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Area forecast discussion for western South Dakota and northeastern Wyoming 
National Weather Service Rapid City South Dakota
421 am MST Sat Dec 20 2014

Short term...today through Sunday night
issued at 332 am MST Sat Dec 20 2014

Zonal westerly upper level flow continues across the western
Continental U.S. With several embedded shortwaves crossing the Pacific northwest to
the northern plains. The next wave is crossing central/eastern Montana at this time
producing hardly any precipitation. Surface observation show temperatures in the 20s to
middle 30s across the area. Early morning fog is again staying east
of the County Warning Area over the eastern half of South Dakota. Upper wave will quickly push
through western ND and northwest South Dakota producing mainly middle level cloud cover.
Some very light precipitation may make it to the ground...and if any
precipitation does fall...some of it could be in the form of freezing
rain very early this morning before temperatures rise above freezing. No
impacts are expected. Temperatures will rise this afternoon into the middle
30s to middle 40s.

A weak upper trough will push into the County Warning Area later tonight/early
Sunday ahead of a jet streak over the Pacific northwest/northern rockies...and
deepen through the day Sunday into Sunday night. Surface low will
cross eastern Montana Sunday and the Dakotas Sunday night. Precipitation will
develop from west to east through the day. Temperatures will be mostly in
the 40s...so precipitation will be rain except the highest elevations of
the Black Hills. As the surface low crosses the Dakotas Sunday
night...cooler air will push in with temperatures dropping enough for
precipitation to change to snow overnight. Upslope conditions will develop
over the northern Black Hills with snow developing. This is likely to
initially produce light snow accumulations through the night.

Long term...Monday through Friday
issued at 332 am MST Sat Dec 20 2014

Fast progressive westerly flow will be interrupted by two
amplified troughs in the period...supporting colder weather and
chances for snow. The first of this wave series will arrive
Monday...and will support surface cyclogenesis over the Midwest and
eastern plains. A period of ridging will follow middle week before
the next long wave trough arrives near the end of the week. The
latter trough/S details still remain sketchy at this time...with some
potential looming for a large portion of the middle level vorticity maximum to
dig well south of the region. Have adjusted probability of precipitation and temperatures in the
period...raising probability of precipitation Monday /mainly over the western and Northern
Hills/ with a slight downtrend in temperatures most of the week.

Monday...widespread lsa/moist northwest flow/and increasing momentum will
support precipitation chances over much of the area...especially when coupled with
a strong upper level jet streak. A good setup is in the works for a
period of upslope snow in the western and northern Black Hills.
Plenty of lingering Pacific moisture will be in place with steep ll
lapse rates and increasing deep layer flow. Winds will have more of
a westerly component Monday...with veering profiles into Monday
night. This will favor best snows more so toward The Four
Corners-BUckhorn area at first...trending into the Northern Hills as
possibly far north as Deadwood and lead. Current forecasts suggest 6+
inches with locally higher amounts certainly possible. Have raised
probability of precipitation in these areas. Outside of the hills a mix of fgen and
convectively induced rain showers will be possible...with better
fgen/deformation type lift over eastern areas including scentral South Dakota
Monday night. Depending on the low track...a period of snow accums
will be possible there. Further west across the South Dakota plains lighter
amounts of snow are expected at this time...least in the Lee of the bh where
downslope shadowing will be in place...especially from Rapid City south.
The other story will be the wind. Ongoing pressure falls east of the
forecast area in a cold air advection regime will support increasing ll flow...indicated well
in model sounding forecasts. Wind advection conds can be expected across much
of western South Dakota...especially from 2wx to rap.

Tue-Wed...mainly dry weather expected save for some lingering
morning snow showers over the bh and far scentral. Colder conds
expected Tuesday...although shortwave ridging may be strong enough to
support a decent day Wednesday.

Thur-Fri...the next longwave trough will affect the western Continental U.S..
have retained chances probability of precipitation all places until trends in wave
strength/track can be attained. Did opt for the cool side of guidance
given potential downstream blocking from the early week wave...which
would allow a stronger meridional component to the flow locally.

&&

Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 412 am MST Sat Dec 20 2014

Mainly VFR conditions through the period. A few MVFR ceilings with a
slight chance of rain/snow are possible across far northwestern
South Dakota/the northern-central Black Hills/and parts of
northeastern Wyoming this morning with a fast moving trough.
However...probs for ceiling restrictions are low..

&&

Unr watches/warnings/advisories...
South Dakota...none.
Wyoming...none.
&&

$$

Short term...13
long term...jc
aviation...jc

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