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ACUS11 KWNS 211954
SWOMCD
SPC MCD 211953
SDZ000-NDZ000-MTZ000-WYZ000-212230-

Mesoscale Discussion 1123
NWS Storm Prediction Center Norman OK
0253 PM CDT Sat Jul 21 2018

Areas affected...Northeast Wyoming...Southeast Montana...and the
Western Dakotas

Concerning...Severe potential...Watch possible

Valid 211953Z - 212230Z

Probability of Watch Issuance...40 percent

SUMMARY...Storms are beginning to develop across northeast Wyoming
and storm coverage should increase over northeast Wyoming, southeast
Montana, and the western Dakotas through the afternoon. Severe
hail/wind are possible.

DISCUSSION...An increasingly agitated Cu field over northeast
Wyoming indicates destabilization of the air mass and storms should
develop out of this Cu field into the evening. A surface pressure
trough is acting to focus/increase surface convergence stretching
from northeast Montana through eastern Wyoming roughly on a line
from OLF to CYS. An approaching upper-level trough should also
provide ascent aloft aiding the vertical development of the expected
storms. Moisture advection from the southeast and strong daytime
heating have helped destabilize the airmass although a cap remains
mostly in place. Steep low/mid-level lapse rates (8.5+ C/km), MLCAPE
of 1000-3000 J/kg, and DCAPE of 1200+ J/kg per mesoanalysis and RAP
soundings all indicate the potential for severe hail/wind. Shear
(effective bulk shear of 30-50 knots) is also supportive for severe
storm development/organization.

There is a relatively narrow window for severe storm development as
instability declines rapidly across the western Dakotas, and storms
may struggle to develop over portions of southeast Montana where
mid-level clouds have streamed over the area. Chances for a watch
are better across far southeast Montana, through eastern Wyoming,
and into the western Dakotas where storm coverage should be greater
and the CAPE/shear environment is more favorable for severe storms.

..Nauslar/Hart.. 07/21/2018

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

ATTN...WFO...BIS...UNR...BYZ...GGW...RIW...

LAT...LON 44290613 45110629 45880652 46860671 47020568 47180326
45740282 44800272 44030263 43970349 43980451 43960608
44290613



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