of the Black Hills and from southwestern into south
central South Dakota. 0.10-0.25" of ice accumulation from freezing
rain is possible in those areas. Of course as noted above, minor
changes in the thermal profile of this storm or the storm track
will cause big changes in the snowfall/ice potential. Will
continue the Winter Storm Watch.

Saturday looks relatively quiet with a return to near normal
temperatures. Temperatures will probably be colder on Saturday
than currently forecast, being highly dependent on the area and
amount of snow cover.

Sunday and Monday, another upper trough moves into the region.
Guidance shows split energy with wide range in upper low solutions
including potential for another shot of snow.


.AVIATION...(For the 18Z TAFS Through 18Z Thursday)
Issued At 255 PM MDT Wed Mar 14 2018

VFR conditions are expected across the CWA through this afternoon
and into the early evening. Late this evening, fog will begin to
develop across northwestern and central SD, reducing conditions to
MVFR/IFR. There is also a chance for some light precipitation
across northeastern WY and the Black Hills area late tonight into
Thursday morning, which could be in the form of rain, freezing
rain, and/or sleet.


SD...Winter Storm Watch from late Thursday night through Friday
     afternoon for SDZ012-024>032-041>044-046-047-072>074.

WY...Winter Storm Watch from late Thursday night through Friday
     afternoon for WYZ056>058-071.



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