An outflow boundary from previous convection triggered several clusters of strong thunderstorms across north-central Illinois during the late afternoon and evening of August 8th. 25 in the town of Danvers.
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Deaths and injuries are direct and indirect together, as the offices filed them. Damage is property and crops in the dollars of the year, not adjusted for inflation, and only where an office estimated it.
The episode as NWS ILX wrote it up.
An outflow boundary from previous convection triggered several clusters of strong thunderstorms across north-central Illinois during the late afternoon and evening of August 8th. Thanks to copious atmospheric moisture and the slow-movement of the storms, very heavy rainfall of 6 to 8 inches was observed across northwestern McLean County...including 8.25 in the town of Danvers. As a result, significant flash flooding occurred from Danvers to Carlock just northwest of Bloomington-Normal. In addition, a Mesoscale Convective Vortex (MCV) generated by convection earlier in the day across the Plains tracked into west-central Illinois, triggering scattered severe thunderstorms along and west of the Illinois River. Isolated wind damage occurred with these cells.
The local office's own words before the first event, as it sent them, from the Iowa Environmental Mesonet's archive of National Weather Service text.
NWS Lincoln, IL wrote its last area forecast discussion before the first event at Aug 8, 2:20 PM CST, 40 minutes earlier.
Area Forecast Discussion National Weather Service Lincoln IL 320 PM CDT Sun Aug 8 2021 .SYNOPSIS... Issued at 212 PM CDT Sun Aug 8 2021 Showers and thunderstorms will push across central Illinois through Monday as an upper- level disturbance pivots across the region. Drier conditions return Tuesday and Wednesday along with uncomfortable heat index values (98F - 108F). This will likely summon a Heat Advisory by mid-week. The stretch of hot and humid conditions will be relatively short-lived, however, as a surface cold front drops through the region later in the week, brining additional showers and thunderstorms. Very pleasant weather arrives by next weekend. && .SHORT TERM...(This evening through Monday) ISSUED AT 212 PM CDT Sun Aug 8 2021 Latest satellite trends continue to show an eroding capping inversion across central and west central Illinois ahead of a decaying MCV. A very moist and unstable boundary layer has evolved this afternoon over the region, characterized by mid 70s surface dewpoints and MLCAPE values around 2500 J/kg. The expectations for today's severe weather potential remain little changed after reviewing the latest suite of hi-res models. Stronger thunderstorms are still anticipated to arrive just prior to 21z with the arrival of our forcing mechanism; a mid- level shortwave trough pivoting across the Mid- Mississippi Valley. The main limiting factor for today's convective setup is the lack of sufficient deep-layer shear, with the latest RAP analysis revealing below 20 kts. This value should increase later this afternoon as the decaying MCV and mid- level shortwave trough approach the Illinois River Valley, but we are still looking at less than 30 kts of deep layer shear through this convective event. Updrafts should struggle to stay organized in such an environment. Nevertheless, we continue to monitor a few strong-to-severe multicell clusters that have already developed west of the Illinois River. Given the orientation of the shear vectors, this activity is expected to grow upscale into a MCS/squall line as it moves into our outlook area over the next few hours. The line should quickly become outflow dominant as the UDCZ struggles to stay balanced, which is once again a result of insufficient shear. Locations west of I-55 will stand the best chance for severe weather through this evening, with damaging downdraft winds up to 60 mph being the prime hazard. Localized heavy rainfall will also be possible with these storms. While the global ensembles (GEFS and ENS) do not suggest upper echelon PWATs or QPF with this convective system, some of the hi- res models do offer localized pockets of 1-3 inches mainly west of I-55. Shower and thunderstorm activity should begin to wane after sunset. Despite a healthy LLJ veering into the region around that time, we think ongoing convection across the Ozarks will rob us of some of the rich theta-e air needed to sustain convection. CAMs have been supporting this over the past few runs. The forecast for Monday is a messy one and may offer another opportunity for severe convection. The threat at this point is conditional, and depends on how much convective debris is leftover from tonight. If the cap can erode quickly enough on Monday, there will likely be a pre- existing boundary lurking. With the mid- level shortwave still centered over northern Illinois, there may be enough instability and forcing to ignite thunderstorms across central Illinois. && .LONG TERM...