Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 182038
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
138 PM MST Mon Dec 18 2017

.DISCUSSION...(This Evening Through Monday)
Issued at 121 PM MST Mon Dec 18 2017

A low-amplitude ridge continues to build into the northern
plains. At the surface, a weak trough covers the region. Skies are
mostly cloudy, and winds are west-southwesterly around 10 to 20
kts. Temperatures early this afternoon are in the 40s to lower
50s.

Cold air will begin filtering into the CWA tonight. Parts of
northwestern SD may see some light snow along the baroclinic zone,
but most areas will remain dry. Some fog may develop over parts of
northeastern WY early in the morning, but winds become less
upsloping as RHs increase, so anything that does develop would
be patchy. Lows tonight will be mainly in the 20s.

The upper ridge will cross the northern plains Tuesday into
Wednesday, as a trough pushes into the Pacific Northwest. Warmer air
will advect into the southern parts of the CWA. Temperatures will
depend on where the warm front sets up, especially on Wednesday,
when highs are forecast to range from the 30s across northwestern SD
to near 60 across southwestern SD.

Chances for snow will increase Wednesday night into Thursday. Both
the GFS and ECMWF are showing the 700 mb low weakening Thursday as
it crosses CO, but the GFS has a more southerly track (and therefore
less snow for us). Currently forecasting up to 3 inches of snow for
the CWA. A strong cold front will also cross the northern plains
Wednesday night, bringing significantly colder air to the region,
along with breezy northwest winds. Blowing snow may reduce
visibility at times.

A reinforcing shot of even colder air is expected this weekend, as a
larger secondary trough sets up over much of the CONUS. Models are
showing this cold air sticking around through the weekend and into
next week, with the ECMWF a bit colder than the GFS.
Additionally, shortwaves periodically moving through cyclonic flow
will bring some chances for light snow through the weekend.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Tuesday)
Issued At 1028 AM MST Mon Dec 18 2017

There is a slight chance of rain or snow over far northwest South
Dakota this evening with local MVFR conditions. Otherwise, VFR
conditions expected across the area through Tuesday morning.

&&

.UNR WATCHES/WARNINGS/ADVISORIES...
SD...None.
WY...None.
&&

$$

DISCUSSION...Pojorlie
AVIATION...7



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