Mesoscale Discussion 605

Issued

May 13, 2021, 1:38 PM MDT (19:38 UTC)

Expired

May 13, 2021, 4:15 PM MDT

In force

3 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch unlikely

Chance of a watch

20%

Counties Arthur County, NE, Banner County, NE, Box Butte County, NE, Brown County, NE, Cherry County, NE, Dawes County, NE, Garden County, NE, Grant County, NE, Hooker County, NE, McPherson County, NE, Morrill County, NE, Scotts Bluff County, NE, Sheridan County, NE, Sioux County, NE, Fall River County, SD

The Rapid City radar (KUDX), 179 km from the middle of the area, began a volume at 19:38 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.

597 
ACUS11 KWNS 131938
SWOMCD
SPC MCD 131938 
NEZ000-SDZ000-WYZ000-132215-

Mesoscale Discussion 0605
NWS Storm Prediction Center Norman OK
0238 PM CDT Thu May 13 2021

Areas affected...Parts of northern/western Nebraska and adjacent
portions of southeastern Wyoming and southwestern South Dakota

Concerning...Severe potential...Watch unlikely 

Valid 131938Z - 132215Z

Probability of Watch Issuance...20 percent

SUMMARY...Strong thunderstorm development, possibly including an
evolving cluster of storms, appears underway.  This may pose a risk
for hail and strong surface gusts which could occasionally approach
or exceed severe limits into early evening.  Due to the marginal
anticipated risk, it is not clear that a severe weather watch is
needed, but trends will be monitored.

DISCUSSION...Boundary-layer destabilization is underway across
central portions of the high plains, modest due to low moisture
levels, but becoming maximized as far north as the Nebraska
Panhandle vicinity.  This is generally north and east of a
developing weak surface low, and along a strengthening zone of
differential surface heating, which is becoming a focus for
deepening convective development.  This is probably being supported
by increasing large-scale ascent, largely due to low/mid-level warm
advection, downstream of a low-amplitude mid-level perturbation
digging within northwesterly flow across the northern Rockies.

Deep-layer ambient mean flow only appears to be strengthening from
20-30 kt, but associated vertical shear, aided by veering profiles
in low to mid-levels, is probably becoming conditionally supportive
of organized convection.  With mixed-layer CAPE, particularly around
the Alliance, NE vicinity, as high as 500 J/kg, but up to around
1000 J/kg for surface-based parcels, there appears potential for at
least a few stronger storms capable of producing small to marginally
severe hail.  

Given the steep low-level lapse rates associated with the modestly
deep, well mixed boundary-layer, locally strong surface gusts might
be the more prominent convective hazard, aided by sub-cloud cooling
associated with melting and evaporating precipitation.  It is
possible that forcing associated with consolidating surface cold
pools, coupled with the larger-scale ascent, could support an
upscale growing, southeastward propagating cluster of storms by
early evening.

..Kerr/Thompson.. 05/13/2021

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

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

LAT...LON   43560347 42309958 41460155 41340384 42450404 42920451
            43560347

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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.