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ACUS11 KWNS 172047
SWOMCD
SPC MCD 172047
SDZ000-NEZ000-COZ000-MTZ000-WYZ000-172245-

Mesoscale Discussion 1068
NWS Storm Prediction Center Norman OK
0347 PM CDT Tue Jul 17 2018

Areas affected...Far southeastern Montana...Eastern Wyoming...Black Hills...western Nebraska Panhandle...portion of northeast Colorado

Concerning...Severe potential...Watch possible

Valid 172047Z - 172245Z

Probability of Watch Issuance...60 percent

SUMMARY...Storms have formed across the higher terrain of the Big
Horn Mountains, Black Hills, and Medicine Bow Mountains. Scattered strong/severe storms are expected for the next few hours. Severe
wind gusts and large hail are the primary risks. With time, some
storm clusters may grow upscale where wind will become the primary
threat. A WW may be needed by 21-22Z.

DISCUSSION...With the approach of a mid-level shortwave trough,
thunderstorms have begun to form and move off of the higher terrain
from southeast to north-central Wyoming and the Black Hills of South
Dakota. Effective bulk shear ranges from 25 kts in the northern
portions of the discussions area to around 40 kts in the southern
portions -- where a few low-topped supercell storms were noted on
KCYS radar imagery. Large hail will be possible with moderate
mid-level lapse rates of 7-8 C/km in place. Steep low-level lapse
rates will also favor strong to severe wind gusts. Some uncertainty
exists with regard to evolution of these clusters of thunderstorms.
The current thinking is that storm longevity will be greatest for
storms currently near the Big Horn Mountains as the best large-scale
forcing will exist in this region. However, any clusters of storms
that remain sustained should eventually congeal given the large
surface temperature/dewpoint spreads. Once that transition occurs,
severe wind gusts will become the primary threat. A watch may be
needed by 21-22Z.

..Wendt/Guyer.. 07/17/2018

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

ATTN...WFO...UNR...BOU...CYS...BYZ...RIW...

LAT...LON 45130734 45300659 45300530 44390347 43890328 41910296
40680286 40270299 39920327 39810359 39990435 40740525
41840604 42410657 43020697 43890746 44710753 45130734



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