South Dakota Mesoscale Discussion

Areas affected…Extreme Southeast MT…Southwest to South-Central
ND…Northwest SD

Concerning…Heavy rainfall…Flash flooding possible

Valid 112145Z – 120345Z

SUMMARY…A strong complex of heavy showers and thunderstorms is
expected to evolve across southwest to south-central ND and at
least far northwest ND going into the evening hours. Some flash
flooding will be possible with a threat of locally a few inches of

DISCUSSION…The latest radar imagery shows a gradually expanding
cluster of heavy showers and thunderstorms across southwest ND
with somewhat of a broken extension down across extreme northwest
SD and into far southeast MT. The convection is developing in
association with an increasingly divergent flow pattern aloft
downstream of an anomalously strong upper low crossing the
northern Rockies. Additionally, there is a substantial amount of
low-level moisture pooled out across the northern Plains along
with a nose of moderate boundary layer instability. MLCAPE values
are on the order of 1500 j/kg with the aid of surface dewpoints in
the low to mid 60s.

The latest hires model guidance shows the evolution of what should
be a strong MCS across southwest to south-central ND and at least
a portion of northwest SD going through the evening hours. A
surface trough is noted across this region right now based the
latest 21Z analysis, and it is expected that the trough axis will
be the dominant focus/track of the MCS. Facilitating the upscale
growth of convection in addition to increasing shear profiles and
forcing aloft will be a strengthening of low-level southeast flow
along and north of a front farther south that should begin to
attempt to lift northward as a warm front this evening ahead of
the upstream height falls. Much of this energy will be intercepted
by the aforementioned trough axis.

Anomalously high moisture transport and plenty of instability
pooling along the boundary should lead to strong convection with
impressively high rainfall rates that may locally exceed 2
inches/hr. The last few runs of the HRRR have been advertising
rainfall totals of 3 to 5 inches with isolated heavier totals
going through the late-evening time frame.

It is possible the HRRR could be a tad on the high side since the
MCS is expected to be rather progressive, but these amounts will
be possible prior to MCS maturity as there may be a period of some
repeating convective cell activity. A few cell-mergers will be
possible as well. Also, given the latest radar trends which show
convection developing across parts of far southeast MT and even
northeast WY, it is possible the HRRR may not have enough
convection impacting areas of northwest SD to the north of the
Black Hills.

Based on the expected rainfall totals, some flash flooding will be


…Please see for graphic product…



LAT…LON 47190148 47190064 46950007 46519968 45939997
45470082 45030234 44820349 44780411 45000441
45420444 45830415 46460344 46930260

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