Mesoscale Discussion 1453

Issued

Aug 9, 2021, 5:53 PM CDT (22:53 UTC)

Expired

Aug 9, 2021, 7:00 PM CDT

In force

67 min

Issued by

the Storm Prediction Center

Concerning

Tornado watch 417

Counties Bureau County, IL, Cook County, IL, DeKalb County, IL, DuPage County, IL, Grundy County, IL, Kane County, IL, Kendall County, IL, Lake County, IL, LaSalle County, IL, Lee County, IL, Putnam County, IL, Will County, IL

The Chicago radar (KLOT), 55 km from the middle of the area, began a volume at 22:54 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.

039 
ACUS11 KWNS 092253
SWOMCD
SPC MCD 092253 
ILZ000-100000-

Mesoscale Discussion 1453
NWS Storm Prediction Center Norman OK
0553 PM CDT Mon Aug 09 2021

Areas affected...Parts of northern Illinois

Concerning...Tornado Watch 417...

Valid 092253Z - 100000Z

The severe weather threat for Tornado Watch 417 continues.

SUMMARY...Vigorous thunderstorm development, including discrete
supercell structures with the potential to produce relatively
short-lived tornadoes, will continue into the 7-9 PM time frame. 
This could impact western and southwestern portions of the Greater
Chicago metro area, before activity possibly consolidates into an
organizing, southward and southwestward moving cluster.

DISCUSSION...To the south of a weak surface low/low-level cyclonic
circulation centered near/east of Madison, WI, a corridor of
stronger low-level surface heating and confluent low-level flow will
remain the focus for vigorous thunderstorm development into the
evening hours.  Mid-level temperatures above this corridor are
relatively warm (including 500 mb temps around -6C), but
boundary-layer moisture content is seasonably high, with surface dew
points in the lower/mid 70s F.  

Beneath northwesterly mid-level flow (30-35+ kt around 500 mb) to
the west of the mid-level trough axis, 30+ kt southwesterly flow
around 850 mb along this corridor is contributing to sufficient
shear for supercells.  While the severe hail risk appears low, the
high boundary-layer moisture content coupled with modest, clockwise
curved low-level hodographs have been conducive to occasional brief
tornadic development, and this may remain the case another hour or
two.

Into the 01-03Z time frame, at least some model output suggests that
convection could begin to congeal into a small organizing cluster of
storms near or to the west/southwest of the Greater Chicago area. If
this occurs, stronger thunderstorm activity should tend propagate to
the south and southwest, with at least some increase in potential
for a strong downburst or two, while the risk for tornadoes
diminishes.

..Kerr.. 08/09/2021

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

ATTN...WFO...LOT...DVN...

LAT...LON   41598991 42018904 42268860 42498798 42338771 41878746
            41268784 41088895 41268969 41598991

Mesoscale Discussions in Illinois, 2021Every mesoscale discussion in 2021

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.