Mesoscale Discussion 1170

Issued

Jul 6, 2021, 6:40 PM MDT (00:40 UTC)

Expired

Jul 6, 2021, 8:15 PM MDT

In force

95 min

Issued by

the Storm Prediction Center

Concerning

Severe thunderstorm watch 339

Counties Blaine County, MT, Fergus County, MT, Garfield County, MT, Hill County, MT, Musselshell County, MT, Petroleum County, MT, Phillips County, MT, Rosebud County, MT, Treasure County, MT, Yellowstone County, MT

The Glasgow radar (KGGW), 161 km from the middle of the area, began a volume at 00:41 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

149 
ACUS11 KWNS 070041
SWOMCD
SPC MCD 070040 
MTZ000-070215-

Mesoscale Discussion 1170
NWS Storm Prediction Center Norman OK
0740 PM CDT Tue Jul 06 2021

Areas affected...Parts of central/eastern Montana

Concerning...Severe Thunderstorm Watch 339...

Valid 070040Z - 070215Z

The severe weather threat for Severe Thunderstorm Watch 339
continues.

SUMMARY...Widely scattered strong storms may pose a continuing risk
for severe hail and locally strong surface gusts through mid to late
evening.  While there appears some potential for the evolution of a
small upscale growing cluster, particularly near/east of Havre into
areas northeast of Lewistown, this threat still appears relatively
low and it is not yet clear that another severe weather watch is
needed.

DISCUSSION...Easterly to southeasterly near surface flow, to the
cool side of a zone of stronger differential surface heating and
deeper boundary-layer mixing to the east of the higher terrain, has
maintained sufficient boundary-layer moisture to contribute to CAPE
on the order of 1000-1500 J/kg.  This is not particularly large, but
even within this regime, the boundary-layer is still relatively
deeply mixed.  And perhaps more prominently, beneath a belt of 40+
kt northwesterly flow around 500 mb, deep-layer shear is strong.

The environment has become conducive to widely scattered supercell
development as far north as southern Alberta and Saskatchewan,
perhaps aided by weak warm advection around the 700 mb level.  It is
possible that this forcing could contribute to some continuing
upscale growth of ongoing convection into the 03-06Z time frame. 
However, synoptic forcing for ascent appears otherwise weak, and it
might be just as possible that much of ongoing activity could begin
to wane as boundary-layer cooling progresses, given the already
somewhat marginal nature of the thermodynamic profiles.

..Kerr.. 07/07/2021

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

ATTN...WFO...BYZ...GGW...TFX...

LAT...LON   48770857 47020557 45590716 45930854 47731007 48841125
            48770857

Mesoscale Discussions in Montana, 2021Every mesoscale discussion in 2021

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.