Mesoscale Discussion 1326

Issued

Jul 22, 2021, 1:50 PM MDT (19:50 UTC)

Expired

Jul 22, 2021, 4:15 PM MDT

In force

2 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

40%

Counties Broadwater County, MT, Fergus County, MT, Garfield County, MT, Golden Valley County, MT, Judith Basin County, MT, Meagher County, MT, Musselshell County, MT, Petroleum County, MT, Phillips County, MT, Sweet Grass County, MT, Wheatland County, MT

The Great Falls radar (KTFX), 141 km from the middle of the area, began a volume at 19:51 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.

• Severe Thunderstorm Watch 386Jul 22, 4:05 PM MDT

392 
ACUS11 KWNS 221950
SWOMCD
SPC MCD 221950 
MTZ000-222215-

Mesoscale Discussion 1326
NWS Storm Prediction Center Norman OK
0250 PM CDT Thu Jul 22 2021

Areas affected...Much of central Montana

Concerning...Severe potential...Watch possible 

Valid 221950Z - 222215Z

Probability of Watch Issuance...40 percent

SUMMARY...Thunderstorms are expected to gradually increase and
intensify across southwestern into central Montana through 3-6 PM
MDT, accompanied by at least some risk for severe hail and wind. 
More substantive strengthening and organization, accompanied by
increasing potential for strong surface gusts, may not occur until
after 6 PM MDT, generally northeast of the Lewistown area.

DISCUSSION...Forcing for ascent, associated with a weak perturbation
migrating around the periphery of mid-level ridging centered over
the Four Corners, appears to be aiding deepening convective
development and the initiation of thunderstorms across the mountains
of southwestern into central Montana.  This is occurring near the
southern fringe of stronger flow associated with the westerlies,
which includes west-southwesterly winds around 500 mb on the order
of 40-50 kt.

Lower/mid-tropospheric lapse rates across this region continue to
steepen in response to insolation and perhaps weak mid-level
cooling, and moisture appears sufficient to support weak to modest
CAPE on the order of 500-1000 J/kg.  This should be enough to
support a gradual further increase in convective coverage and
intensity through the next few hours, with at least some potential
for stronger storms to pose a risk for hail and locally strong
surface gusts.

In time, a more substantive risk for organized severe weather may
gradually develop, as consolidating convectively driven cold pools
contribute to the evolution of an organizing convective system in
the presence of the strong deep-layer shear.  This seems most
probable as activity spreads northeast of the Lewistown area by or
shortly after 00Z, with severe weather potential increasingly more
rapidly thereafter, as strong northeasterly low-level inflow
increasingly emanates from a hotter and more moist environment
across northeastern Montana.

..Kerr/Hart.. 07/22/2021

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

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

LAT...LON   46741198 48450859 48130723 46850687 45461098 46741198

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.