Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 140818

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

.DISCUSSION...(Today Through Tuesday)
Issued at 215 AM MDT Wed Mar 14 2018

08z surface analysis had frontal boundary from central MT into
central SD, roughly on the southern edge of the snowpack over SD.
Some fog has developed over northwest SD per advanced nighttime
microphysics RGB, but SDDOT cams not showing significant
reductions is visibilities at this time. Water vapour had upper
ridge from AB/SK into CO and upper trough just off the northwest

Today, upper/thermal ridge peak over the CWA with spring-like
temperatures and plenty of sunshine as aforementioned frontal
boundary shifts northeast as a warm front. Temperatures will again
be subdued over the snowpack.

Tonight, upper ridge starts to breakdown as upper trough moves
onshore over the northwest CONUS.  Warm front stalls and becomes a
back door cold front in response to pressure falls ahead of upper
trough over MT/WY. Southeast upslope flow expected to create stratus
and fog late, especially over the snowpack. Can`t rule out a bit of
mixed precipitation, but latest guidance has backed off some.
Temperatures will be quite varied, staying up over the higher
terrain and dropping off on the SD plains.

Thursday, back door cold front sinks southwest across western SD and
will hangup on the southern edge of the snowpack. Surface low
develops over eastern CO as upper trough moves into the Rockies.
Temperatures spread will be impressive with 30s over northwest SD
and lower 60s over parts of northeast WY/southwest SD. Large
temperature busts likely.

Thursday night into Friday, models are in decent agreement with
portrayal of impactful late winter storm. Main shortwave trough
ejects into the central/northern plains with main piece of energy
closing off over eastern NE and a weaker piece breaking off into
eastern MT. System will be moisture laden as surface low moves
into eastern NE by 00z Saturday and inverted trough moves across
the CWA. Synoptic lift well-defined and will link with tight
baroclinic zone and subsequent low level frontogenesis. Thermal
profiles suggest mainly wet heavy snow west third of the CWA with
a wintry mix east of the Black Hills. Wet heavy snow possible from
the Black Hills into southern SD. 0.10-0.25" of ice possible east
of the Black Hills over southern SD. Of course, minor changes in
the thermal profile of this storm or the storm track will cause
big changes in the snowfall/ice potential. Felt confident enough
to issue Winter Storm Watch for southeast half of the CWA.

Saturday looks relatively quiet with a return to near normal
temperatures - antecedent new snowfall dependent of course.

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.

For the middle of next week, perhaps a new upper ridge will grace
the region with milder and drier weather on tap.


.AVIATION...(For the 06Z TAFS Through 06Z Wednesday Night)
Issued At 1047 PM MDT Tue Mar 13 2018

Areas of BR/FG will bring MVFR/IFR conditions to portions of the
northwestern SD plains overnight. Some of this BR/FG may linger
through midday Wednesday, but generally, VFR conditions are
expected Wednesday morning through evening across the area.

With weak upslope flow developing over the SD plains Wednesday
night ahead of our next low pressure system, BR/FG will likely
again be an issue. For now, think any impacts to cigs/vsbys at
KRAP will be after 06z Wednesday night.


SD...Winter Storm Watch from late Thursday night through Friday
     afternoon for SDZ024>032-041>044-046-047-072>074.

WY...Winter Storm Watch from late Thursday night through Friday
     afternoon for WYZ057-058.



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