Severe Storm Outlook Narrative (AC)
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802
ACUS11 KWNS 211612
SWOMCD
SPC MCD 211612
MNZ000-IAZ000-NEZ000-SDZ000-211745-

Mesoscale Discussion 1540
NWS Storm Prediction Center Norman OK
1112 AM CDT Mon Aug 21 2017

Areas affected...Southeast SD...Northeast NE...Far northwest
IA...Far southwest MN

Concerning...Severe Thunderstorm Watch 459...

Valid 211612Z - 211745Z

The severe weather threat for Severe Thunderstorm Watch 459
continues.

SUMMARY...Intense thunderstorm cluster with a threat of severe hail
and wind is expected to persist into early afternoon. Downstream
watch issuance may be required.

DISCUSSION...At 16Z, an intense thunderstorm cluster with embedded
supercell structures was ongoing across south-central SD. Very steep
midlevel lapse rates feeding into the system combined with
sufficient buoyancy and shear downstream should sustain this cluster
into at least early afternoon, with a continued threat of severe
hail and wind as it moves east-southeastward across southeast SD and
potentially into northeast NE/northwest IA. Gradual heating and
steepening low-level lapse rates downstream may result in some
increase in the damaging wind threat with time, while the steep
lapse rates will continue to support a severe hail threat with
embedded supercell structures.

Additional convection has recently developed near a low-level
baroclinic zone across southeast SD. This activity may pose a large
hail risk in the short term, while the baroclinic zone will likely
aid in sustenance of the longer-lived cluster approaching from the
west. Watch issuance downstream of WW 459 may be required to cover
these threats.

..Dean.. 08/21/2017

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

ATTN...WFO...FSD...OAX...ABR...LBF...UNR...

LAT...LON   43170067 43940018 44129846 44009679 43599585 42189611
            42329711 42559832 43079988 43170067




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