Mesoscale Discussion 989

Issued

Jun 17, 2021, 2:57 PM CDT (19:57 UTC)

Expired

Jun 17, 2021, 4:00 PM CDT

In force

63 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch likely

Chance of a watch

80%

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• Severe Thunderstorm Watch 276Jun 17, 3:50 PM CDT

677 
ACUS11 KWNS 171957
SWOMCD
SPC MCD 171957 
WIZ000-ILZ000-IAZ000-MNZ000-172100-

Mesoscale Discussion 0989
NWS Storm Prediction Center Norman OK
0257 PM CDT Thu Jun 17 2021

Areas affected...portions of far southeast Minnesota...far northeast
Iowa...western Wisconsin

Concerning...Severe potential...Watch likely 

Valid 171957Z - 172100Z

Probability of Watch Issuance...80 percent

SUMMARY...Isolated thunderstorm development is underway, and a
gradual increase in coverage and intensity is expected through the
afternoon into early evening. Large hail and damaging winds are the
primary threats. Timing of the more substantial severe threat is
uncertain, with current storm trends being monitored for the need of
a WW issuance.

DISCUSSION...Latest visible satellite and MRMS mosaic radar data
show isolated thunderstorm development ongoing across northern IA,
southern MN, and far western WI. Modified 18Z RAP forecast soundings
in proximity to the region of convective initiation show a
near-saturation zone close to 700 mb, where the developing storms
are likely rooted. As such, these elevated storms are expected to
progress eastward over the next couple of hours in an 8+ C/km
deep-layer lapse rate environment, contributing to 2000 J/kg MUCAPE
above the convective inhibition layer. Though large-scale ridging is
in place, embedded 700 mb perturbations cresting the ridge are
likely contributing to enough deep-layer ascent for convective
initiation and possibly sustenance. 

While diurnal mixing has reached up to 700 mb, low-level
lee-troughing has resulted in the preservation of 25+ kt 850 mb
southwesterly flow, overspread by 40-60 kts of westerly 500 mb winds
pivoting around the upper ridge. As such, up to 50 kts of effective
bulk shear is overlapping the 2000 J/kg MUCAPE environment based on
19Z mesoanalysis, with multicells and supercell structures likely
the dominant convective mode initially. Large hail and damaging
gusts will be the main threats, with 2+ inch diameter stones and a
few 65+ kt wind gusts possible with the stronger, more organized
supercells given the very steep deep-layer lapse rates. A brief
tornado or two is possible before storms become more outflow
dominant given relatively weak low-level shear and evaporative
cooling within a dry sub-cloud layer leading to outflow dominant
storm modes. With time, cold pool mergers should contribute to MCS
development, with damaging winds becoming the primary threat. While
timing of supercell and MCS development remains uncertain, the
gradually increasing risk suggests that a WW issuance will be needed
soon.

..Squitieri/Grams.. 06/17/2021

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

ATTN...WFO...GRB...MKX...DVN...ARX...MPX...DMX...

LAT...LON   43079481 43649387 44229266 44599186 44739092 44689031
            44348975 43708946 43028961 42609011 42349092 42419213
            42539401 43079481

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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.