Severe Storm Outlook Narrative (AC)
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ACUS11 KWNS 211328
SPC MCD 211327

Mesoscale Discussion 1539
NWS Storm Prediction Center Norman OK
0827 AM CDT Mon Aug 21 2017

Areas affected...Central into southeast SD

Concerning...Severe potential...Watch possible

Valid 211327Z - 211530Z

Probability of Watch Issuance...40 percent

SUMMARY...The severe hail risk continues with ongoing thunderstorm
cluster, and some damaging wind risk may evolve with time. While
uncertainty remains regarding the evolution of this cluster, watch
issuance is possible.

DISCUSSION...Intense thunderstorms are ongoing at 1315Z across
west-central SD. The character of this system has changed somewhat
over the last 1-2 hours, evolving from primarily discrete elevated
supercells into more of a cluster with some upscale growth noted,
though with some embedded supercell structures occasionally still
observed. Very steep midlevel lapse rates (in excess of 9 C/km per
12Z RAP sounding) are helping to maintain this activity, with some
large-scale support being provided by a low amplitude midlevel
trough moving through the Dakotas.

The downstream environment is characterized by moderate/strong
MUCAPE (2000-3000 J/kg per recent mesoanalysis) and effective shear
of 35-45 kt, more than sufficient to maintain storm organization
through the morning. The primary uncertainty relates to the elevated
nature of this cluster and whether it can become rooted closer to
the surface and begin propagating more to the southeast into a more
moist and unstable environment. If continued upscale growth is noted
and some southeastward propagation is observed, a hail and
increasing damaging wind threat will develop into southeast SD.
Another possible scenario is that convection remains rooted above
850 mb and propagates more to the east, which would result in an
occasional hail threat and less damaging wind potential.

Depending on convective trends over the next hour or so, watch
issuance is possible downstream of the ongoing cluster into portions
of central and southeast SD.

..Dean/Grams.. 08/21/2017

...Please see for graphic product...


LAT...LON   43780130 44580064 44529886 44399735 43899679 43529671
            43179689 43059791 43269953 43500033 43780130 is the U.S. government's official web portal to all federal, state and local government web resources and services.