Hart Ranch Mesoscale Discussion

Areas affected…much of SD into southeast ND

Concerning…Severe potential…Severe Thunderstorm Watch likely

Valid 171731Z – 171930Z

Probability of Watch Issuance…80 percent

SUMMARY…Severe thunderstorms are expected to increase across South
Dakota into southeast North Dakota through the afternoon. Large hail
and damaging wind gusts will be possible with these storms.

DISCUSSION…An isolated supercell has developed early this
afternoon near SPF in the Black Hills in moist, upslope flow regime
and 1.5 inch hail was recently reported. Surface temperatures across
far western SD to the south of a cold front have only warmed into
the low 70s, resulting in weak instability. However, further east
into central SD, temperatures are rapidly warming into the 80s with
surface dewpoints in the mid to upper 60s. With additional heating,
instability should continue to increase with MLCAPE values
approaching 2000 J/kg in the next few hours. EML should also rapidly
erode as forcing for ascent continues to increase, both along the
southeastward-advancing cold front and as midlevel shortwave impulse
brushes the area. As a result, a gradual increase in thunderstorms
development along/ahead of the front is expected over the next few
hours across western into central SD and southeast ND.

Effective shear around 40 kt and steep midlevel lapse rates will
support rotating updrafts and supercell structures capable of
producing large hail initially. However, convection may quickly
become more linear or clustered, which could limit a larger/greater
hail threat. Steep low level lapse rates and a well-mixed sub-cloud
layer will support strong downdrafts and damaging winds are
possible. With time, convection may congeal into one or more surging
lines either along the cold front or through storms mergers/outflow
interactions, further increasing severe/damaging wind potential.
Given the expected increase in storm coverage/intensity over the
next 1-3 hours, a severe thunderstorm watch will likely be needed in
the next hour or two.

..Leitman/Grams.. 08/17/2019

…Please see www.spc.noaa.gov for graphic product…


LAT…LON 45880070 46679871 46969809 46929757 46769712 46189707
45319749 44419819 43869891 43479981 43240228 43390378
43650407 44120420 44410409 44760367 45880070

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