(Tuesday through Sunday) ISSUED AT 212 PM CDT Sun Aug 8 2021 The bigger concern heading into this week is the heat. Mid- level heights will begin to increase behind the departing shortwave trough late Monday evening in response to low- level warm air advection. By Tuesday afternoon, surface temperatures will surge near 90 degrees with dewpoints in the mid 70s. This combination of heat and humidity will yield triple digit heat index values and will likely summon a Heat Advisory. By the middle-to-late stages of next week, ensemble guidance is in excellent agreement of strong shortwave energy digging into the Upper Great Lakes region. This will help drive a surface cold front through the Midwest. Shower and thunderstorm activity is anticipated ahead of this front, especially on Wednesday and Thursday, but it's not clear yet on whether this activity may be severe. Cooler and drier weather will return in the wake of frontal passage, with the NBM offering afternoon temperatures in the lower 80s with lows near 60 throughout next weekend. && .AVIATION...(For the 18Z TAFS through 18Z Monday afternoon) Issued at 1241 PM CDT Sun Aug 8 2021 Diurnal cumulus with bases around 4 kft will begin to bubble up across the regional terminals this afternoon beneath mostly clear skies. Vicinity thunderstorms are likely to develop between 20z- 02z, and as reflected in the TEMPO groups, a few of the regional terminals may experience a direct hit from this convective activity. Otherwise, scattered showers with VFR cloud bases should persist through 09z. Beyond that time, low stratus will begin to build down into the MVFR range. Additional shower and thunderstorm activity is possible beyond 12z Monday, but confidence remains low at this time. Surface winds will blow from south to north through the TAF period. Breezy conditions are anticipated through today with sustained speeds 10-15 kts with occasional gusts to 25 kts, diminishing after sunset. .ILX WATCHES/WARNINGS/ADVISORIES... && .ILX WATCHES/WARNINGS/ADVISORIES... NONE. && SYNOPSIS...MJA SHORT TERM...MJA LONG TERM...MJA AVIATION...MJA
Hazardous Weather Outlook National Weather Service Lincoln IL 346 PM CDT Sun Aug 8 2021 ILZ027>031-036>038-040>043-047>053-091100- Cass-Christian-De Witt-Fulton-Knox-Logan-Macon-Marshall-Mason- McLean-Menard-Morgan-Peoria-Sangamon-Schuyler-Scott-Stark- Tazewell-Woodford- 346 PM CDT Sun Aug 8 2021 This Hazardous Weather Outlook is for portions of central Illinois and west central Illinois. .DAY ONE...TONIGHT... Thunderstorms are expected to increase in western Illinois this afternoon and evening, and move into eastern Illinois later in the evening. A few of the storms may be strong to locally severe, especially late afternoon and early evening, with damaging winds the main concern, but isolated hail possible as well. Ahead of the storms, the heat index is expected to reach around 100 degrees. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY... Periodic thunderstorms are expected from Monday through mid week. A few of the storms Monday and Tuesday may be on the strong side, though the risk on Wednesday is more uncertain. Heat index values of 95 to 100 degrees are expected Monday and around 105 Tuesday, followed by near 100 degrees Wednesday and Thursday. .SPOTTER INFORMATION STATEMENT... Spotter activation may be required this afternoon and evening, with the most likely time frame from 4 pm to 9 pm. ILZ044>046-054>057-061-062-066-067-071-091100- Champaign-Clay-Coles-Cumberland-Douglas-Edgar-Effingham-Jasper- Moultrie-Piatt-Shelby-Vermilion- 346 PM CDT Sun Aug 8 2021 This Hazardous Weather Outlook is for portions of central Illinois, east central Illinois and southeast Illinois. .DAY ONE...TONIGHT... Thunderstorms are expected this evening. A few of the storms may be strong to locally severe, with damaging winds the main concern, but isolated hail is possible as well. Ahead of the storms, the heat index is expected to range from 95 to 100 degrees this afternoon. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY... Periodic thunderstorms are expected from Monday through mid week. A few of the storms Monday and Tuesday may be on the strong side, though the risk on Wednesday is more uncertain. Heat index values near 100 to 105 degrees are expected Tuesday, and will be close to 100 degrees Wednesday and Thursday. .SPOTTER INFORMATION STATEMENT... Spotter activation may be needed this evening, with the most likely time frame from 5 pm to 10 pm. ILZ063-068-072-073-091100- Clark-Crawford-Lawrence-Richland- 346 PM CDT Sun Aug 8 2021 This Hazardous Weather Outlook is for portions of east central Illinois and southeast Illinois. .DAY ONE...TONIGHT... Thunderstorms are expected to move into the area tonight. At the current time, it appears the higher severe weather threat will be further west. .DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY... Periodic thunderstorms are expected from Monday through mid week. A few of the storms Monday and Tuesday may be on the strong side, though the risk on Wednesday is more uncertain. Heat index values of 100 to 105 degrees are expected Tuesday, and will be close to 100 degrees Wednesday and Thursday. .SPOTTER INFORMATION STATEMENT... Spotter activation is not anticipated through tonight. 37
Special Weather Statement
National Weather Service Lincoln IL
221 PM CDT Sun Aug 8 2021
ILZ031-038-081945-
Woodford-McLean-
221 PM CDT Sun Aug 8 2021
...Strong thunderstorms will impact portions of southeastern Woodford
and western McLean Counties through 245 PM CDT...
