Area Forecast Discussion
Issued by NWS Rapid City, SD

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FXUS63 KUNR 172329
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
429 PM MST Sun Dec 17 2017

.DISCUSSION...(This Evening Through Sunday)
Issued at 243 PM MST Sun Dec 17 2017

Current surface analysis shows weak high pressure over eastern
portions of the Northern Plains with light southerly winds across
much of the area. Upper level analysis shows shortwave energy now
east of the region, with cutoff area of low pressure over AZ.
Ridge is beginning to rebuild over the Pacific NW. Skies have
become mostly clear across much of the area now, except for south
central SD. Temps are in the 30s and lower 40s.

Mild and mainly dry weather will return for the first half of the
week as the building ridge to the west gradually shifts east into
the Plains. All models generally agree on a significant change in
the weather taking place Wednesday night and Thursday as cold arctic
air pushes into the Plains and then persists through the long
holiday weekend.

For tonight, skies will be mostly clear with lows dropping into the
20s. West-southwest winds will gradually increase later tonight as
milder air pushes in. Could see rising temps toward morning across
northeast WY and the Black Hills. Clouds will increase late tonight
and Monday morning as warm front moves through the area. Highs on
Monday will mostly be in the 50s, with 40s over the higher Black
Hills. Breezy west-southwest winds will develop in the morning,
especially from northeast WY to northwest SD. A weak cold front will
slide through the region Monday night, bringing about a 10 degree
cooldown for Tuesday. A few showers could clip northwest SD Monday
evening as shortwave energy passes mostly north of the CWA.
Wednesday will remain mild with highs in the 40s to mid 50s.

A strong Canadian cold front will slide south through the area
Wednesday night, ushering in a much colder airmass, gusty winds, and
a round of snowfall. Many areas will likely see at least a light
accumulation of snowfall later Wednesday night into Thursday, up to
a couple of inches, with some areas possibly seeing more depending
on the track of the upper level shortwave. Cold air is expected to
remain entrenched across the area late week and over the weekend,
with highs well below average. Weak disturbances could bring
additional light snow/flurries at times to some areas, especially
northeast WY and southwest SD. Longer range models still hinting at
a reinforcing shot of arctic air later in the weekend and Christmas
day, with lows near or below zero and highs potentially struggling
to get much above zero.

&&

.AVIATION...(For the 00Z TAFS Through 00Z Monday Evening)
Issued At 428 PM MST Sun Dec 17 2017

VFR conditions expected through the period. Gusty southwest winds
will develop over northeast WY after 06z and then spread into
northwest SD during the day Monday.

&&

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

$$

DISCUSSION...26
AVIATION...Helgeson



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