Mesoscale Discussion 1929
NWS Storm Prediction Center Norman OK
0213 PM CDT Tue Aug 11 2026
Areas affected...portions of western South Dakota and southwestern
North Dakota
Concerning...Severe potential...Watch likely
Valid 111913Z - 112115Z
Probability of Watch Issuance...80 percent
SUMMARY...Widely scattered thunderstorm development is expected this
afternoon across portions of the western Dakotas. A supercellular
storm mode is likely, with the primary threats large to very large
hail and severe wind gusts.
DISCUSSION...Recent visible satellite imagery depicts agitated
cumulus developing along the Black Hills in western South Dakota.
Widely scattered thunderstorm development is likely across this
region and within a weak surface confluence zone noted just west of
the ND/SD/MT border. The 18z UNR observed sounding sampled 2000 J/kg
MLCAPE and steep mid-level lapse rates of 8-9 C/km, with further
destabilization expected through the afternoon (MLCAPE forecast to
exceed 3000 J/kg by peak heating). Enhanced westerly flow aloft is
also yielding elongated, relatively linear hodographs and 40-50 kts
of effective shear, which will support splitting supercells. Ample
buoyancy within the hail growth zone, steep mid-level lapse rates,
and a favorable storm mode will promote a threat for large to very
large hail of 2-3+ inches in diameter. Isolated giant hail (4+
inches in diameter) cannot be ruled out with any persistent,
longer-lived supercell; however, supercell longevity remains
somewhat uncertain. Severe wind gusts and perhaps a brief tornado
are also possible. While the exact timing of convective initiation
is uncertain, a Severe Thunderstorm Watch will likely be needed
within the next 1-2 hours to cover this potential.
Some recent CAM solutions also indicate some potential for upscale
growth later this evening; however, this scenario remains unclear.
Should this occur, a transition towards damaging wind gusts as the
primary threat could be expected.
..Chalmers/Hart.. 08/11/2026
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...ABR...BIS...UNR...
LAT...LON 43670380 44270382 45380379 46210362 46710300 46810243
46770204 46680185 46390149 45990126 45050118 44130123
43690131 43350154 43140196 43100233 43090289 43240340
43670380
MOST PROBABLE PEAK TORNADO INTENSITY...UP TO 90 MPH
MOST PROBABLE PEAK WIND GUST...55-70 MPH
MOST PROBABLE PEAK HAIL SIZE...2.00-3.50 IN
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