Updated August 15, 2008
A total of 5 tornadoes was confirmed by the National Weather Service as having occurred during the severe thunderstorm episode Monday evening, August 4th, across the area.
The cluster of thunderstorms which moved across northern Illinois and northwest Indiana on this day can be described as a mesoscale convective system (MCS). This is a continuous line of thunderstorms spanning many miles, for several hours, affecting many locations. This meteorological phenomenon is not uncommon for northern Illinois and northwest Indiana for this time of year. The pattern is distinguished by upper level winds orienting themselves in a northwest flow pattern. This orientation allows waves of energy (upper level disturbances) to traverse the region. Combining this with warm, moist, unstable conditions can trigger thunderstorm development and lead to a storm complex similar to Monday's occurrence. As this pattern persists, multiple waves of energy move across the region, providing several rounds of MCS activity.
Low level circulations that can accompany a Bow Echo can also lead to significant damage. These low level circulations are generally near the leading edge of the line of storms, but north of the bow apex. It is these low level circulations that can create brief spin ups, or non-supercell tornadoes. Although these tornadoes are generally brief and weak in nature, they can still create damage as high as EF2. The environmental shear as discussed earlier is what can be the impetus for tornadogenesis along the leading edge of this type of storm. The environmental shear creates a horizontal circulation similar to a pinwheel. This circulation, or vorticity, gets tilted into the vertical, and helps create these low level circulations.
The widespread severity of wind damage with this unusual event will mark Monday’s storm as an historical one. The Bow Echo that moved across northern Illinois and northwest Indiana produced winds to 94 MPH at one point, while creating widespread damage and at least three confirmed tornadoes.
National Weather Service storm survey teams immediately assessed the damage which occurred with this devastating MCS. This attempted to validate any tornadoes indicated by radar or by reports sent in via the spotter network. Any tornado confirmed by the NWS gets a strength rating based on the damage inflicted. This rating is based off of the Enhanced-Fujita scale. Included with the rating, are the length and width of the tornado's path. This information is used by the NWS for agency use, but is also used to progress the science. Storm surveys used in conjunction with case studies can be a valuable tool to provide knowledge and insight each occurrence. Doing this allows meteorologists to better understand storm scale features that can lead to another significant event similar to Monday's. The goal here is to provide a better service to the public and the meteorological community.
The Chicago Weather Forecast Office would like to extend great thanks for all who helped with this event. This includes trained spotters, emergency managers, the media, law enforcement, and amateur radio network.
Image displaying preliminary storm reports for portions of Northern Illinois and northwest Indiana. Multiple high wind damage reports were observed across much of the area and indicated by the blue W's. A few reports of tornadoes and funnel clouds were also called into the Weather Service and indicated by the red T's. A full list of the preliminary local storm reports can be accessed through the NWS website (weather.gov/chicago).
Radar image at 745 PM of the Mesoscale Convective System which was in its mature bow echo stage as it moved through Cook, Dupage, and Will counties. Worthy to note are the circular looking portions north and south of the bowing segment. These are referred to as bookend vortices, which can enhance the rear inflow jet creating stronger winds at the surface.
Radar Image at 758 PM displaying the very strong winds associated with the Bow Echo. Cool colors (green) represent inbound winds toward the radar and hot colors (red) represent winds away from the radar. Note the dark blue and purple colors in Kane county. Radar indicated possible winds near 90 MPH across this location at this time.
Bloomingdale, IL -------------------------------------------------------------
Radar image at 745 PM displaying storm relative motion. In this image, rotation is observed near Bloomingdale, IL. This is distinguished through the green colors representing strong velocities directed toward the radar, while the adjacent brighter red colors represent velocities away from the radar. This couplet suggests a cyclonic, or counter-clockwise circulation.
A broad area of straight line wind damage occurred across northern DuPage County with a concentrated area of stronger winds produced by what meteorologists refer to as a mesovortex. The mesovortex is apparent in the Doppler radar velocity images as a inbound-outbound couplet, indicated by the small area of bright greens immediately next to a small area of bright reds. This mesovortex persisted along the north edge of the bow echo for many miles stretching all the way from Kane county near Elburn to the Chicago lakefront near Fullerton Ave. A large area of straight line wind damage was evident from north of the DuPage Airport across Bloomingdale and Addison to Bensenville.
Damage to trees and rooftops was widespread across a portion of the Bloomingdale, Glendale Heights, and Addison areas where straight line winds were estimated to have been greater than 76 MPH. Embedded within this zone was evidence of a tornado touchdown in a light industrial area from Fox Court and across South Gary Drive ending near Stratford Square Mall where there was a downburst wind. With wind speeds estimated to 110 MPH, the tornado was rated at EF1. A large warehouse had a wall blow out near Fox Court and Madsen drive.
The same tornado did a second touchdown at an apartment near Century Point Lane. There was roof and window damage at two apartment buildings. There was significant damage to roofs on homes and tree damage from Cardinal Drive east across Bloomingdale Road through Norton Lane and into the area around Sterling Drive. We think this tornado was also followed by a downburst. The damage was also rated as an EF1.
