Light to moderate snow will continue into Saturday over the Great Lakes, Central Appalachians, and Northeast. This weekend into next week, a series of atmospheric rivers will bring gusty winds, periods of heavy rain, and mountain snow to northern California and the Pacific Northwest. Colder temperatures are in store for the weekend from the Great Lakes to East Coast. Read More >
Overview
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Fast Facts:
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Preliminary Storm Reports
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| September 7th Preliminary Storm Reports |
PRELIMINARY LOCAL STORM REPORT...SUMMARY
NATIONAL WEATHER SERVICE CHICAGO IL
938 PM CDT THU SEP 9 2021
..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON...
..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE....
..REMARKS..
1232 PM TSTM WND DMG 2 NW SHIRLAND 42.46N 89.22W
09/07/2021 WINNEBAGO IL PUBLIC
PICTURE OF TREE DOWN SENT VIA SOCIAL MEDIA.
TIME IS ESTIMATED FROM RADAR.
1236 PM HAIL ROCKTON 42.45N 89.07W
09/07/2021 E0.75 INCH WINNEBAGO IL TRAINED SPOTTER
CORRECTS TIME OF PREVIOUS HAIL REPORT FROM
ROCKTON.
1255 PM HAIL 1 NNW TIMBERLANE 42.35N 88.87W
09/07/2021 E1.75 INCH BOONE IL TRAINED SPOTTER
TIME ESTIMATED FROM RADAR.
1259 PM HAIL HARVARD 42.42N 88.62W
09/07/2021 E1.50 INCH MCHENRY IL PUBLIC
TIME ESTIMATED BY RADAR.
0113 PM TSTM WND DMG WONDER LAKE 42.39N 88.35W
09/07/2021 MCHENRY IL PUBLIC
SMALL TREE ABOUT 6 INCHES IN DIAMETER
SNAPPED. TIME ESTIMATED FROM RADAR.
0113 PM HAIL 2 NNE NELSON 41.82N 89.60W
09/07/2021 U1.00 INCH LEE IL PUBLIC
QUARTER SIZE HAIL REPORT FROM MPING. TIME
ESTIMATED FROM RADAR.
0116 PM HAIL 2 E RICHMOND 42.48N 88.26W
09/07/2021 E1.00 INCH MCHENRY IL TRAINED SPOTTER
0120 PM HAIL DIXON 41.85N 89.48W
09/07/2021 E2.00 INCH LEE IL PUBLIC
TIME ESTIMATED FROM RADAR.
0120 PM HAIL SPRING GROVE 42.44N 88.24W
09/07/2021 E1.00 INCH MCHENRY IL PUBLIC
0123 PM HAIL 1 NW JOHNSBURG 42.39N 88.25W
09/07/2021 U1.00 INCH MCHENRY IL PUBLIC
QUARTER SIZE HAIL REPORT FROM MPING. TIME
ESTIMATED FROM RADAR.
0126 PM HAIL 1 ESE FOX LAKE 42.39N 88.16W
09/07/2021 E0.70 INCH LAKE IL TRAINED SPOTTER
0127 PM HAIL WOODSTOCK 42.32N 88.45W
09/07/2021 E1.50 INCH MCHENRY IL TRAINED SPOTTER
0129 PM HAIL ROUND LAKE 42.35N 88.09W
09/07/2021 E1.00 INCH LAKE IL PUBLIC
TIME ESTIMATED BY RADAR. LOCATION
APPROXIMATED BY WFO BGM, CALLER ONLY GAVE
ZIP CODE.
0129 PM TSTM WND GST 2 SE BULL VALLEY 42.29N 88.32W
09/07/2021 M68 MPH MCHENRY IL TRAINED SPOTTER
MEASURED ON PERSONAL WEATHER STATION.
