683 
FXUS63 KUNR 211116
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
516 AM MDT Sat Aug 21 2021

.DISCUSSION...(Today Through Friday)
Issued at 250 AM MDT Sat Aug 21 2021

Current surface analysis shows cold front well south and east of 
the region, with weak high pressure over eastern MT into WY. Upper
level analysis shows trough moving northeast into Canada. Weak 
shortwave ridge is located over the Rockies, with another trough 
located over the Pacific NW. Skies are mostly clear over the area,
with some patchy clouds lingering over parts of the Black Hills 
and northwest SD. Temps are on the chilly side across a good 
portion of the area, especially northeast WY and far western SD, 
where temps are in the 40s and lower 50s. Some 30s are noted 
across the highest elevations of the Black Hills. Winds are mostly
on the light side from the west to northwest. 

Cooler conditions will continue today, with high temperatures in the 
70s to around 80 degrees under mostly sunny skies. A few showers and 
thunderstorms could develop toward late afternoon and early evening 
across northeast WY, with chances spreading east across the rest of 
the area overnight into Sunday as the Pacific NW shortwave moves 
quickly into the region and fairly robust warm advection takes 
place. Most of the activity should move east of the forecast area by 
Sunday afternoon as the trough moves quickly east-northeast and a 
Pacific cool front pushes through. Breezy conditions will develop
behind the frontal passage, with highs ranging from the 70s over 
the Black Hills to the lower 90s toward south central SD. Some 
haze and smoke will likely return behind the front on Sunday. 

Monday will be the warmest day of the week as hot air returns 
briefly ahead of the next cold front. Highs will reach the 90s 
across much of the plains with south to southwest winds developing. 
The mainly dry cold front will move through Monday night and early 
Tuesday, with near to slightly below average temps expected for mid 
to late week. The pattern could become more active again later in 
the week as west to southwest flow aloft likely develops ahead of a 
trough over the Pacific NW.  

&&

.AVIATION...(For the 12Z TAFS Through 12Z Sunday Morning)
Issued At 514 AM MDT Sat Aug 21 2021

Mostly VFR conditions expected through the period. -shra/ts are
possible late this afternoon over northeast WY, spreading into
western SD in the evening. Local MVFR conditions may occur with
the strongest echoes. LLWS is likely east of the Black Hills
tonight due to a 50kt southerly low level jet.

&&

.FIRE WEATHER...
Issued At 250 AM MDT Sat Aug 21 2021

A weak Pacific cold front will move through most of the area 
Sunday morning, which will produce breezy westerly winds gusting 
to 25 to 35 mph across much of the area on Sunday. The lowest 
humidities Sunday afternoon will develop across far southwest SD, 
where values will likely drop into the upper teens. A period of 
elevated to near critical fire weather conditions is likely there 
later in the afternoon. The grassland fire danger will be very 
high across much of western SD.

Monday will be the hottest day of the period, ahead of another cold 
front, with elevated fire weather conditions likely over more of the 
area. Highs will be in the upper 80s to mid/upper 90s for most 
places, and relative humidities will drop to the teens across 
much of northeastern Wyoming and western South Dakota. The best 
potential for gusty winds and near critical fire weather 
conditions still looks to be across the northeast WY plains, where
deep mixing could produce southwest winds gusting to around 25 
mph in the afternoon. Will continue to monitor model guidance to 
see if fire weather headlines may be needed on either day. 

&&

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

$$

DISCUSSION...26
AVIATION...Helgeson
FIRE WEATHER...26