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ACUS11 KWNS 282035
SWOMCD
SPC MCD 282035
NDZ000-SDZ000-MTZ000-WYZ000-282300-

Mesoscale Discussion 0923
NWS Storm Prediction Center Norman OK
0335 PM CDT Thu Jun 28 2018

Areas affected...Eastern Mont aha...Western North Dakota...and far
Northeastern South Dakota

Concerning...Severe potential...Tornado Watch likely

Valid 282035Z - 282300Z

Probability of Watch Issuance...95 percent

SUMMARY...Supercell thunderstorms are expected to form later this
afternoon and into the evening. Very large hail, significant
damaging winds, and a few tornadoes are possible this evening. A
tornado watch will likely be issued in the next couple of hours.

DISCUSSION...Latest HRRR guidance and timing of the upper-wave on
6.2 micron water vapor imagery suggest convective initiation is
likely between 22Z and 23Z this evening in eastern Montana. Strong
surface heating and low-level moisture advection has increased
surface temperatures into the mid to upper 80s and dewpoints into
the low 70s. This warm, moist boundary layer combined with very
steep mid-level lapse rates (over 9 C/km per UNR 19Z sounding and
RAP mesoanalysis) has led to extreme instability across the area
(Over 4000 J/kg MLCAPE). The GGW 19Z sounding showed a 50 knot wind
speed at 500 mb which is above the 90th percentile for late June.
This leads to 50 to 60 knots of effective shear, which combined with
the extreme instability will support supercells as the primary storm
mode initially. The extreme values of instability, deep-layer shear,
and mid-level lapse rates will support very large hail, especially
for the first few hours after storm development.

Low-level east-southeasterly flow, veering to west-southwesterly
flow at 3 km will also favor rotating low-level mesocyclones. While
the speed shear in the lowest 1 km will initially be a limiting
factor for tornadic production, 850 mb winds will strengthen
significantly after 00Z. Current thinking is that storm mode may
become less discrete around the time of the improving low-level
shear profile which could limit the tornado threat. However, the
last few HRRR runs have hinted that a discrete storm mode may
persist long enough into the evening to support the tornado threat.

..Bentley/Hart.. 06/28/2018

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

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

LAT...LON 44930413 44750495 44680581 44790683 45120806 45950825
46760836 47340760 47590715 47740647 47880562 47980487
48000446 48080408 48040367 47820337 47150317 45910344
45220372 44930413



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