Mesoscale Discussion 1768

Issued

Sep 20, 2021, 12:38 PM CDT (17:38 UTC)

Expired

Sep 20, 2021, 2:45 PM CDT

In force

2 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch likely

Chance of a watch

80%

The Des Moines radar (KDMX), 128 km from the middle of the area, began a volume at 17:39 UTC, the scan nearest the moment this went out.

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• Severe Thunderstorm Watch 507Sep 20, 2:10 PM CDT
• Severe Thunderstorm Watch 508Sep 20, 3:25 PM CDT

881 
ACUS11 KWNS 201738
SWOMCD
SPC MCD 201738 
IAZ000-WIZ000-MNZ000-MOZ000-201945-

Mesoscale Discussion 1768
NWS Storm Prediction Center Norman OK
1238 PM CDT Mon Sep 20 2021

Areas affected...portions of eastern Minnesota...western
Wisconsin...central into eastern Iowa

Concerning...Severe potential...Watch likely 

Valid 201738Z - 201945Z

Probability of Watch Issuance...80 percent

SUMMARY...The severe threat is expected to gradually increase over
the next several hours across portions of the Mid to Upper
Mississippi Valley. Damaging gusts and large hail will be the
primary severe threats and a Severe Thunderstorm Watch will likely
be needed.

DISCUSSION...A surface cold front, coupled with the approaching
mid-level trough and associated deep-layer ascent, is gradually
progressing eastward towards an open warm sector with 75-80F surface
temperatures (given adequate diurnal heating) and 65-70F dewpoints.
Additional surface heating and convergence along the front will
support a gradual increase in convective development and
intensification through the afternoon. 

Given largely unidirectional tropospheric flow/vertical shear and
limited forcing for ascent in the warm sector, organized linear
storm modes along/immediately ahead of the cold front are expected.
However, brief instances of transient low-level rotation are
possible given modest surface-700 mb directional shear evident in
RAP forecast soundings. Despite modest (i.e. 6-6.5 C/km) low and
mid-level lapse rates, 500-1000 J/kg MLCAPE is expected given the
aforementioned surface temperatures/dewpoints. The stronger line
segments will be capable of producing damaging wind gusts and
perhaps large hail, especially for updrafts acquiring any
appreciable rotation. A Severe Thunderstorm Watch will likely be
needed within the next couple of hours to address the severe threat.

..Squitieri/Hart.. 09/20/2021

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

ATTN...WFO...DVN...ARX...MPX...DMX...EAX...

LAT...LON   41129180 40539267 40449321 40379389 40509475 40789481
            41219464 43499376 44899280 44799219 44589159 44319107
            43929073 43339057 42779063 42259088 41579147 41129180

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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.