Tornado rated EF1 (Time - 745PM)
Bolingbrook, IL ---------------------------------------------------------------
Radar image at 741 PM displaying storm relative motion. In this image, rotation is observed just west of Bolingbrook, IL. Note the green colors representing strong velocities directed toward the radar and the adjacent brighter red colors representing velocities away from the radar. This couplet suggests a cyclonic, or counter-clockwise circulation.
A segment of the bow shaped line of storms moved through Naperville and Bolingbrook. This part of the storm produced a path of wind damage and a brief tornado. Starting near Plainfield-Naperville Road and Boughton Road on the west side of Bolingbrook, there were scattered tree limbs down. The tornado apparently developed about 1/2 mile east of Plainfield-Naperville Road and about a block south of Boughton Road. There was a narrow path of more intense damage in a subdivision where several homes had sections of roofing torn off, siding and soffits torn loose, trees snapped, and limbs down. The most intense damage was on Sparrow Lane, Maroon Bells Lane, and Silverado Street. The tornado was rated EF1 at this point with winds 86 to 110 MPH. Here one house will likely be a total loss due to structural damage to the roof, covered porch, garage, and one wall. A garage was destroyed at another home. There was evidence of rotation and uplift in this area. The tornado weakened to EF0 east of this area from Kings Road to Indian Chase Meadows. Winds were estimated at 65 to 85 MPH. There was sporadic damage to trees and limbs, and a few homes had damage to shingles, siding, and soffits. A small section of roof was removed from a house just east of Indiana Chase Meadows. At this point the tornado apparently dissipated but strong straight-line winds fanned out. The damage path widened to 1/4 to 1/2 mile wide just south of Clow Airport and between Weber Road and Veterans Parkway, just south of Lilly Cache Lane. Fences were blown down and sporadic minor tree damage continued. In an older subdivision with larger more mature trees, there were a lot of limbs down and a few trees snapped or uprooted. But the houses were more sheltered by the big trees and there was very little damage to homes in this area. The strong winds continued east of Veterans Parkway. Small limbs were down at Jane Adams School. From the school east southeast to Schmidt Road, a few homes had siding peeled back, or soffit damaged. There was sporadic damage to small trees, and loose objects such as trampolines and portable basketball sets were blown over. Winds were likely 60 to 75 MPH in this area. East of Schmidt Road, along Remington Road and the Frontage Road, damage to tree limbs was very minor. A construction trailer on the Frontage Road, east of Woodcreek Drive was destroyed. Winds were estimated to be 85 to 95 MPH at this point. South of I-55, a large metal hotel sign was broken and a wooden pole holding a tornado siren was snapped. There was damage to an air conditioning unit on the roof of an industrial building. There was minor damage to trees along Old Chicago Road, east of Route 53 to Joliet Road. Large trees were damaged on Joliet Road. East along Bluff Road from Joliet Road to near I-355, only sporadic minor damage to tree limbs was found with winds around 50 to 60 MPH.
Tornado rated EF1 (Time - 745) with a path 1.1 miles long and 50 yards wide.
Griffith, IN ----------------------------------------------------------------------
Radar image at 828 PM displaying storm relative motion. In this image, rotation is noted in northern Lake county near Griffith, Indiana. This is once again observed through the green colors representing strong velocities directed toward the radar with adjacent brighter red colors representing velocities away from the radar. This couplet suggests a cyclonic, or counter-clockwise circulation.
Map of tornado near Griffith, IN.
Storm Survey #3 focused on the storm circulation that caused the brief Bolingbrook tornado as it tracked eastward into northwest Indiana. This circulation tightened up and strengthened as it reached far northwest Lake County just to the north of Munster. The strongest radar velocity signatures appeared as the storm passed through Griffith. It was at this point that we received reports of funnel clouds and measured 80 mph winds and widespread damage across the city.
The damage path continued east for approximately a total of 5 miles along and north of Ridge Rd until the damage appeared to become less around Chase Rd and Ridge Rd. Scattered 1 to 3 inch diameter tree branches were still noted to be down further east from here, but this was likely due to the line of storms rolling through.
Tornado rated EF2 (Time - 828 PM CDT) with a path 5 miles long and 30 yards wide.
Boswell, IN -------------------------------------------------------------------------------------
A three mile long track of wind damage occurred across south central Benton county beginning around 12:20 AM EDT. Wind damage was evident beginning 1 mile south of Boswell on U.S. 41 and extending southeast along a fairly continuous path for about 3 miles. Most of this damage was due to straight line winds, likely a microburst. However, evidence of a small F0 tornado touchdown was apparent in the middle of the track with perpendicular debris left where a tool shed was destroyed. There was also crop damage indicative of swirling winds. Total path length of this tornado was 100 yards, path width was 10 yards. Other damage due to the microburst Included approximately 20 pine trees all snapped off laying in the same direction. These trees ranged from 18 to 24 inches in diameter and were located toward the beginning of the damage path. Damage extended southeastward to Maridian Road near 850 South. There was also wind damage at the Benton County Fairgrounds near Boswell.
