Alert archive / Illinois / Mesoscale Discussions in 2021
Mesoscale Discussion 1341
The Storm Prediction Center issued this mesoscale discussion at Jul 24, 2021, 3:00 PM CDT for 23 counties in Illinois; 5 counties in Iowa. It was in force for 2 hr.
The area
The outline as issued.
The facts
- Issued
- Jul 24, 2021, 3:00 PM CDT (20:00 UTC)
- Expired
- Jul 24, 2021, 5:00 PM CDT
- In force
- 2 hr
- Issued by
- the Storm Prediction Center
- Concerning
- Severe potential...watch possible
- Chance of a watch
- 60%
All 28 counties
Clinton County, IA, Des Moines County, IA, Louisa County, IA, Muscatine County, IA, Scott County, IA, Boone County, IL, Bureau County, IL, Carroll County, IL, Cook County, IL, DeKalb County, IL, DuPage County, IL, Henderson County, IL, Henry County, IL, Kane County, IL, Kendall County, IL, Knox County, IL, Lake County, IL, LaSalle County, IL, Lee County, IL, McHenry County, IL, Mercer County, IL, Ogle County, IL, Rock Island County, IL, Stark County, IL, Warren County, IL, Whiteside County, IL, Will County, IL, Winnebago County, IL
The radar
The Quad Cities radar (KDVN), 88 km from the middle of the area, began a volume at 20:02 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.
The watches that followed
Watches SPC issued over any of the same counties within four hours of this discussion.
The nearest sounding before it
Davenport, (KDVN), launched 12:00Z, 8.0 hours before the alert went out and 81 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.
- Mixed-layer CAPE
- 267 J/kg
- Mixed-layer CIN
- -45 J/kg
- Mixed-layer LCL
- 1158 m above ground
- 0-6 km bulk shear
- 32 kt
- 0-1 km storm-relative helicity
- 108 m²/s²
- 0-3 km storm-relative helicity
- 64 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.7
The text
As transmitted.
187
ACUS11 KWNS 242001
SWOMCD
SPC MCD 242000
INZ000-ILZ000-IAZ000-242200-
Mesoscale Discussion 1341
NWS Storm Prediction Center Norman OK
0300 PM CDT Sat Jul 24 2021
Areas affected...Parts of southern Lake Michigan...northern Illinois
and adjacent northwestern Indiana...into parts of southeastern Iowa
Concerning...Severe potential...Watch possible
Valid 242000Z - 242200Z
Probability of Watch Issuance...60 percent
SUMMARY...Widely scattered to scattered strong thunderstorm
development appears increasingly possible through 5-7 PM CDT,
particularly around the Greater Chicago metropolitan area, posing a
risk for a few strong downbursts.
DISCUSSION...Deepening convective development is ongoing along a
zone of boundary-layer confluence near/south of a slowly southward
advancing convective outflow boundary, with thunderstorms beginning
to initiate near/northwest of the Greater Chicago metropolitan area.
This coincides with a low-level thermal ridge axis, along which
strong daytime heating has contributed to steepening near surface
lapse rates and mixed-layer CAPE on the order of 2000+ J/kg. This is
also near the southwestern periphery of larger-scale mid-level
troughing shifting across and to the east of the Great Lakes
vicinity, where 30-40 kt northwesterly flow around 500 mb is
contributing to strong deep-layer shear.
Although forcing for ascent, particularly in mid/upper levels, may
only allow for widely scattered thunderstorm development to the
west-southwest of southern Lake Michigan late this afternoon, much
of the Greater Chicago metro may be impacted. This could include an
upscale growing line or clusters of storms with the potential to
produce at least a few strong downbursts.
..Kerr/Hart.. 07/24/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...IWX...LOT...ILX...DVN...
LAT...LON 41619143 42308965 42558661 41638688 41228927 40569141
41619143SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Jul 24, 3:32 PM CDT | Severe Thunderstorm Watch | SPC | Bureau, Cook, DeKalb, DuPage, Ford, Grundy and 17 more | 6 hr |
| Jul 24, 5:21 PM CDT | Mesoscale Discussion | SPC | Bureau, Cook, DuPage, Ford, Grundy, Iroquois and 12 more | 114 min |
Nearby
Mesoscale Discussions in Illinois, 2021Mesoscale Discussions in Iowa, 2021Every mesoscale discussion in 2021
Sources
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.