845
FXUS63 KUNR 240450
AFDUNR

Area Forecast Discussion For Western SD and Northeastern WY
National Weather Service Rapid City SD
1050 PM MDT Sun Aug 23 2026

.KEY MESSAGES...

- Isolated severe thunderstorms possible Monday afternoon and
  evening, especially from the Black Hills east and south.

- Upper ridge will dominate the weather with subtle disturbances
  through next week with temperatures about 4-8F above normal

&&

.UPDATE...
Issued at 1050 PM MDT Sun Aug 23 2026

Another round of at least isolated thunderstorms is expected to
initiate off the Black Hills as early as 19z/1pm Monday afternoon,
with the I-90 corridor and points south to the Nebraska border
then favored through the evening as cells push southeast. Forecast
soundings suggest this corridor will support the best combination
of moisture, instability, and shear, with SBCAPE values
potentially over 2,500 J/kg and convective temperatures likely to
be reached to break the cap. Overall shear will be lower than
today, and a mix of pulse, multicell, and transient supercell
structures are possible. PoPs have been massaged to support this
scenario based on the latest round of CAMs. Across most of the
rest of the CWA, mostly dry and breezy conditions are expected.
Fire weather conditions will be a touch elevated due to the heat
and dry/breezy conditions, but wind gust and RH values are not
expected to reach Red Flag criteria.

&&

.DISCUSSION...(This Afternoon Through Saturday)
Issued at 1043 AM MDT Sun Aug 23 2026

16z surface analysis had frontal boundary from eastern MT to
eastern CO and a trough from eastern CO into northeastern NE.
Water vapour loop had upper ridge over the southwest CONUS with a
shortwave over north-central WY. Overnight MCS about to fully exit
the CWA with leftover low stratus burning off per 0.5km 1-minute
visible imagery. Main concern this forecast will be effect of
north-central WY shortwave.

This afternoon/tonight, shortwave moves into the Dakotas with
slight increase in mid-level flow resulting in 30-45kt effective
bulk shear later this afternoon. Td/s in the 60s will be sustained
per breezy southeast winds ahead of frontal boundary. Band of
2-3KJ/kg MLCAPE forecast this afternoon over far western SD with
minimum cap. Convective initiation (CI) will likely be over the
Black Hills around 20z with supercells moving south/southeast at
10-20kts per Bunkers storm motion prognostications. Another area
of strong/severe CI may develop over northwestern SD along the
frontal boundary/trough interaction late this afternoon. CAMs
handle this scenario in different fashion with some emphasizing
MLCIN east of the Black Hills with some members not so much.
Other strong/isolated severe convection may occur over the rest
of the CWA. Sufficient buoyancy/bulk shear for marginal/slight
(1-2 out of 5) of severe thunderstorms. Large hail/damaging wind
the main threats with CIG1 magnitudes over western SD.
Temperatures will be a bit below guidance this afternoon per low
level moisture/dissipating stratus.

Monday, diurnally driven convection possible in the
afternoon/evening, but the upper support less certain compared to
today. Temperatures will be near guidance.

The rest of the forecast will be dominated by the upper ridge
over the southwest CONUS. Ridge amplification into the northern
Plains for the second half of the week suggests somewhat quieter
and drier weather.

&&

.AVIATION...(For the 06Z TAFS Through 06Z Monday Night)
Issued At 1032 PM MDT Sun Aug 23 2026

VFR conditions are expecting through tonight. Tomorrow has more
storm chances in the evening, primarily over the Black Hills and
east. A PROB30 has been retained in the KRAP forecast to reflect
this. Prior to any storms, winds across the area may get
gusty, with gusts up to 20kts expected.

&&

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

&&

$$

UPDATE...KSmith
DISCUSSION...Helgeson
AVIATION...Schweigert