Tornado rated EF0 (Time - 1220 AM EDT) with a path 100 yards long and 10 yards wide.
Orland Park, IL -------------------------------------------------------------------------------
A storm damage survey was conducted by NWS meteorologists in Orland Park on Wednesday evening. It was determined that a very brief EF0 tornado touched down and was immediately followed by a larger splat of microburst straight line winds. The storm circulation that caused this damage (referred to by meteorologists as a mesovortex) was the same storm circulation responsible for tornadoes in Griffith, IN as well as Bolingbrook, IL.
The tornado initiated in Orland Park along the south side of 135th Street one block east of La Grange Road. It tracked southeast through the neighborhood along Howe Drive snapping off numerous tree tops, downing three or four large trees, blowing down a fence, sending a gazebo swirling 30 feet into the air before depositing it in a neighbor’s yard, and damaging a roof on an auto shop along Southwest Highway. At this point, the weakening tornado was wiped out by a microburst as it crossed the railroad tracks just east of Southwest Highway about a quarter mile south of 135th Street. The entire path of the tornado was ¼ mile long and 30 yards wide. The microburst that followed widened the damage path to a quarter mile wide and was about a third of a mile long. The microburst downed numerous trees in the area of Carolina Lane, 92nd Ave., Elm Street, and 137th and 138th Streets. Trees fell on rooftops, shingles were blown off a shed, there was damage to eaves on homes, and there was porch and eave damage due to falling trees and branches. Power lines were downed due to falling tree branches.
Additional, but more sporadic, tree damage extended along the same line to the southeast, crossing 140th Street near Clearview Drive and along 144th Street between 87th and 84th Avenues.
The type of damage observed in Orland Park was first classified by Dr. Ted Fujita (same researcher who invented the F scale for tornado classification – now used as the EF [Enhanced Fujita] scale) as a wide-end tornado while he worked at the University of Chicago during the 1970s. A wide end tornado by definition includes both tornado and downburst. It is believed that a strong downburst behind the weakening tornado undercuts the tornado circulation, in effect wiping out the swirling motion. This scenario is consistent with what eyewitnesses observed in the area on the evening of the 4th.
Tornado rated EF0 (Time - 800 PM CDT) with a path 0.25 miles long and 30 yards wide.
Watseka and Kankakee, IL --------------------------------------------------------------
A NWS storm survey team of meteorologists investigated the damage at Watseka and Kankakee in northeast Illinois on Wednesday afternoon. They determined that straight line winds of 80 to 110 MPH occurred near the intersection of county roads 2100N and 2100E approximately 2 to 3 miles north of Watseka. A machine shed was completely destroyed and the surveyed damage was suggestive of straight line winds. Another pocket of significant straight line wind damage occurred near Route 1 just south of Route 24 approximately 1 mile east of Watseka. Here significant tree damage and its resultant damage debris caused broken windows to a glass patio area. Based on damage across this location, wind gusts were estimated to be in excess of 80 MPH. Tree damage continued eastward to Sheldon. Most of the wind damage that occurred across these areas was in association with thunderstorms that moved through Iroquois county during the early morning hours on Tuesday, just after 6 AM. The map below highlights the areas of most significant wind damage that were surveyed in the Watseka area.
The following is a radar image of storm relative velocity at 616 AM Tuesday morning from the storm that affected Watseka. The radar from the NWS in Romeoville is located to the northwest of this location. The green colors represent velocities toward the radar, and the red colors represent velocities away from the radar. The combination of these colors suggests the presence of a mesovortex, or a small scale counter-clockwise rotation in the low levels of the atmosphere. This mesovortex was likely responsible for the straight line wind damage observed near Watseka as it moved from north of Watseka to the Sheldon area in eastern Iroquois county.
An area of widespread damage also was observed in the southern portions of Kankakee. This wind damage was more widespread in nature. The following image shows an approximate location of the more extensive wind damage:
The extensive wind damage began along W. Jeffrey Street and extended to east of the Cobb Park area. Many roads were impassable across these locations on Tuesday due to downed trees. Several very large trees were uprooted and power lines were downed. One very large tree landed on the roof of a home across the western portion of the main damage area. The damage across southern Kankakee came in two rounds, the first during Monday evening and then another round of storms affected this area early Tuesday morning. The most significant damage was reported to have occurred with the intial round of storms on Monday evening. Based on the observed damage, wind gusts were estimated to be from 80 to 100 MPH.
The following is a radar image of base velocity from 922 PM Monday evening from the storms that affected the southern parts of Kankakee. In this image, the NWS Romeoville radar is located to the north northwest. Note the presence of velocities toward the radar (green colors) north of the velocities going away from the radar (red colors). This signature suggests divergent winds near the ground indicative of a microburst. During a microburst, strong downburst winds hit the ground and spread out in circular fashion. The damage observed in Kankakee is consistent with a microburst.