Mesoscale Discussion 1165
NWS Storm Prediction Center Norman OK
0337 PM CDT Tue Jul 06 2021
Areas affected...southern Montana far northern Wyoming.
Concerning...Severe potential...Watch possible=20
Valid 062037Z - 062200Z
Probability of Watch Issuance...40 percent
SUMMARY...Isolated storms including a few supercells may pose a risk
for damaging winds and hail this evening. A weather watch is
possible but uncertain.
DISCUSSION...Afternoon visible satellite imagery showed towering
cumulus developing off the higher terrain of southern Montana and
northern Wyoming. Along the western fringes of a northwesterly flow
shortwave trough, storm development appears probable in the next
hour or two as remaining inhibition weakens. Within a warming and
destabilizing environment with around 1000 J/kg of MLCAPE, and 30-40
kts of effective shear, supercells are expected to be the primary
storm mode. Mid-level lapse rates while not overly steep around
7-7.5 C/km should be sufficient for the development of large hail,
especially with stronger supercells. Damaging wind gusts will also
be likely given the well-mixed boundary layer with temperature
dewpoint spreads near 30 degrees F.
High-res guidance has been inconsistent on the coverage of storms
that develop later this evening. This is likely due to weak forcing
for ascent within the upslope flow regime limiting model-based
convective initiation. Given satellite trends, it appears possible
that isolated to widely scattered storms may develop and track
southeastward across Montana and portions of northern Wyoming. The
convective parameter space suggests a weather watch is possible,
though uncertainty remains.
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