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

Short term...this evening through Sunday night
issued at 319 PM MST Sat Dec 20 2014

Current surface analysis shows warm front just to the east of the
forecast area. Upper level analysis depicts near zonal flow across
the region...with a strong storm system and upper jet pushing
across the Pacific northwest. Water vapor imagery shows a weak shortwave
has moved east of the area...with partly cloudy skies behind it.
Temperatures range from the upper 30s across far south central South Dakota to near
50 in the Rapid City area. Winds are mostly on the light side from
the west and northwest.

A unsettled pattern will take shape for the end of the weekend and
into early next week as a developing low pressure system moves
southeast across the region. However...it will remain mild into
Sunday before this system pushes in. The storm will bring at least
some light rain and snow to most areas...along with strong northwest
winds early next week.

For tonight...skies will remain partly cloudy in general as west to
southwest winds develop. The winds will become breezy overnight over
portions of northeast Wyoming...especially Campbell Colorado. Relatively mild
temperatures can be expected...with lows in the 20s and lower 30s...a bit
warmer across the eastern slopes of the Black Hills.

Clouds will begin to increase from the west Sunday morning...with
skies becoming mostly cloudy across the entire area by middle
afternoon. Milder air will remain across the region through much of
the day...with highs in the 40s and lower 50s...warmest again across
the eastern slopes of the Black Hills and adjacent plains.
Developing low pressure will move across northern Montana during the day
and into ND/northern South Dakota Sunday night...dragging a cold front across
the County Warning Area during the afternoon and evening hours. Rain showers will
develop along and behind the cold front...mostly during the
afternoon...as upper level energy accompanies the front. The best
chances during the afternoon look to be across far northeast
Wyoming...southeast Montana...far northwest South Dakota...and eventually the northern
Black Hills area. Northwesterly winds will increase behind the front
late Sunday and Sunday night...becoming breezy to windy in many
areas overnight.

Upper trough and developing low pressure system will track from Montana on
Sunday to somewhere over the eastern Dakotas on Monday morning.
Various models continue to be in some disagreement on track and
strength of this system. For Sunday night...will keep the best
chances for precipitation over northeastern portions of the County Warning Area...as well as
the northern Black Hills area. Precipitation will gradually change over to
snow Sunday night...with the quickest change over the Black Hills.
Not expecting much in the way of snow accumulation over the plains
as most of the precipitation will be out of the area by the time temperatures drop
off enough. However...forecast soundings and Froude analysis suggest
favorable conditions for at least occasional snow over the northern
Black Hills Sunday night into Monday as upslope northwesterly flow
increases. Have raised probability of precipitation over these areas Sunday night and have 1
to 3 inches of accumulation across the higher terrain through early
Monday morning. Lows Sunday night will be in the middle 20s to lower
30s across the forecast area.

Long term...Monday through Saturday
issued at 321 PM MST Sat Dec 20 2014

Upper trough will dig over the Dakotas Monday and remain
over the upper Midwest through Tuesday. While models initially have
a similar placement with the trough axis over the eastern Dakotas
Monday morning...they are quite different with the strength as the
GFS closes a low over southeastern South Dakota with the NAM farther north and
the European model (ecmwf) only a trough...eventually forming a closed low but not as
strong. With 120-140kt jet coming over West Coast ridge and
diving into the upper trough...the stronger GFS would be a better
solution. Deep vertically stacked low will have a strong pressure
gradient across western South Dakota...with gusts likely to reach 50kt...as
forecast soundings show >50kt momentum Transfer. Precipitation will
spread across forecast area Monday...could still see some rain/snow
mix early before changing to snow. Froude number increases to more
than 2 Monday morning...along with 90-100 percent relative humidity and strong
winds...so upslope snow is likely over the northern Black
Hills...while downslope winds will keep precipitation to a minumum from
Rapid City to Hot Springs and Edgemont. Depending on the amount of
precipitation...a Winter Weather Advisory may be more appropriate
than a High Wind Warning as blowing snow would have the greater
impact. Snow and gusty winds may linger Tuesday as a secondary low
rotates around main storm center. Brief ridge will build on
Wednesday before the next system comes across the northern plains on
Christmas day. Models in good agreement with a fast-moving longwave
trough. Cyclonic northwest flow remains behind the trough Friday...with
upper ridging slow to build toward next weekend.

&&

Aviation...for the 00z tafs through 00z Sunday evening
issued at 321 PM MST Sat Dec 20 2014

Clouds continue to clear behind a short wave trough crossing the
northern plains...with hi clouds advancing form the west. Winds will be light through
the even...then SW winds will increases over northestern Wyoming.

&&

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

$$

Short term...26
long term...55
aviation...55

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