0130 PM TSTM WND DMG 2 SSW PRAIRIE GROVE 42.26N 88.28W
09/07/2021 MCHENRY IL TRAINED SPOTTER
NUMEROUS TREE BRANCHES DOWNED AND SNAPPED,
LARGE BRANCH BLOCKING WESTBOUND LANE OF
IL-176. TIME ESTIMATED FROM RADAR.
0130 PM HAIL ROUND LAKE 42.36N 88.09W
09/07/2021 E1.00 INCH LAKE IL PUBLIC
TIME ESTIMATED FROM RADAR.
0135 PM HAIL 4 N AMBOY 41.77N 89.33W
09/07/2021 E1.50 INCH LEE IL PUBLIC
REPORT RELAYED BY WTVO/WQRF. TIME AND
LOCATION ESTIMATED FROM RADAR.
0136 PM TSTM WND DMG 1 SSW ISLAND LAKE 42.26N 88.20W
09/07/2021 MCHENRY IL PUBLIC
MULTIPLE LARGE BRANCHES AND TREES DOWN ALONG
ROBERTS RD. TIME ESTIMATED FROM RADAR.
0140 PM HAIL 1 NE GAGES LAKE 42.36N 87.96W
09/07/2021 E0.88 INCH LAKE IL TRAINED SPOTTER
0142 PM TSTM WND DMG LIBERTYVILLE 42.28N 87.95W
09/07/2021 LAKE IL UNKNOWN
REPORT RELAYED VIA NWS CHAT OF BRANCHES
DOWN, UNKNOWN SIZE. TIME ESTIMATED BY RADAR.
0145 PM TSTM WND DMG VERNON HILLS 42.23N 87.96W
09/07/2021 LAKE IL PUBLIC
14 INCH DIAMETER TREE SNAPPED AT BASE. TIME
ESTIMATED BY RADAR.
0148 PM HAIL 3 NNW PAW PAW 41.73N 89.01W
09/07/2021 E1.00 INCH LEE IL BROADCAST MEDIA
0148 PM HEAVY RAIN 2 SE BULL VALLEY 42.29N 88.32W
09/07/2021 M1.10 INCH MCHENRY IL TRAINED SPOTTER
RAINFALL MEASURED ON PERSONAL WEATHER
STATION OVER A 45 MINUTE PERIOD.
0152 PM TSTM WND GST 2 SE WHEELING 42.12N 87.90W
09/07/2021 M58 MPH COOK IL ASOS
PWK ASOS PEAK GUST.
0154 PM TSTM WND DMG 1 SE ISLAND LAKE 42.26N 88.18W
09/07/2021 LAKE IL PUBLIC
PICTURE OF LARGE TREE DOWN AND BLOCKING
DARREL RD SOUTH OF IL-176 RELAYED VIA SOCIAL
MEDIA.
0155 PM TSTM WND DMG DEERFIELD 42.17N 87.85W
09/07/2021 LAKE IL PUBLIC
VIDEO SHARED ON SOCIAL MEDIA SHOWS PART OF A
LARGE TREE GETTING SNAPPED OFF AT ITS TOP.
TIME ESTIMATED FROM RADAR.
0155 PM TSTM WND DMG 1 NNE NORTHBROOK 42.14N 87.83W
09/07/2021 COOK IL TRAINED SPOTTER
SPOTTER REPORTS MULTIPLE 3 TO 4 INCH
BRANCHES DOWN.
0158 PM HAIL WEST BROOKLYN 41.69N 89.15W
09/07/2021 E2.00 INCH LEE IL PUBLIC
PHOTO RECEIVED OF HAIL. ALSO REPORTED SMALL
DENTS ON VEHICLES FROM THE HAIL IN TOWN.
0158 PM HAIL COMPTON 41.69N 89.09W
09/07/2021 E1.00 INCH LEE IL PUBLIC
TIME ESTIMATED FROM RADAR. IMAGE WAS SENT IN
VIA SOCIAL MEDIA.
