Area Forecast Discussion
Issued by NWS Rapid City, SD

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000
FXUS63 KUNR 042137
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
237 PM MST Sun Dec 4 2016

.SHORT TERM...(This Evening Through Monday Night)
Issued at 232 PM MST Sun Dec 4 2016

Upper level analysis shows a broad trough over much of the CONUS,
with closed lows over Mexico and Saskatchewan. Mid/high clouds are
pushing into the northern plains ahead of the next wave associated
with the Canadian low. At the surface, low pressure is over
northern MT, with ridging over the Dakotas and southwest flow over
the CWA. Temperatures are warming into the 40s across much of the
area.

Energy associated with upper trough will approach the region
tonight, while surface low moves through western SD. Behind the low,
a strong cold front will begin pushing through the region, bringing
with it a band of snow, strong winds, and much colder air.

Snow will begin moving into northeastern WY and the Black Hills this
evening, expanding over parts of northwestern SD Monday morning
behind the cold front. Snow is most likely to occur during the early
morning hours over northeastern WY and the northern Hills. Models
differ on snow amounts, with the SREF and ECM showing 2 inches or
less, and the NAM and GFS reaching 5 inches. Current forecast has 2-
4 inches for northeastern WY and the northern Hills, with a half
inch or so across northwestern SD.

Another factor with this system will be gusty northwest winds, which
will be aided by cold air advection and pressure rises around 5
mb/3hr. ECMWF and GFS have 850 mb winds around 40 kts over the
western SD plains, but only 15-20 kts over northeastern WY. Expect
some areas of blowing snow over northeastern WY,  with some patchy
blowing snow over the windier but less snowy western SD plains.
Considered Winter Weather Advisory for northwestern SD, but lighter
snow amounts shouldn`t reduce visibility as much. Also considered
Wind Advisory for northwestern SD, but forecast winds only
marginally meet criteria.

Monday will begin a very cold week as arctic air begins sinking into
the region. Lows tonight will be in the 20s, with highs Monday not
much warmer, and falling during the afternoon. High temperatures
will range from the mid 20s across northeastern WY and
northwestern SD, to the 30s across south central SD. Some places
in the Hills may remain in the teens. Lows Monday night will be
mainly in the single digits.

.LONG TERM...(Tuesday Through Sunday)
Issued at 232 PM MST Sun Dec 4 2016

The upper low will move into western Ontario on Tuesday
with the leading edge of the upper trough arcing from near Duluth to
Omaha. Another upper trough will dive southward in the western
CONUS. A very weak upper ridge will develop over the forecast area
on Tuesday between the influence of the upper troughs. There will be
a slight chance of light snow on Tuesday night over the southern
parts of our northeastern Wyoming zones, the srn Black Hills and far
southwestern South Dakota as the next upper disturbance passes to
the south of the forecast area and period of mid level WAA develops.
Arctic air will continue flowing southward into the forecast area
out of western Canada on Tuesday and Wednesday and high temperatures
will be around 30 degrees colder than average over most of the
forecast area on Wednesday and Thursday, with only slightly warmer
temperatures over parts of northeastern Wyoming on Thursday
afternoon. The surface high will shift east from Friday into next
weekend as another upper level trough moves off to the east and an
upper ridge builds over the western CONUS. Warmer air will begin to
move into northeastern Wyoming and parts of far western South Dakota
on Friday but the low level easterly component to wind over western
SD will keep the cold air in place longer on the plains east of the
Black Hills. The easterly component to the sfc wind will be created
by low pressure moving along the edge of arctic airmass across WY
into NEB Friday night and Saturday which will create small chances
of light snow as the relatively warmer air aloft overruns the colder
air at the surface.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Monday Evening)
Issued At 232 PM MST Sun Dec 4 2016

VFR conditions are expected through this evening. An
arctic cold front, followed by gusty northwest winds, will move into
northeastern Wyoming and western South Dakota late tonight/early
Monday morning. Areas of MVFR/IFR conditions will develop with low
stratus, -SN and possibly areas of blowing snow.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...Winter Weather Advisory from midnight tonight to 5 PM MST Monday
     for WYZ054>058.

&&

$$

SHORT TERM...Pojorlie
LONG TERM...10
AVIATION...10



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