Mesoscale Discussion 1017

Issued

Jun 19, 2021, 3:58 PM MDT (21:58 UTC)

Expired

Jun 19, 2021, 5:30 PM MDT

In force

92 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch unlikely

Chance of a watch

20%

Counties Carter County, MT, Fallon County, MT, Dawes County, NE, Butte County, SD, Custer County, SD, Fall River County, SD, Harding County, SD, Lawrence County, SD, Meade County, SD, Oglala Lakota County, SD, Pennington County, SD, Crook County, WY, Weston County, WY

The Rapid City radar (KUDX), 84 km from the middle of the area, began a volume at 21:58 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.

058 
ACUS11 KWNS 192158
SWOMCD
SPC MCD 192158 
SDZ000-NEZ000-NDZ000-MTZ000-WYZ000-192330-

Mesoscale Discussion 1017
NWS Storm Prediction Center Norman OK
0458 PM CDT Sat Jun 19 2021

Areas affected...Southeastern Montana...northeast Wyoming...and
portions of the Dakotas.

Concerning...Severe potential...Watch unlikely 

Valid 192158Z - 192330Z

Probability of Watch Issuance...20 percent

SUMMARY...Storm initiation appears possible in the next 1 to 2 hours
across the central and northern High Plains. A narrow corridor of
buoyancy will support a few hours of isolated severe potential with
high-based storms. Uncertainty on storm coverage and severity
remains high.

DISCUSSION...Ahead of a deepening mid-level trough observed on
afternoon water vapor imagery, towering cumulus has become
progressively more agitated from far southeastern Montana into
northeastern Wyoming. Incipient updrafts along the crest of the Big
Horns and higher terrain in southeast Montana suggests the airmass
across the High Plains is likely nearing minimal convective
inhibition. SPC mesoanalysis indicates between 500-1500 J/kg of
MLCAPE have developed in a narrow corridor from the Nebraska
Panhandle northward, owing to steep mid-level lapse rates and
surface mixing ratios between 8 and 10 g/kg. Weak upslope flow,
diurnal heating, and background ascent from the approaching
mid-level wave should further diminish remaining inhibition, and may
allow for convective development in the next couple of hours. CAM
guidance remains unreliable in the weak ascent regime, but does hint
at one or more clusters of storms evolving off the higher terrain
this evening. The primary severe threats will be damaging outflow
winds and hail, given moderate buoyancy and steep low/mid-level
lapse rates. 

Across the Black Hills of western South Dakota, cloud debris from
earlier convection is inhibiting strong surface heating casting
doubt on storm initiation in this region. Here, deeper surface
moisture and higher buoyancy may support a locally greater, but more
conditional severe threat for wind and hail into this evening. Given
the narrow corridor of destabilization and the uncertainty on storm
coverage/intensity, a weather watch is unlikely, though trends will
be monitored.

..Lyons/Guyer.. 06/19/2021

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

ATTN...WFO...BIS...LBF...UNR...CYS...BYZ...GGW...

LAT...LON   42540325 42570417 43430464 44650494 45680493 46490480
            46640474 46820434 46590388 45590312 44680263 43960228
            43370213 42610256 42540325

Mesoscale Discussions in South Dakota, 2021Mesoscale Discussions in Montana, 2021Mesoscale Discussions in Wyoming, 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.