0159 PM TSTM WND DMG HIGHLAND PARK 42.18N 87.80W
09/07/2021 LAKE IL PUBLIC
PICTURE OF A TREE DOWN NEAR HIGHLAND PARK
RELAYED VIA SOCIAL MEDIA.
0201 PM HAIL COMPTON 41.69N 89.09W
09/07/2021 E2.00 INCH LEE IL PUBLIC
UPDATES PREVIOUS HAIL REPORT FROM COMPTON.
HAIL WAS AT A MINIMUM 2 INCHES IN DIAMETER.
AT LEAST SIX AUTO WINDSHIELDS SMASHED. ONE
RESIDENCE HAD MULTIPLE SKYLIGHTS SMASHED.
TIME ESTIMATED FROM RADAR.
0203 PM HAIL 2 SE COMPTON 41.68N 89.06W
09/07/2021 E1.75 INCH LEE IL PUBLIC
HAIL DAMAGE TO SIDING AT FARMSTEAD. TIME
ESTIMATED BY RADAR.
0205 PM TSTM WND DMG HIGHWOOD 42.20N 87.81W
09/07/2021 LAKE IL TRAINED SPOTTER
20 INCH DIAMETER TREE DOWNED ONTO ROAD.
0205 PM TSTM WND GST HIGHWOOD 42.20N 87.81W
09/07/2021 M56 MPH LAKE IL TRAINED SPOTTER
PEAK GUST. SMALL BRANCHES DOWNED.
0210 PM MARINE TSTM WIND WILMETTE BUOY 42.14N 87.66W
09/07/2021 M54 MPH LMZ740 IL BUOY
0212 PM HAIL PAW PAW 41.69N 88.98W
09/07/2021 M2.00 INCH LEE IL PUBLIC
0218 PM HAIL 4 WNW EARLVILLE 41.61N 89.00W
09/07/2021 E1.75 INCH LA SALLE IL BROADCAST MEDIA
TIME ESTIMATED FROM RADAR.
0225 PM HAIL EARLVILLE 41.59N 88.92W
09/07/2021 E2.00 INCH LA SALLE IL PUBLIC
MULTIPLE REPORTS OF HAIL UP TO 2 INCHES IN
DIAMETER IN AND NEAR EARLVILLE. TIME
ESTIMATED FROM RADAR.
0225 PM HEAVY RAIN 1 NNW LIBERTYVILLE 42.30N 87.96W
09/07/2021 M1.32 INCH LAKE IL EMERGENCY MNGR
0230 PM MARINE TSTM WIND HARRISON-DEVER CRIB 41.92N 87.57W
09/07/2021 M42 MPH LMZ741 IL C-MAN STATION
0252 PM TSTM WND DMG 4 SSW LELAND 41.56N 88.82W
09/07/2021 LA SALLE IL TRAINED SPOTTER
REPORT VIA NWS CHAT FROM SPOTTER OF 3 INCH
DIAMETER TREE BRANCH DOWN NEAR LELAND, IL.
0254 PM TSTM WND DMG 6 NE SENECA 41.37N 88.53W
09/07/2021 GRUNDY IL TRAINED SPOTTER
NUMEROUS TREES DOWNED AND ONE SNAPPED IN
HALF ALONG US ROUTE 6 BETWEEN MORRIS AND
SENECA. POWER POLE ALSO DAMAGED. TIME
ESTIMATED FROM RADAR.
0255 PM TSTM WND DMG MORRIS 41.36N 88.42W
09/07/2021 GRUNDY IL TRAINED SPOTTER
NUMEROUS TREES AND WIRES DOWNED IN AND
AROUND MORRIS. TIME ESTIMATED FROM RADAR.
0255 PM TSTM WND DMG MORRIS 41.36N 88.42W
09/07/2021 GRUNDY IL TRAINED SPOTTER
REPORT OF 16 INCH DIAMETER TREE SNAPPED.
