Mesoscale Discussion 1450

Issued

Aug 9, 2021, 12:18 PM CDT (17:18 UTC)

Expired

Aug 9, 2021, 2:45 PM CDT

In force

2 hr

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

60%

Counties Boone County, IL, Carroll County, IL, Cook County, IL, DeKalb County, IL, DuPage County, IL, Grundy County, IL, Kane County, IL, Kankakee County, IL, Kendall County, IL, Lake County, IL, LaSalle County, IL, Lee County, IL, McHenry County, IL, Ogle County, IL, Will County, IL, Winnebago County, IL, Lake County, IN, Porter County, IN

The Chicago radar (KLOT), 61 km from the middle of the area, began a volume at 17:18 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 417Aug 9, 2:40 PM CDT

303 
ACUS11 KWNS 091719
SWOMCD
SPC MCD 091718 
INZ000-ILZ000-091945-

Mesoscale Discussion 1450
NWS Storm Prediction Center Norman OK
1218 PM CDT Mon Aug 09 2021

Areas affected...Portions of northern IL and extreme northwestern IN

Concerning...Severe potential...Watch possible 

Valid 091718Z - 091945Z

Probability of Watch Issuance...60 percent

SUMMARY...A threat for large hail, damaging winds, and a couple of
tornadoes should increase this afternoon. Watch issuance is
possible.

DISCUSSION...A well defined MCV is located over southern WI early
this afternoon. Convection that has occurred through the morning
across parts of northern IL on the southern flank of this feature is
in the process of moving over the northern suburbs of Chicago and
southern Lake Michigan. In the wake of this activity, modest diurnal
heating of a moist low-level airmass, but muted by persistent
low-level cloud cover, should support increasing instability by late
afternoon. The development of 1500-3000 J/kg of MLCAPE still appears
probable by 22-23Z along/south of the outflow left by the morning
storms across northern IL. Recent VWPs from KLOT show southerly
low-level winds veering and strengthening with height through mid
levels, with around 30-40 kt of effective bulk shear present per
mesoanalysis estimates. This should be enough to support a mix of
supercells and multicells this afternoon as renewed convection
develops across northern IL, with continued low-level warm advection
atop the outflow boundary and ascent associated with the MCV
providing lift. Large hail may occur with any supercell, and the
potential for a couple tornadoes is also apparent with effective SRH
around 150-250 m2/s2 across northern IL this afternoon and early
evening owing to the presence of a 25-30 kt southwesterly low-level
jet. A small cluster may eventually develop as storms move slowly
south-southeastward off the boundary. If this occurs, then
severe/damaging winds may eventually become this primary threat. The
potential for surface-based storms should increase over the next
couple of hours as surface temperatures warm into the low 80s, and
watch issuance is being considered in this time frame.

..Gleason/Thompson.. 08/09/2021

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

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

LAT...LON   41828994 42198997 42468933 42458773 41608695 41058708
            41018866 41418949 41828994

Mesoscale Discussions in Illinois, 2021Mesoscale Discussions in Indiana, 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.