FXUS63 KUNR 231642
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1042 AM MDT Sun Aug 23 2026

.KEY MESSAGES...

- Upper ridge will dominate the weather through next week.

- Isolated strong/severe thunderstorms may occur during the
  afternoon/evening hours today and Monday.

- Temperatures will average 4-8F above normal.

&&

.UPDATE...
Issued at 1051 PM MDT Sat Aug 22 2026

Updated PoPs and QPF overnight to better capture latest model
guidance and current radar trends. Models continue to show
significant disagreement on placement of potential thunderstorms
Sunday afternoon and evening, and have been changing from run to
run as well. Have taken a broad-brush approach with low PoPs for
now, favoring the Black Hills and most of our South Dakota plains
(especially north of I-90 and then east of the Badlands). We'll
continue to try to refine this over the next 36 hours. Parameters
continue to support the potential for a couple severe
thunderstorms with damaging wind and large hail risks.

&&

.DISCUSSION...(This Evening Through Saturday)
Issued at 1236 PM MDT Sat Aug 22 2026

18z surface analysis had weak high over the northern Plains into 
the middle MO river valley. Weak frontal boundary behind it from 
central MT into eastern CO. Patch of low stratus over western NE 
has been pushed northwestward on southeasterly winds along 
boundary leading to upper 50s Td/s across parts of northeastern 
WY. Water vapour matinee depicted shortwave ridge over the CWA, 
but shortwave trough hot on it's heels over eastern ID/western WY.
Lift ahead of wave leading to some weak radar returns over 
northeastern WY, but nothing below 12kft per KGCC observation. 
Main concern this forecast is convection.

Later this afternoon/evening, around 1500J/kg MLCAPE pools along 
frontal boundary with weak MLCIN. Shortwave overtops, which will 
assist convective initiation. Perhaps 20-30kt effective shear will
be present, enough to support isolated strong/severe 
thunderstorms where SPC Day 1 Marginal in place. Storms will 
slowly move eastward across the CWA tonight, but in weaker fashion
over western SD given less favorable environment. Temperatures 
will be near guidance.

Sunday, aforementioned frontal boundary ends up over the western 
SD plains, influenced by tonight's convection/outflow. The 
further east it goes, the less the effect on the CWA. Right now, 
HREF suggests it'll settle over the eastern quarter of the CWA. 
2KJ/kg CAPE pools along it with 30-40kts of 0-6km shear sufficient
for strong/severe thunderstorms. Some CAM guidance suggests a 
supercell or two dropping south along the UNR/ABR border area late
in the day. Temperatures will be near guidance.

For Monday/Tuesday, disturbances rotating around the upper ridge 
will support additional chances for strong/severe convection given
forecast shear/buoyancy profiles. The rest of next week looks 
less active as upper ridge/height rises more dominant.

&&

.AVIATION...(For the 18Z TAFS Through 18Z Monday)
Issued At 1036 AM MDT Sun Aug 23 2026

Isolated/scattered thunderstorms will develop this afternoon 
(around 20Z) with hail, lightning, heavy rain, and gusty, erratic 
winds expected. Transient MVFR/IFR conditions in stronger cores is
expected. Storms should clear the area by late evening (05Z), 
however, lingering moisture may result in some lower clouds across
the western South Dakota plains through 14Z.

&&

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

$$

UPDATE...KSmith
DISCUSSION...Helgeson
AVIATION...MLS