Mesoscale Discussion 1309

Issued

Jul 17, 2021, 2:40 PM MDT (20:40 UTC)

Expired

Jul 17, 2021, 5:15 PM MDT

In force

3 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

40%

The Cheyenne radar (KCYS), 189 km from the middle of the area, began a volume at 20:43 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.

353 
ACUS11 KWNS 172041
SWOMCD
SPC MCD 172040 
NEZ000-SDZ000-COZ000-WYZ000-172315-

Mesoscale Discussion 1309
NWS Storm Prediction Center Norman OK
0340 PM CDT Sat Jul 17 2021

Areas affected...Western South Dakota...Nebraska Panhandle...and
Northeastern Colorado

Concerning...Severe potential...Watch possible 

Valid 172040Z - 172315Z

Probability of Watch Issuance...40 percent

SUMMARY...Storms developing over the high terrain are expected to
eventually move eastward into the High Plains, posing a risk for
severe hail and wind.  The coverage of severe storms remains
uncertain, so convective trends will be monitored for the
possibility of a severe thunderstorm watch.

DISCUSSION...Even though midlevel heights are rising across the
area, an impulse rounding the top of the ridge has contributed to
thunderstorm initiation over the high terrain of central
Colorado/Wyoming and Black Hills of South Dakota.  Substantial
convective inhibition remains over the plains, as indicated by the
20Z UNR sounding (170 J/kg of MLCIN), which will likely delay the
threat and limit the eastward extent. Nevertheless, strong heating
will continue to remove the convective inhibition through the
afternoon, yielding ~1000 J/kg of MLCAPE with 30-40 knots of
effective bulk shear by late afternoon/evening (per latest RAP). 
This environment should support organized storm modes that pose a
threat for severe hail initially (midlevel lapse rates around 8
C/km), followed by a transition to a wind threat as storms merge
into one or more southward-moving clusters, perhaps aided by a
southeasterly low-level jet after 02Z.  Significant uncertainty
remains regarding storm coverage, longevity, and evolution, so
convective trends will be monitored for the possibility of a watch.

..Jirak/Dial.. 07/17/2021

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

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

LAT...LON   40000377 41000413 42510419 43440414 44100404 44300386
            44280333 44180286 43910261 43570235 43140226 42370220
            41650215 41130210 40650207 40000207 39730276 40000377

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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.