Alert archive / Illinois / Mesoscale Discussions in 2021
Mesoscale Discussion 1492
The Storm Prediction Center issued this mesoscale discussion at Aug 11, 2021, 7:35 PM CDT for 5 states. It was in force for 85 min.
The area
The outline as issued.
The facts
- Issued
- Aug 11, 2021, 7:35 PM CDT (00:35 UTC)
- Expired
- Aug 11, 2021, 9:00 PM CDT
- In force
- 85 min
- Issued by
- the Storm Prediction Center
- Concerning
- Severe potential...watch possible
- Chance of a watch
- 40%
All 38 counties
Clinton County, IA, Jackson County, IA, Scott County, IA, Boone County, IL, Bureau County, IL, Carroll County, IL, Cook County, IL, DeKalb County, IL, DuPage County, IL, Grundy County, IL, Henry County, IL, Kane County, IL, Kendall County, IL, Knox County, IL, Lake County, IL, LaSalle County, IL, Lee County, IL, Marshall County, IL, McHenry County, IL, Mercer County, IL, Ogle County, IL, Putnam County, IL, Rock Island County, IL, Stark County, IL, Stephenson County, IL, Warren County, IL, Whiteside County, IL, Will County, IL, Winnebago County, IL, Lake County, IN, LaPorte County, IN, Porter County, IN, Saint Joseph County, IN, Berrien County, MI, Cass County, MI, Van Buren County, MI, Kenosha County, WI, Walworth County, WI
The radar
The Chicago radar (KLOT), 96 km from the middle of the area, began a volume at 00:37 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 00:00Z, 0.6 hours before the alert went out and 114 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
- 3462 J/kg
- Mixed-layer CIN
- -49 J/kg
- Mixed-layer LCL
- 1034 m above ground
- 0-6 km bulk shear
- 28 kt
- 0-1 km storm-relative helicity
- 103 m²/s²
- 0-3 km storm-relative helicity
- 233 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.0
The text
As transmitted.
478
ACUS11 KWNS 120036
SWOMCD
SPC MCD 120035
MIZ000-INZ000-ILZ000-WIZ000-IAZ000-120200-
Mesoscale Discussion 1492
NWS Storm Prediction Center Norman OK
0735 PM CDT Wed Aug 11 2021
Areas affected...Eastern IA...Northern IL...Far Southern
WI...Northwest IN...Southwest Lower MI
Concerning...Severe potential...Watch possible
Valid 120035Z - 120200Z
Probability of Watch Issuance...40 percent
SUMMARY...Storm coverage is expected to increase this evening, with
at least an isolated severe threat into late tonight. New watch
issuance is possible by 01-02Z.
DISCUSSION...Earlier supercells over eastern IA, southwest WI, and
northwest IL have weakened over the last hour, likely due to
generally weak large-scale ascent and modest capping noted in 00Z
DVN sounding. However, regional soundings and VWPs also depict a
rather strong southwesterly low-level jet across the area, which is
generally forecast to intensify further tonight as it veers to a
more westerly direction. Storm coverage is expected to increase with
time later this evening within this warm advection regime, as
airmass recovery continues across northern IL/IN and a surface
boundary moves southeastward across the region.
Even if storms remain somewhat elevated tonight, steep midlevel
lapse rates and rich low-level moisture will still support large
MUCAPE, with sufficient deep-layer shear for some organized storm
structures. A mix of cells and clusters will be possible, posing a
threat of at least isolated hail and damaging wind. A conditional
tornado threat will also be present with any storm that can become
rooted in the boundary layer, which would be most likely for a few
hours this evening before MLCINH becomes increasingly prohibitive
late tonight.
While some uncertainty remains regarding the convective evolution
and coverage of the severe threat later tonight, watch issuance is
possible by 01-02Z, in advance of the expected increase in storm
coverage.
..Dean/Goss.. 08/12/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...IWX...GRR...LOT...ILX...MKX...DVN...
LAT...LON 42149072 42459001 42748844 42788754 42408622 41758578
40968857 40739068 41069116 41759107 42149072SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Aug 11, 8:22 PM CDT | Severe Thunderstorm Watch | SPC | Bureau, Cook, DeKalb, DuPage, Ford, Grundy and 11 more | 8 hr |
| Aug 11, 11:06 PM CDT | Mesoscale Discussion | SPC | Boone, Bureau, Carroll, Cook, DeKalb, DuPage and 15 more | 84 min |
| Aug 12, 3:16 AM CDT | Mesoscale Discussion | SPC | Bureau, Ford, Fulton, Grundy, Iroquois, Kankakee and 16 more | 119 min |
| Aug 12, 2:22 PM CDT | Mesoscale Discussion | SPC | Bond, Cass, Champaign, Christian, Clark, Clay and 26 more | 83 min |
| Aug 12, 5:19 PM CDT | Severe Thunderstorm Watch | SPC | Calhoun, Clinton, Greene, Jersey, Macoupin, Madison and 7 more | 5 hr |
| Aug 12, 6:06 PM CDT | Mesoscale Discussion | SPC | Calhoun, Jersey, Madison, Monroe, Saint Clair | 84 min |
| Aug 11, 9:17 PM CDT | Mesoscale Discussion | SPC | Genesee, Lapeer, Livingston, Macomb, Oakland, Saint Clair and 3 more | 88 min |
| Aug 12, 5:38 AM CDT | Mesoscale Discussion | SPC | Hillsdale, Jackson, Lenawee, Monroe, Oakland, Washtenaw and 1 more | 97 min |
Nearby
Mesoscale Discussions in Illinois, 2021Mesoscale Discussions in Indiana, 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.