FXUS63 KUNR 230439
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1039 PM MDT Sat Aug 22 2026

.KEY MESSAGES...

- Upper ridge will dominate the weather through next week.

- Thunderstorm areal coverage will increase tonight and hang 
  about into next week, although in the hit or miss variety.

- Isolated strong/severe thunderstorms may occur during the later
  afternoon/evening hours today through Tuesday.

- Temperatures will average 4-8F above normal.

&&

.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 06Z TAFS Through 06Z Sunday Night)
Issued At 1038 PM MDT Sat Aug 22 2026

Showers are moving eastward out of WY and over the Black Hills
onto the SD plains. These should move out by sunrise. A front will
pass through northeastern WY early Sunday afternoon, shifting to
north winds. Assuming appreciable surface heating, TS conditions
are possible across most of the area, primarily over the Black
Hills and east. Confidence in timing/placement is still low.
Regardless, have introduced a PROB30 for KRAP for Sunday evening. 

&&

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

$$

DISCUSSION...Helgeson
AVIATION...Schweigert