At 220 PM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near El Paso to McLean. Movement was northeast
at 20 mph.
HAZARD...Winds in excess of 40 mph and half inch hail.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects. Minor damage to outdoor objects is
possible.
Strong thunderstorms will be near...
Hudson around 225 PM CDT.
El Paso around 230 PM CDT.
Other locations in the path of these storms include Bloomington and
Gridley.
This includes the following highways...
Interstate 39 between mile markers 0 and 13.
Interstate 55 between mile markers 143 and 169.
Interstate 74 between mile markers 123 and 136.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
If outdoors, consider seeking shelter inside a building.
These storms may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.
&&
LAT...LON 4075 8893 4075 8883 4029 8903 4029 8923
4076 8901 4076 8893
TIME...MOT...LOC 1920Z 226DEG 16KT 4069 8901 4031 8918
MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPH
SBSpecial Weather Statement
National Weather Service Lincoln IL
330 PM CDT Sun Aug 8 2021
ILZ038-082100-
McLean-
330 PM CDT Sun Aug 8 2021
...A strong thunderstorm will impact portions of southwestern McLean
County through 400 PM CDT...
At 330 PM CDT, Doppler radar was tracking a strong thunderstorm near
Bloomington, moving northeast at 25 mph.
HAZARD...Winds in excess of 40 mph and half inch hail.
SOURCE...Radar indicated.
IMPACT...Gusty winds could knock down tree limbs and blow around
unsecured objects. Minor damage to outdoor objects is
possible.
This strong thunderstorm will be near...
Bloomington around 335 PM CDT.
Downs around 340 PM CDT.
Normal around 350 PM CDT.
Other locations in the path of this storm include Hudson.
This includes the following highways...
Interstate 39 between mile markers 0 and 5.
Interstate 55 between mile markers 150 and 169.
Interstate 74 between mile markers 126 and 140.
PRECAUTIONARY/PREPAREDNESS ACTIONS...
If outdoors, consider seeking shelter inside a building.
Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.
This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.
&&
LAT...LON 4034 8907 4041 8916 4061 8900 4047 8877
TIME...MOT...LOC 2030Z 219DEG 21KT 4043 8904
MAX HAIL SIZE...0.50 IN
MAX WIND GUST...40 MPH
SBThe Lincoln radar (KILX), 69 km away, was scanning. The volume nearest the first event began at 21:02 UTC.
4 thunderstorm wind, 1 flash flood. Local standard time, as filed.
A large tree limb was blown down along Highway 123 just south of Prentice.
A large pecan tree branch was blown onto a roof, causing damage to the roof.
Six to eight inches of rain fell in a short amount of time, resulting in significant flash flooding. The Carlock-Danvers Blacktop was under water. Numerous roads in both Carlock and Danvers were flooded, with vehicles reported flooded on S West Street in Carlock. In addition, several vehicles were flooded along Highway 150 between Carlock and Bloomington.
Several 4 to 5-inch diameter tree branches were blown down.
A small tree was snapped.
After NWS Directive 10-1605, the rules offices file Storm Data by.
Every storm in Illinois in 2021Every state in 2021This day in weather historyWarnings in Illinois, August 2021
NOAA National Centers for Environmental Information, Storm Events Database, compiled from the National Weather Service’s monthly Storm Data. Complete through 2026-06; NCEI adds each month about 75 days after it ends and revises past years a few times a year. Rebuilt 2026-09-24. Each event’s entry at NCEI: 978262, 978264, 978266, 978265, 978263.