TIME ESTIMATED BY RADAR. LOCATION IS
APPROXIMATE.
0255 PM TSTM WND GST 3 E LISBON 41.47N 88.42W
09/07/2021 M72 MPH KENDALL IL TRAINED SPOTTER
0255 PM TSTM WND GST 5 SE LISBON 41.43N 88.42W
09/07/2021 M61 MPH GRUNDY IL ASOS
WIND GUST OF 61 MPH (53 KTS) REPORTED BY C09
ASOS.
0259 PM TSTM WND DMG 3 WSW PLATTVILLE 41.51N 88.43W
09/07/2021 KENDALL IL TRAINED SPOTTER
REPORT OF TREE BRANCHES AND LIMBS DOWNED BY
WINDS ESTIMATED TO BE 60 TO 65 MPH.
0302 PM TSTM WND GST MINOOKA 41.46N 88.26W
09/07/2021 E85 MPH GRUNDY IL NWS STORM SURVEY
AN NWS STORM DAMAGE SURVEY TEAM FOUND
WIDESPREAD TREE DAMAGE, INCLUDING NUMEROUS
DOWNED, ACROSS MUCH OF AND NEAR MINOOKA.
SOME OF THESE TREES WERE DOWNED ON
STRUCTURES. THE DAMAGE WAS CONSISTENT WITH
THE HIGHEST WIND GUSTS TO AROUND 85 MPH.
0302 PM TSTM WND DMG MINOOKA 41.46N 88.26W
09/07/2021 GRUNDY IL STORM CHASER
PICTURES OF TREE BRANCHES DOWNED ON A HOUSE
VIA SOCIAL MEDIA. TIME ESTIMATED BY RADAR.
0307 PM TSTM WND DMG 1 WSW MINOOKA 41.45N 88.28W
09/07/2021 GRUNDY IL STORM CHASER
REPORT VIA SOCIAL MEDIA OF TRACTOR TRAILERS
OVERTURNED AT A COLD STORAGE FACILITY. TIME
IS ESTIMATED FROM RADAR.
0308 PM TSTM WND DMG 1 NE CHANNAHON 41.45N 88.21W
09/07/2021 WILL IL NWS EMPLOYEE
SEVERAL LARGE TREE LIMBS SNAPPED IN
CHANNAHON. TIME ESTIMATED FROM RADAR.
0310 PM TSTM WND GST CHANNAHON 41.43N 88.23W
09/07/2021 E85 MPH WILL IL NWS STORM SURVEY
AN NWS STORM DAMAGE SURVEY TEAM FOUND
WIDESPREAD TREE DAMAGE, INCLUDING NUMEROUS
DOWNED, ACROSS MUCH OF AND NEAR CHANNAHON.
SOME OF THESE TREES WERE DOWNED ON
STRUCTURES. THE DAMAGE WAS CONSISTENT WITH
THE HIGHEST WIND GUSTS TO AROUND 85 MPH.
0310 PM TSTM WND DMG CHANNAHON 41.43N 88.22W
09/07/2021 WILL IL EMERGENCY MNGR
CHANNAHON VILLAGE OFFICIAL REPORTS
WIDESPREAD DAMAGE TO TREES AND POWERLINES.
ESTIMATE OF 100 OR MORE TREES DAMAGED ACROSS
THE VILLAGE WITH SOME TREES UPROOTED. A
DOZEN OR MORE TREES ON HOUSES. SIDING DAMAGE
TO AT LEAST 2 HOUSES. WORST DAMAGE APPEARS
TO BE NEAR THE DUPAGE RIVER SOUTHWEST OF THE
JUNIOR HIGH SCHOOL. TIME ESTIMATED BY RADAR.
0310 PM TSTM WND DMG 2 ENE CHANNAHON 41.44N 88.20W
09/07/2021 WILL IL LAW ENFORCEMENT
CORRECTS TIME OF PREVIOUS REPORT FROM 2 ENE
CHANNAHON. REPORT OF A TREE DOWN ON SB I-55
AT US 6 AND OVERTURNED SEMI TRUCK ON I-55 AT
ARSENAL.
