Mesoscale Discussion 1954
NWS Storm Prediction Center Norman OK
0145 PM CDT Wed Aug 12 2026
Areas affected...portions of northeastern Wyoming and far
southeastern Montana into the Black Hills and western South Dakota
Concerning...Severe potential...Watch likely
Valid 121845Z - 122045Z
Probability of Watch Issuance...80 percent
SUMMARY...Scattered thunderstorms will develop eastward into the
Black Hills/western South Dakota vicinity through this afternoon,
bringing a threat for large to very large hail and severe wind
gusts.
DISCUSSION...Latest radar imagery depicts a developing supercell in
close proximity to a surface low across northeastern Wyoming. Aided
by weak ascent ahead of a mid-level perturbation and easterly,
upslope flow (surface dewpoints ranging from the low 60s F to near
70 farther east), convection will expand in coverage through the
afternoon as it evolves eastward into the Black Hills/western South
Dakota vicinity. While the 18z UNR observed sounding depicted
lingering inhibition, filtered diurnal heating should gradually
reduce inhibition through the afternoon. For ongoing/developing
convection, steep mid-level lapse rates (8+ C/km), 40-50+ kts of
effective shear, and relatively straight, elongated hodographs will
support supercells as the initial storm mode, with large to very
large hail the primary hazard. Increasing storm coverage lends
uncertainty to how long discrete supercells will be maintained, with
eventual upscale growth/cluster development expected to yield a
transition towards severe wind gusts as the primary hazard. Should a
discrete mode be maintained for longer, however, very large hail to
3 inches in diameter would be possible. A Severe Thunderstorm Watch
will likely be needed for a portion of the area within the hour.
..Chalmers/Thompson.. 08/12/2026
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...ABR...LBF...UNR...CYS...BYZ...
LAT...LON 44660525 44950506 45510373 45500236 45040131 44360096
43600107 43130139 43010175 42850299 42810383 42870451
43100512 43300535 43680538 44220535 44660525
MOST PROBABLE PEAK WIND GUST...65-80 MPH
MOST PROBABLE PEAK HAIL SIZE...2.00-3.50 IN
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