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ACUS11 KWNS 181945
SWOMCD
SPC MCD 181945
SDZ000-WYZ000-182215-

Mesoscale Discussion 1318
NWS Storm Prediction Center Norman OK
0245 PM CDT Sat Aug 18 2018

Areas affected...Portions of western/central SD and far northeastern
WY

Concerning...Severe potential...Watch unlikely

Valid 181945Z - 182215Z

Probability of Watch Issuance...5 percent

SUMMARY...Thunderstorms should continue developing along a surface
trough/front this afternoon, with very isolated large hail and
strong/gusty winds possible. Due to the marginal nature of the
severe threat, watch issuance is not expected.

DISCUSSION...Initial convective development has occurred this
afternoon across the Black Hills region of western SD as an upper
trough/low centered over western WY continued eastward. Additional thunderstorms may develop over the next couple of hours along a
surface trough/front evident in 19Z surface observations across this
region. Even with increasing mid/upper-level cloudiness spreading in
from the west, surface temperatures have generally warmed into the
mid 80s to lower 90s, and MLCAPE increases from around 500 J/kg west
to 2000 J/kg east across western/central SD. The 12Z sounding from
UNR showed lapse rates around 8.0 C/km in the 700-500 mb layer, and
several cores of potentially large hail have been recently noted
over the Black Hills per MRMS MESH estimates. The main limiting
factor that will likely preclude a greater severe risk is
displacement of stronger mid-level winds and related shear to the
west of the greater instability. Even so, very isolated large hail
will be possible with any thunderstorms given the steep mid-level
lapse rates present, and strong/gusty downdraft winds could also
occur along and east of the surface trough/front where low-level
lapse rates are steeper and instability is greater.

..Gleason/Hart.. 08/18/2018

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

ATTN...WFO...ABR...UNR...

LAT...LON 43050308 43860361 44450432 44980399 45710349 45880304
45920189 45710107 44900037 43860038 43030059 43050308



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