South Dakota Mesoscale Discussion

Areas affected…Southeast MT…Northern WY…Northwest
SD…Southwest ND

Concerning…Severe potential…Watch likely

Valid 111815Z – 111945Z

Probability of Watch Issuance…95 percent

SUMMARY…Severe thunderstorm development is likely across some
portion of the area this afternoon. Very large hail and severe wind
gusts will be the primary hazards, though a tornado or two will also
be possible. One or more watches are likely by 19-20Z.

DISCUSSION…A strong thunderstorm has recently developed near
Sheridan, WY, while a gradual intensification has been noted for
embedded cells within an ongoing elevated cluster across southeast
MT. Thunderstorms are expected to increase in coverage and intensity
through the afternoon, as further destabilization occurs and a
seasonably strong upper-level low/trough ejects across the northern
Rockies. Evolution of the ongoing elevated cluster remains
uncertain, but there is some potential for a transition to
surface-based convection as the downstream airmass destabilizes into
southwest ND/northwest SD. Meanwhile, additional thunderstorms are
likely to develop across the higher terrain of northern/eastern WY
later this afternoon.

Moderate to locally strong buoyancy (MLCAPE 1500-3000 J/kg) is
forecast to develop later this afternoon, as heating of a moist
low-level environment occurs beneath steep midlevel lapse rates.
Effective shear in the 35-45 kt range will favor supercell
development with initial deep convection, with some cell
mergers/upscale growth possible later in the convective evolution.
Very large hail will the primary initial threat, along with isolated
severe wind gusts. Any upscale growth later this afternoon would
favor somewhat greater wind potential. A tornado or two will also be
possible with any supercells, though relatively weak low-level flow
may temper the tornado threat to some extent.

Given the expected increase in the severe threat with time, one or
more watches are likely across some portion of this region by

..Dean/Grams.. 08/11/2019

…Please see for graphic product…


LAT…LON 46890656 46850435 46320328 45840290 44750289 44070323
43710360 43600416 43660480 43790536 43990621 44600786
44810922 45001013 46120828 46740690 46890656

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