0310 PM TSTM WND GST 2 NNW MAZON 41.27N 88.45W
09/07/2021 M52 MPH GRUNDY IL PUBLIC
PRIVATE WEATHER STATION KILMAZONIO REPORTED
A WIND GUST OF 52 MPH AT 3:10 PM CDT.
0315 PM TSTM WND DMG MINOOKA 41.46N 88.26W
09/07/2021 GRUNDY IL TRAINED SPOTTER
SPOTTER REPORTS TREE BRANCHES DOWN. WINDS
WERE ESTIMATED TO BE 60 MPH.
0318 PM TSTM WND DMG 3 E ELWOOD 41.40N 88.06W
09/07/2021 WILL IL TRAINED SPOTTER
DAMAGE TO BUILDINGS AT A FARM ON THE CORNER
OF W HOFF RD AND ROWELL AVE. ALSO TREES
UPROOTED AND CORN FLATTENED. TIME ESTIMATED
FROM RADAR.
0321 PM TSTM WND GST ELWOOD 41.40N 88.11W
09/07/2021 E60 MPH WILL IL AMATEUR RADIO
REPORT OF ESTIMATED WIND GUSTS OF 50 TO 60
MPH WITH SOME TREE LIMBS DOWN. TREE LIMBS
ARE ESTIMATED TO BE 2 INCHES IN DIAMETER.
0324 PM TSTM WND DMG 1 WNW MANHATTAN 41.43N 88.00W
09/07/2021 WILL IL STORM CHASER
PICTURE OF UTILITY POLLS SNAPPED AT THE BASE
RELAYED VIA SOCIAL MEDIA. TIME IS ESTIMATED
FROM RADAR.
0340 PM TSTM WND DMG 3 E PEOTONE 41.34N 87.73W
09/07/2021 WILL IL FIRE DEPT/RESCUE
10 TO 12 POWER POLES DOWNED.
0342 PM TSTM WND GST 5 NW GRANT PARK 41.30N 87.70W
09/07/2021 E60 MPH KANKAKEE IL TRAINED SPOTTER
TIME ESTIMATED FROM RADAR.
0342 PM TSTM WND DMG PEOTONE 41.34N 87.79W
09/07/2021 WILL IL TRAINED SPOTTER
8 INCH DIAMETER TREE LIMBS DOWNED. TIME
ESTIMATED FROM RADAR.
0345 PM TSTM WND DMG 1 WSW KANKAKEE 41.11N 87.88W
09/07/2021 KANKAKEE IL EMERGENCY MNGR
REPORT OF A 6 TO 12 INCH DIAMETER TREE
BRANCH DOWN.
0401 PM LIGHTNING 2 ENE LIMESTONE 41.15N 87.93W
09/07/2021 KANKAKEE IL TRAINED SPOTTER
SPOTTER REPORTS LIGHTNING STRUCK A HOME
UNDER CONSTRUCTION AND CAUSED ROOF DAMAGE.
0405 PM HAIL REDDICK 41.10N 88.25W
09/07/2021 E0.70 INCH LIVINGSTON IL TRAINED SPOTTER
SPOTTER REPORTS DIME SIZE HAIL WHILE DRIVING
NEAR REDDICK ALONG ROUTE 17.
0418 PM TSTM WND DMG 2 WSW FLANAGAN 40.87N 88.90W
09/07/2021 LIVINGSTON IL BROADCAST MEDIA
LARGE METAL OUTBUILDING COLLAPSED. RELAYED
BY WMBD. TIME ESTIMATED FROM RADAR.
0420 PM TSTM WND DMG FLANAGAN 40.88N 88.86W
09/07/2021 LIVINGSTON IL PUBLIC
LARGE TREE LIMBS DOWNED IN FLANAGAN. RELAYED
BY WMBD. TIME ESTIMATED FROM RADAR.
