Mesoscale Discussion 1770

Issued

Sep 20, 2021, 3:55 PM CDT (20:55 UTC)

Expired

Sep 20, 2021, 5:30 PM CDT

In force

95 min

Issued by

the Storm Prediction Center

Concerning

Severe thunderstorm watch 507

The La Crosse radar (KARX), 71 km from the middle of the area, began a volume at 20:57 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.

• Tornado Watch 509Sep 20, 5:15 PM CDT

585 
ACUS11 KWNS 202056
SWOMCD
SPC MCD 202055 
WIZ000-MIZ000-IAZ000-MNZ000-202230-

Mesoscale Discussion 1770
NWS Storm Prediction Center Norman OK
0355 PM CDT Mon Sep 20 2021

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

Concerning...Severe Thunderstorm Watch 507...

Valid 202055Z - 202230Z

The severe weather threat for Severe Thunderstorm Watch 507
continues.

SUMMARY...The severe threat continues with Severe Thunderstorm Watch
507, and should persist through the afternoon. Damaging gusts and
occasional large hail are expected to remain the primary threats,
though a brief tornado cannot be completely ruled out. A new WW may
be needed to the northeast of Severe Thunderstorm Watch 507.

DISCUSSION...Temperatures have reached or exceeded 80F across much
of the warm sector ahead of the ongoing line of storms, south of a
warm front draped across northern WI, contributing up to 1500 J/kg
MLCAPE in spots. Storms along/ahead of the eastward advancing cold
front have obtained predominantly linear structures, posing a threat
for mainly damaging gusts, though a couple instances of marginally
severe hail remain possible, especially across northern portions of
Severe Thunderstorm Watch 507 given 45+ kts of effective bulk shear
in place.

While the tornado threat is expected to remain limited overall, over
200 m2/s2 of effective SRH has been noted in proximity to the warm
front by the 20Z mesoanalysis, with the 2024Z ARX VWP showing well
over 200 m2/s2 SRH within the 0-1 km layer alone. Despite the linear
storm mode, any segment which can become more northwest-southeast
aligned (oriented relatively more normal to the 45+ kt effective
bulk shear vector) would support some potential for a brief tornado
given adequate buoyancy.

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

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

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

LAT...LON   40629466 42649403 44469346 45559311 46119259 46489176
            46509074 46318999 45898955 44719025 42379077 40859166
            40619188 40629466

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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.