Mesoscale Discussion 1171
NWS Storm Prediction Center Norman OK
0328 PM CDT Wed Jun 28 2017
Areas affected...Eastern WY...Western SD...Eastern MT
Concerning...Severe potential...Watch unlikely
Valid 282028Z - 282230Z
Probability of Watch Issuance...20 percent
SUMMARY...Isolated severe thunderstorms are possible this afternoon,
with a threat of hail and locally damaging wind.
DISCUSSION...Widely scattered thunderstorms have developed from
eastern MT/WY into western SD, in conjunction with diurnal heating
and in advance of a low-amplitude upper trough moving through the
The strongest convection within the MCD area has thus far been
across northeast MT, in the vicinity of a weak surface boundary
where winds are favorably backed out of the east, resulting in
effective shear of 35-45 kt and effective helicity of 100-150 m2/s2.
In conjunction with SBCAPE of 500-1000 J/kg, the wind profile in
this area will continue to support organized convection in the short
term, including transient supercell structures with a threat of
large hail and locally damaging wind. Strong drying/mixing across
far eastern MT and western ND should limit the severe threat with
eastward extent across this area.
Further south, additional clusters of thunderstorms are moving
across southeast MT and eastern WY, and an isolated strong cell has
recently developed south of Rapid City. While wind profiles are not
quite as favorable compared to northeast MT, effective shear of
25-35 kt in conjunction with SBCAPE of 500-1250 J/kg will support
the potential for modestly organized clusters and perhaps a
supercell or two, capable of large hail and locally damaging wind.
In general, the magnitude and coverage of the severe threat across
these areas are expected to remain too low for watch issuance.
...Please see www.spc.noaa.gov for graphic product...
LAT...LON 42970449 43020476 43200505 43760506 44570517 46620596
47940612 48090449 45770401 44050259 43140254 43100273
43050316 43030410 42970449