Mesoscale Discussion 692

Issued

May 20, 2021, 12:58 PM MDT (18:58 UTC)

Expired

May 20, 2021, 3:30 PM MDT

In force

3 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

40%

The Rapid City radar (KUDX), 165 km from the middle of the area, began a volume at 19:02 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

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914 
ACUS11 KWNS 201859
SWOMCD
SPC MCD 201858 
SDZ000-NEZ000-COZ000-MTZ000-WYZ000-202130-

Mesoscale Discussion 0692
NWS Storm Prediction Center Norman OK
0158 PM CDT Thu May 20 2021

Areas affected...Portions of eastern WY...southeastern MT...the
western NE Panhandle...and western SD

Concerning...Severe potential...Watch possible 

Valid 201858Z - 202130Z

Probability of Watch Issuance...40 percent

SUMMARY...An isolated threat for marginally severe hail and
strong/gusty winds should gradually increase this afternoon. Watch
issuance is possible.

DISCUSSION...Visible satellite imagery shows convection is deepening
over and just downstream of the higher terrain in north-central CO
and central/eastern WY, with a couple of lightning flashes recently
noted. Stronger mid-level winds associated with a large, closed
upper low centered over the Pacific Northwest are expected to
generally remain displaced to the west of the northern/central High
Plains. But, around 20-30 kt of south-southwesterly flow at 500 mb
should be present by late afternoon. Weak low-level
south-southeasterly winds have transported modest boundary-layer
moisture to the lee of the CO Front Range and Laramie/Bighorn
Mountains in WY, with surface dewpoints generally ranging from the
mid 40s to low 50s. Steep low and mid-level lapse rates are present
across this region, which combined with the limited low-level
moisture and strong diurnal heating should support MLCAPE generally
in the 500-1500 J/kg range by late afternoon.

Current expectations are for storms to gradually move off the higher
terrain and into parts of the High Plains through the remainder of
the afternoon and continuing into the early evening. Both the
limited low-level moisture and rather weak mid-level flow are
concerns for overall storm intensity and organization. Still, about
25-30 kt of effective bulk shear is forecast across the warm sector
by most short-term guidance, which will probably be enough for some
convective organization. Storms may have a tendency to cluster as
they move northeastward with time, especially in eastern WY and
southeastern MT. Isolated strong to severe wind gusts will be a
possibility if this occurs, as the very well mixed boundary layer
will promote efficient downdraft accelerations. Marginally severe
hail may also occur with any initially discrete storms. At this
point, the need for a Severe Thunderstorm Watch this afternoon
remains unclear, but observational trends will be monitored for
potential storm clustering which may locally increase the severe
wind threat.

..Gleason/Kerr.. 05/20/2021

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

ATTN...WFO...UNR...BOU...CYS...BYZ...RIW...

LAT...LON   45120677 45870625 46350519 46240463 45910404 45420334
            43810303 41380274 41160294 41020337 40960398 41000462
            41100501 42550534 43810610 44410673 45120677

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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.