South Dakota Mesoscale Discussion

Areas affected…Much of SD…Northern NE

Concerning…Severe potential…Tornado Watch likely

Valid 091847Z – 092015Z

Probability of Watch Issuance…80 percent

SUMMARY…Scattered severe thunderstorm development is expected
later this afternoon. The most likely hazards will be severe wind
and possibly a couple of tornadoes, though some hail will also be
possible. Tornado Watch issuance is likely by 20Z.

DISCUSSION…Earlier elevated convection is gradually shifting
eastward across north-central SD, with a recent attempt at
surface-based initiation across Jackson County, SD. The convective
evolution remains uncertain this afternoon, with subsidence noted in
WV imagery in the wake of the elevated storm cluster, but also some
indication of increasing low-level convergence along an effective
warm front and near a frontal wave east of Rapid City. Midlevel
lapse rates are rather modest (generally less than 7 C/km), but rich
low-level moisture is supporting moderate buoyancy (MLCAPE of
1500-2000 J/kg) near and south of the boundary.

Elongated hodographs will support an initial supercell mode with any
sustained surface-based convection this afternoon. An increase in
the low-level jet is forecast later this afternoon, which will
strengthen low-level shear, especially near and just to the cool
side of the surface boundary. There is potential for one or more
supercells to develop later this afternoon and track
east-southeastward within an environment that will be increasingly
supportive of tornadoes, though this potential will be contingent on
the timing/location of initiation with respect to the surface
boundary. Initial supercell development would also pose some hail
risk, though this should be modulated by modest midlevel lapse rates
and relatively warm temperatures aloft. Eventual development of an
upscale-growing cluster will be possible later on in the convective
evolution, which would pose an increasing threat of severe wind.

While the timing of the primary threat remains uncertain, Tornado
Watch issuance is likely by 20Z to cover what is likely to be an
increasing severe threat with time.

..Dean/Grams.. 08/09/2019

…Please see for graphic product…


LAT…LON 43010286 43340288 43840286 44050288 44240291 44420293
44640290 44850285 45010269 45170239 45180178 45170081
45019944 44039876 43609871 43069863 42969935 42780046
42840233 43010286

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