Mesoscale Discussion 1184

Issued

Jul 7, 2021, 2:11 PM MDT (20:11 UTC)

Expired

Jul 7, 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 Box Butte County, NE, Dawes County, NE, Morrill County, NE, Scotts Bluff County, NE, Sioux County, NE, Custer County, SD, Fall River County, SD, Niobrara County, WY, Weston County, WY

The Rapid City radar (KUDX), 156 km from the middle of the area, began a volume at 20:14 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.

983 
ACUS11 KWNS 072012
SWOMCD
SPC MCD 072011 
NEZ000-SDZ000-WYZ000-072215-

Mesoscale Discussion 1184
NWS Storm Prediction Center Norman OK
0311 PM CDT Wed Jul 07 2021

Areas affected...eastern Wyoming southwestern South Dakota and
portions of the Nebraska Panhandle.

Concerning...Severe potential...Watch possible 

Valid 072011Z - 072215Z

Probability of Watch Issuance...40 percent

SUMMARY...Model guidance and observational trends continue to
suggest that isolated storm development may take place this
afternoon and evening. A supercell or two will be capable of mainly
a hail and damaging wind threat this evening. A tornado or two may
also be possible. A weather watch is possible, but uncertain given
the low expected storm coverage.

DISCUSSION...Within a weak upslope flow regime north of a warm
front, towering cumulus has been observed percolating along western
portions of the Pine Ridge and southern Black Hills across eastern
Wyoming and southwestern South Dakota. Under the influence of a weak
shortwave trough around the periphery of a mid-level ridge,
background ascent and surface heating should continue to weaken
inhibition enough for convective initiation by mid to late
afternoon. Surface temperatures in the 80s and dewpoints in the mid
50s to low 60s F along with steep lapse rates around 8 C/km will
contribute to moderate buoyancy with 1500-2000 J/kg of MLCAPE
developing by late this afternoon. Model soundings and VWPS show
enhanced mid-level flow associated with the shortwave trough
overspreading much of the eastern Wyoming supporting 30-40 kts of
effective shear. Relatively long but straight hodographs will
support supercells as the primary storm mode. Given favorable
kinematic and thermodynamic profiles, these storms may pose a risk
for large hail (some 2+ inches) and damaging wind gusts through this
evening. 

Thunderstorm initiation along the western Pine Ridge escarpment in
Converse, Niobrara, and Campbell Counties, Wyoming and across the
southern Black Hills appears probable by 21-22z. Within this
corridor and the southern Black Hills Convergence Zone, locally
backed winds may also support a risk for a brief tornado with more
well-developed supercells as they track southeastward. The threat
for tornadoes does appear low however, given high-based storms and
weak low-level shear. 

Severe weather appears increasingly likely across the area though
uncertainty on storm coverage remains high. Trends will be monitored
for a possible weather watch, but it remains unclear if one will be
needed.

..Lyons/Guyer.. 07/07/2021

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

ATTN...WFO...LBF...UNR...CYS...

LAT...LON   41550347 42050422 42640474 43460527 43890537 44340489
            44540422 44130355 43410303 42890272 42210250 41600277
            41550347

Mesoscale Discussions in Nebraska, 2021Mesoscale Discussions in South Dakota, 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.