Alert archive / Illinois / Severe Thunderstorm Watches in 2021
Severe Thunderstorm Watch 422
The Storm Prediction Center issued this severe thunderstorm watch at Aug 11, 2021, 8:52 AM CDT for 12 counties in Illinois; 4 counties in Indiana. It was in force for 4 hr.
The area
The outline as issued.
The facts
- Issued
- Aug 11, 2021, 8:52 AM CDT (13:52 UTC)
- Expired
- Aug 11, 2021, 12:48 PM CDT
- In force
- 4 hr
- Issued by
- the Storm Prediction Center
- Largest hail expected
- 2 in
- Strongest gust expected
- 65 kt
Counties 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, Ogle County, IL, Will County, IL, Jasper County, IN, Lake County, IN, Newton County, IN, Porter County, IN
The radar
The Chicago radar (KLOT), 13 km from the middle of the area, began a volume at 13:54 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.
What SPC expected
The probabilities SPC published with the watch, for the watch area over its life.
- Two or more tornadoes
- 20%
- One or more EF2 to EF5 tornadoes
- 5%
- Ten or more severe wind reports
- 60%
- One or more gusts of 65 knots or more
- 60%
- Ten or more severe hail reports
- 40%
- One or more hailstones of 2 inches or more
- 30%
- Six or more combined severe hail and wind reports
- 90%
Warnings issued inside it
8 tornado, severe thunderstorm and flash flood warnings for its counties while it was in force.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Aug 11, 9:05 AM CDT | Severe Thunderstorm Warning | LOT | Grundy, Kane, Kendall | 40 min |
| Aug 11, 9:25 AM CDT | Severe Thunderstorm Warning | LOT | DuPage, Kane, Will | 65 min |
| Aug 11, 9:35 AM CDT | Severe Thunderstorm Warning | LOT | Boone, DeKalb, Ogle, Winnebago | 42 min |
| Aug 11, 9:40 AM CDT | Severe Thunderstorm Warning | LOT | Cook, Will | 65 min |
| Aug 11, 9:46 AM CDT | Severe Thunderstorm Warning | LOT | Kane, Lake, McHenry | 59 min |
| Aug 11, 9:50 AM CDT | Severe Thunderstorm Warning | LOT | Grundy, Kane, Kendall | 40 min |
| Aug 11, 10:12 AM CDT | Severe Thunderstorm Warning | LOT | Lake, Porter | 63 min |
| Aug 11, 11:15 AM CDT | Severe Thunderstorm Warning | LOT | Jasper, Lake, Newton, Porter | 45 min |
Discussions around it
SPC mesoscale discussions over the same counties from four hours before the watch to its end.
The nearest sounding before it
Lincoln, IL (KILX), launched 12:00Z, 1.9 hours before the alert went out and 162 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
- 1930 J/kg
- Mixed-layer CIN
- -174 J/kg
- Mixed-layer LCL
- 1253 m above ground
- 0-6 km bulk shear
- 28 kt
- 0-1 km storm-relative helicity
- 228 m²/s²
- 0-3 km storm-relative helicity
- 185 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 4.4
The text
As transmitted.
646
WWUS20 KWNS 111348
SEL2
SPC WW 111348
ILZ000-INZ000-LMZ000-111800-
URGENT - IMMEDIATE BROADCAST REQUESTED
Severe Thunderstorm Watch Number 422
NWS Storm Prediction Center Norman OK
850 AM CDT Wed Aug 11 2021
The NWS Storm Prediction Center has issued a
* Severe Thunderstorm Watch for portions of
Northern Illinois
Extreme northwestern Indiana
Lake Michigan
* Effective this Wednesday morning and afternoon from 850 AM
until 100 PM CDT.
* Primary threats include...
Scattered damaging winds and isolated significant gusts to 75
mph likely
Scattered large hail and isolated very large hail events to 2
inches in diameter possible
A tornado or two possible
SUMMARY...An intensifying storm cluster in northwest Illinois has
shown signs of increasing organization this morning. The downstream
environment favors maintenance of the storms, and the potential for
swaths of damaging winds with isolated significant gusts to 75 mph,
occasional large hail, and possibly a tornado or two with embedded
circulations.
The severe thunderstorm watch area is approximately along and 35
statute miles north and south of a line from 25 miles west northwest
of Marseilles IL to 5 miles east of Valparaiso IN. For a complete
depiction of the watch see the associated watch outline update
(WOUS64 KWNS WOU2).
PRECAUTIONARY/PREPAREDNESS ACTIONS...
REMEMBER...A Severe Thunderstorm Watch means conditions are
favorable for severe thunderstorms in and close to the watch area.
Persons in these areas should be on the lookout for threatening
weather conditions and listen for later statements and possible
warnings. Severe thunderstorms can and occasionally do produce
tornadoes.
&&
AVIATION...A few severe thunderstorms with hail surface and aloft to
2 inches. Extreme turbulence and surface wind gusts to 65 knots. A
few cumulonimbi with maximum tops to 550. Mean storm motion vector
27045.
...ThompsonSPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Aug 10, 7:38 PM CDT | Severe Thunderstorm Watch | SPC | Kankakee | 5 hr |
| Aug 11, 1:20 AM CDT | Mesoscale Discussion | SPC | Bureau, Ford, Fulton, Grundy, Henry, Iroquois and 15 more | 115 min |
| Aug 11, 4:49 AM CDT | Mesoscale Discussion | SPC | Adams | 86 min |
| Aug 11, 6:26 AM CDT | Mesoscale Discussion | SPC | Adams, Brown, Cass, Fulton, Hancock, Henderson and 12 more | 2 hr |
| Aug 11, 8:34 AM CDT | Mesoscale Discussion | SPC | Bureau, Cook, DeKalb, DuPage, Ford, Grundy and 13 more | 86 min |
| Aug 11, 9:52 AM CDT | Mesoscale Discussion | SPC | Cook, DuPage, Grundy, Kane, Kankakee, Lake and 1 more | 83 min |
| Aug 11, 3:56 PM CDT | Mesoscale Discussion | SPC | Carroll, Hancock, Henderson, Henry, Jo Daviess, Mercer and 2 more | 3 hr |
| Aug 11, 5:40 PM CDT | Mesoscale Discussion | SPC | Carroll, Jo Daviess, Stephenson | 95 min |
| Aug 10, 7:12 PM CDT | Mesoscale Discussion | SPC | DeKalb, Elkhart, Kosciusko, LaGrange, Lake, LaPorte and 6 more | 63 min |
| Aug 10, 9:57 PM CDT | Mesoscale Discussion | SPC | Allen, DeKalb, Elkhart, Fulton, Huntington, Jasper and 8 more | 93 min |
| Aug 11, 10:50 AM CDT | Severe Thunderstorm Watch | SPC | Adams, Allen, Blackford, Cass, DeKalb, Elkhart and 18 more | 4 hr |
| Aug 11, 12:26 PM CDT | Mesoscale Discussion | SPC | Adams, Allen, DeKalb, Elkhart, Huntington, Kosciusko and 7 more | 94 min |
Nearby
Severe Thunderstorm Watches in Illinois, 2021Severe Thunderstorm Watches in Indiana, 2021Every severe thunderstorm watch 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.