0448 PM TSTM WND GST 3 SW BAILEYS CORNER 41.05N 87.05W
09/07/2021 M68 MPH JASPER IN TRAINED SPOTTER
SPOTTER MEASURED WIND GUST OF 68.2 MPH AT
INTERSECTION OF SR 49 AND COUNTY ROAD 255N.
0512 PM HEAVY RAIN 3 W IRWIN 41.05N 88.04W
09/07/2021 M2.51 INCH KANKAKEE IL COCORAHS
THE 2.51 INCHES OCCURRED IN ABOUT 1.75 HOURS
ENDING AT 5:12 PM CDT.
0530 PM TSTM WND GST 4 WNW GIBSON CITY 40.48N 88.45W
09/07/2021 M54 MPH FORD IL MESONET
PRIVATE WEATHER STATION REPORTS 54 MPH JUST
WEST OF GIBSON CITY.
0535 PM TSTM WND DMG GIBSON CITY 40.46N 88.38W
09/07/2021 FORD IL BROADCAST MEDIA
WIDESPREAD TREE AND POWER LINE DAMAGE,
HOSPITAL ROOF DAMAGED, AND BASEBALL DUGOUT
DESTROYED IN GIBSON CITY.
0555 PM HEAVY RAIN 1 SSE GIBSON CITY 40.46N 88.37W
09/07/2021 M1.50 INCH FORD IL BROADCAST MEDIA
CORRECTS TIME OF PREVIOUS HEAVY RAIN REPORT
FROM 1 SSE GIBSON CITY. 1.5 INCHES OF RAIN
FELL IN ABOUT 45 MINUTES IN GIBSON CITY.
TIME IS ESTIMATED BY RADAR.
0558 PM TSTM WND DMG GARDNER 41.19N 88.31W
09/07/2021 GRUNDY IL 911 CALL CENTER
REPORTS OF TREES DOWN ON POWER LINES AND
POWER LINES DOWN ON ROADWAYS.
0601 PM HAIL 3 SW ESSEX 41.16N 88.23W
09/07/2021 E1.00 INCH KANKAKEE IL TRAINED SPOTTER
TIME ESTIMATED FROM RADAR.
Minooka and Channahon Damage
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Public Information Statement
National Weather Service Chicago IL
450 PM CDT Wed Sep 8 2021 /550 PM EDT Wed Sep 8 2021/
...NWS Damage Survey for 09/07/2021 Thunderstorm Wind Event...
.Overview...Thunderstorms produced widespread damaging straight
line winds across southern Kendall, northern Grundy, and eastward
into western Will County. This includes the communities of Lisbon,
Minooka, and Channahon. Much of the damage was consistent with
winds of 60 to 70 mph, however, there were pockets of more intense
damage consistent with 70 to locally greater than 80 mph winds.
Numerous trees were downed, including some into homes, and there
was also some mostly minor structural damage. Extensive ground
surveying did not reveal any conclusive evidence of tornadic
damage, though we will continue to evaluate any additional
information that becomes available.
&&
NOTE:
The information in this statement is preliminary and subject to
change pending final review of the event and publication in NWS
Storm Data.
Photos
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Environment
On September 7, 2021, bands of severe thunderstorms raced through northern Illinois and northwestern Indiana. The thunderstorms were supercells, or storms with rotating updrafts. Supercells are known for producing all hazards of severe weather (damaging hail, winds, and tornadoes), and develop in environments with strong wind shear.
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MRMS Radar Reflectivity at Lowest Altitude (RALA) from 11:00 AM to 9:00 PM CDT, September 7, 2021 |
Let's look through thermodynamic and kinematic variables meteorologists with the National Weather Services often use when assessing the potential for severe weather.
