Mesoscale Discussion 696

Issued

May 21, 2021, 2:54 PM MDT (20:54 UTC)

Expired

May 21, 2021, 5:30 PM MDT

In force

3 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

60%

Counties Logan County, CO, Morgan County, CO, Phillips County, CO, Sedgwick County, CO, Washington County, CO, Yuma County, CO, Banner County, NE, Box Butte County, NE, Cheyenne County, NE, Deuel County, NE, Garden County, NE, Kimball County, NE, Morrill County, NE, Scotts Bluff County, NE, Sheridan County, NE, Bennett County, SD

The North Platte radar (KLNX), 178 km from the middle of the area, began a volume at 20:54 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 192May 21, 4:15 PM MDT

725 
ACUS11 KWNS 212055
SWOMCD
SPC MCD 212054 
NEZ000-SDZ000-COZ000-212330-

Mesoscale Discussion 0696
NWS Storm Prediction Center Norman OK
0354 PM CDT Fri May 21 2021

Areas affected...Portions of northeastern CO...the NE
Panhandle...and southwestern SD

Concerning...Severe potential...Watch possible 

Valid 212054Z - 212330Z

Probability of Watch Issuance...60 percent

SUMMARY...The potential for large hail, strong/gusty winds, and
perhaps a couple of tornadoes should increase late this afternoon.
Watch issuance is possible.

DISCUSSION...20Z surface observations show a cold front extending
from central into southwestern SD, the NE Panhandle, and parts of
northeastern CO. Strong boundary-layer heating has occurred so far
this afternoon to the south of this front and east of a surface lee
trough/dryline in eastern CO, with surface temperatures generally
warming into the upper 70s to mid 80s. Low-level moisture still
remains somewhat limited, but surface dewpoints are mostly holding
in the mid to upper 50s. The presence of steep mid-level lapse rates
across the warm sector is contributing to 1000-1500 J/kg of MLCAPE
per latest mesoanalysis estimates. A large, closed upper low remains
centered over CA and the Great Basin, with a fairly broad swath of
enhanced south-southwesterly mid-level winds extending from the
Southwest across the central Rockies and adjacent High Plains.

As convective inhibition continues to weaken, current expectations
are for isolated storms to develop by 22-23Z (4-5 PM MDT) along both
the cold front and dryline as a subtle vorticity maximum ejects
northeastward over the central Rockies/High Plains. 40-50+ kt of
effective bulk shear will likely support supercells initially, with
mainly a large hail threat given the steep mid-level lapse rates.
There may be some potential for very large hail (2+ inch) for a
brief period before storms begin to interact and merge with one
another along the cold front. A south-southeasterly low-level jet is
forecast to strengthen to around 40-45 kt by early this evening, and
low-level shear will increase as well south of the front. A couple
of tornadoes appear possible in this regime, but there is
uncertainty regarding dominant storm mode (supercells vs clusters)
by this evening as the low-level shear strengthens. Strong/gusty
winds will also be possible if storms grow upscale into one or more
small clusters. Given the increasing severe potential anticipated
over the next couple of hours, watch issuance will probably be
needed.

..Gleason/Guyer.. 05/21/2021

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

ATTN...WFO...LBF...UNR...GLD...BOU...CYS...

LAT...LON   40020212 39510259 39340310 39660349 40320388 41370393
            42280357 43220257 43570185 43550126 43080124 42000194
            40990210 40020212

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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.