Mesoscale Discussion 1771

Issued

Sep 20, 2021, 5:52 PM CDT (22:52 UTC)

Expired

Sep 20, 2021, 7:45 PM CDT

In force

113 min

Issued by

the Storm Prediction Center

Concerning

Severe thunderstorm watch 507...509

The La Crosse radar (KARX), 65 km from the middle of the area, began a volume at 22:57 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.

458 
ACUS11 KWNS 202252
SWOMCD
SPC MCD 202252 
WIZ000-MIZ000-IAZ000-MNZ000-MOZ000-210045-

Mesoscale Discussion 1771
NWS Storm Prediction Center Norman OK
0552 PM CDT Mon Sep 20 2021

Areas affected...Far southeast Minnesota...eastern Iowa...and
western to northern Wisconsin

Concerning...Severe Thunderstorm Watch 507...509...

Valid 202252Z - 210045Z

The severe weather threat for Severe Thunderstorm Watch 507, 509
continues.

SUMMARY...The threat for severe wind, and perhaps a brief tornado,
continues across parts of the upper Mississippi River valley. The
greatest severe risk may reside across west-central and northern
Wisconsin over the next 2 hours.

DISCUSSION...GOES IR imagery over the past two hours has shown a
general cooling trend in convective cloud tops, indicative of
intensifying storms. This trend has manifest at the surface in the
form of a few severe wind reports from northeast IA within the past
hour. An upstream mid-level speed max is noted in recent RAP
analyses and water-vapor imagery, suggesting that deep-layer shear
may improve over the next few hours and aid in further strengthening
prior to the onset of diurnal cooling near sunset. As such, the
severe threat will likely continue for at least the next 1-2 hours. 


A broken line of somewhat organized cells is beginning to congeal
into a more cohesive line across far southeast MN, northeast IA, and
western WI, which may evolve into a more organized wind threat
during this 1-2 hour window. Nearly 40 knot 0-3 km bulk shear
vectors and over 250 m2/s2 0-1 km SRH noted in the KARX VWP suggest
the environment remains supportive of some tornado potential, though
the orientation of the bulk shear vectors with the evolving
line/cold front are not overly ideal for a robust QLCS tornado
threat. Further north, shear vectors remain slightly stronger and
are more favorably aligned with a balanced QLCS along the front. A
few circulations have been noted from the KDLH radar, suggesting a
heightened tornado potential for northern WI. This threat should
persist through the remainder of the evening hours before diurnal
cooling and stabilization occurs.

..Moore.. 09/20/2021

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

ATTN...WFO...MQT...GRB...MKX...DVN...DLH...ARX...MPX...DMX...
EAX...

LAT...LON   45469222 46359196 46729099 46418996 45528964 43929008
            42659070 41119180 40669239 40549303 40629388 41029397
            42209306 43599234 44469219 45469222

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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.