1). Forcing, or sources of lift
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| RAP-analyzed 300 mb analysis from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021. 300 mb heights are contoured in black, wind speeds are shaded and displayed in wind barbs, and upper-level divergence is contoured in magenta. | RAP-analyzed surface analysis from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021. Surface pressure is contoured in black, temperature contoured in red, and dew points shaded and contoured in light blue. |
The main "forcing" or sources of lift for thunderstorms on September 7 were 1). an approaching upper-level trough and associated jet max, and 2). a surface cold front. Both acted in tandem to lift low-level air to the point where it became freely buoyant.
2). Storm Mode
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| RAP-analyzed MLCAPE (J/kg) from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021. | RAP-analyzed Effective Bulk Shear (kt) from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021. |
As the upper-level trough and cold front moved southeast into Illinois, they met increasingly unstable air as analyzed by mixed-layer CAPE (MLCAPE) of 2000 to 3000 J/kg. Such values typically lead to vigorous thunderstorms with strong updrafts. Additionally, the incoming upper-level jet max afforded appreciable deep-layer shear, both with respect to speed and direction. Effective Bulk Shear exceeded 50 kt across much of the area, which is more than sufficient for organized thunderstorms including supercells.
The strong "linear" forcing along the cold front as well as broad synoptic-scale lift afforded by the approaching upper-level trough was supportive of a long line of thunderstorms. However, the deep-layer shear had a large component orthogonal to the cold front, which promotes a semi-discrete "broken" band of thunderstorms. Taken together, the thermal and kinematic environment was supportive of intense, semi-discrete supercells as the dominant storm mode.
3). Severe Weather Hazards
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| RAP-analyzed Max 2 to 6 km Lapse Rate (C/km) from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021. | RAP-analyzed DCAPE (J/kg) from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021. |
When assessing the threat of hail, we typically look at instability and lapse rates. Instability provides an approximation for the potential intensity of an updraft. Aside from looking at traditional measures of instability (e.g. MLCAPE), we often look at the rate at which temperatures increase or decrease with height, which is called the lapse rate. The mid-level lapse rate on September 7 was 8 to 8.5 C/km (meaning the temperature dropped about 8 decrees Celsius every kilometer), values that are considered very "steep" or indicative of considerable instability. Mid-level lapse rates greater than 8 C/km can support updrafts that produce hail larger than 2 inches in diameter, provided the other ingredients for thunderstorms are available.
The environment also had a large amount of Downdraft CAPE (DCAPE), with values analyzed greater than 1000 J/kg. Values greater than 800 are typically favorable for damaging winds, with the threat (and intensity of) greatly increasing as values exceed 1000 J/kg. Taken together, with a supercell storm mode, both large hail greater than 2" in diameter as well as significantly damaging winds were possible in the environment.
4). Composite Parameters
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| RAP-analyzed Significant Hail Parameter from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021. | RAP-analyzed Supercell Parameter from 1700-0000Z (12:00-7:00 PM CDT) September 7, 2021. |
One way to visualize the necessary ingredients for severe weather is to use composite parameters that use calibrated instability and shear variables to make a non-dimensional parameter. We often look at the Significant Hail Parameter, where values greater than 1.5 are typically associated with the potential for thunderstorms to produce hail larger than 2.5 inches in diameter. We also use the Supercell Composite Parameter to assess the threat for supercells. Both parameters demonstrated that the environment was favorable for large hail and organized thunderstorms.
(Want to learn more about these parameters, or see them in real time? Visit the Storm Prediction Center Mesoscale Analysis page!)
5). Put it all together...
Lo and behold, supercells with damaging winds and large hail developed across northern Illinois and northwestern Indiana. One storm was particularly intense and produced accumulating, large hail over the town of Earlville.
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Additional Info
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Media use of NWS Web News Stories is encouraged! Please acknowledge the NWS as the source of any news information accessed from this site. Additional recaps can be found on the NWS Chicago Past Events Page |
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