Mesoscale Discussion 1478

Issued

Aug 11, 2021, 11:48 AM CDT (16:48 UTC)

Expired

Aug 11, 2021, 1:45 PM CDT

In force

117 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...tornado watch likely

Chance of a watch

95%

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• Tornado Watch 424Aug 11, 12:15 PM CDT

738 
ACUS11 KWNS 111648
SWOMCD
SPC MCD 111648 
WIZ000-MIZ000-IAZ000-MNZ000-111845-

Mesoscale Discussion 1478
NWS Storm Prediction Center Norman OK
1148 AM CDT Wed Aug 11 2021

Areas affected...Much of central/eastern WI and parts of far
northeastern IA...southeastern MN...and the U.P. of MI

Concerning...Severe potential...Tornado Watch likely 

Valid 111648Z - 111845Z

Probability of Watch Issuance...95 percent

SUMMARY...The threat for several tornadoes, very large hail, and
significant severe wind gusts will likely increase this afternoon.
Tornado Watch issuance will be needed within an hour.

DISCUSSION...Recent visible satellite imagery shows convection is
beginning to develop along and ahead of a surface cold front across
northern and central WI. Pronounced large-scale forcing for ascent
associated with an upper trough across the Upper Midwest will likely
aid the development of at least scattered severe storms this
afternoon across much of WI. The airmass south of the front is
strongly unstable, with very steep mid-level lapse rates present
atop a rather moist low-level airmass. Any remaining convective
inhibition should dissipate within the next hour or two with
continued robust diurnal heating. MLCAPE will likely reach
3500-4500+ J/kg by peak afternoon heating across this region, and
enhanced mid-level west-southwesterly winds will support 50+ kt of
effective bulk shear across the warm sector. Current expectations
are for supercells to form initially, posing a threat for both very
large (2+ inch) hail and tornadoes. This tornado threat will be
aided by the continued presence of a 30-40 kt southwesterly
low-level jet, which should support 0-1 km SRH of 100-200 m2/s2 and
low-level updraft rotation. There is some signal in short-term
guidance that these supercells may try to grow upscale into an MCS
at some point this afternoon. If this were to occur, then numerous
severe/damaging wind gusts, some of which could be significant,
would be likely given the large buoyancy available. Regardless of
eventual storm evolution, the increasing threat for supercells this
afternoon will require Tornado Watch issuance within an hour.

..Gleason/Thompson.. 08/11/2021

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

ATTN...WFO...MQT...GRB...MKX...ARX...

LAT...LON   43238756 42528773 42528925 42589046 42789105 43099137
            43859143 45218958 45478856 45548744 45368680 44648711
            43868744 43238756

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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.