5 Confirmed Tornadoes for the Chicagoland Area!

August 4th High Wind Event (4 Tornadoes Confirmed)

The cluster of thunderstorms which moved across northern Illinois and northwest Indiana on Monday can be described as what is commonly referred to as a mesoscale convective system (MCS).

This is generally a continuous line of thunderstorms spanning many miles, over a long duration, and during which, will affect many locations.

This is a meteorological phenomenon that is not uncommon for northern Illinois and northwest Indiana during this time of the year. The weather pattern is such that, the upper level winds orient themselves in a northwest flow pattern. This orientation of the upper level winds allows waves of energy (upper level disturbances) to traverse across the region. This, coinciding with very warm, moist, and unstable conditions can trigger thunderstorm development which can lead to a complex of thunderstorms similar to what occurred on Monday. As this weather pattern persists, multiple waves of energy can move across the region providing several rounds of MCS’s for the region.

As what was previously noted above, the atmosphere was very unstable. During the day on Monday, persistent southerly winds across the region allowed a stream of very warm and moist air to advect north into the region. This weather pattern, with clearing skies on Monday, allowed temperatures to rebound to near 90 degrees by the afternoon. With dew point temperatures now in the mid to upper 70s across much of the region, very high humidities were produced. This moist, unstable environment provided the energy needed for thunderstorm development to occur. These same muggy conditions provided heat index values between 100 and 105, also validating a heat advisory that was in place until 800 pm Monday evening for areas just outside of the Chicago Metro area.

During the early evening at around 600 PM, an MCS moved into far northwest Illinois. As it continued into the southern portions of Winnebago county and the western portions of Ogle county, a portion of the storm began to bow. The signature at this point in the storm evolution is what provides the name Bow Echo. This type of thunderstorm complex is capable of producing widespread wind damage and possible tornadoes as it moves very fast across a certain location. This bowing structure is induced by mid-level winds (rear inflow jet) penetrating the surface. It is at this time, that the storm begins to bow and wind damage is produced. What can affect its magnitude and longevity is if the low level wind shear is strong. Wind shear is just a change in wind direction or speed with height. The low level wind shear, generally from the surface up to 3 km, on Monday was sufficient to balance out the bowing segment. Thus, allowing the Bow Echo that occurred on Monday to provide a wide swath of very high winds across northern Illinois and northwest Indiana.

What also can occur with a Bow Echo and can lead to significant damage, are low level circulations. These low level circulations are generally near the leading edge of the line, 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. Chicago being in the northern hemisphere is why the low level circulations are more commonly noted just north of the bow apex.

What occurred on Monday and the magnitude of the event is not unusual, but is definitely an event that will not occur on a frequent basis. The severity of the wind damage and coverage associated with this event is such that will mark Monday’s event a significant one. The Bow Echo that moved across northern Illinois and northwest Indiana provided winds as high as 94 MPH at one point, widespread damage, and at least three confirmed tornadoes.

The NWS conducted storm surveys on Tuesday to assess the damage which occurred with the MCS that moved across the area on Monday. This was needed in order to validate any possible tornadoes either indicated by radar or by reports sent in via the spotter network. Any tornado confirmed by the NWS, gets a rating based on damage from that tornado. This rating is based off of the Enhanced-Fujita scale. Included with the rating, are the length and width of the path of the tornado. This information is used by the NWS for agency use, but it 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 to what occurred. Doing this, allows meteorologists to better understand storm scale features that can lead to a significant event similar to Mondays. Thus, providing a better service to the public and 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, which is indicated by the blue W's. A few reports of tornadoes and funnel clouds were also called into the Weather Service, which is 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 Mesocale 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. Worthy to note are 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 observed through the green colors representing strong velocities directed toward the radar while the adjacent brighter red colors representing 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 area where straight line winds were estimated to have been between greater than 76 mph. Embedded within this zone was evidence of a tornado touchdown in the vicinity of Gary Ave, Scott Drive, and Fox Court, with wind speeds estimated up to 110 mph and the tornado rated at EF1. Roof and structural damage occurred to an apartment complex with windows blown out. Windows were also blown out of retail establishments at the Stratford Mall, pieces of roofing material were blown off the mall, and trees in the parking lot were blown down. Apartment buildings had windows and doors blown out, portions of rooftops peeled back and shingles off, and one wall bowed out. The actual path length and width of this tornado will be finalized by Tuesday morning.

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. This is observed through the green colors representing strong velocities directed toward the radar while 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 EFO 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, 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.

The tornado was rated EF1 (Time - 745) with a damage 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 observed in northern Lake county near Griffith, Indiana. This is once again observed through the green colors representing strong velocities directed toward the radar while the 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 wide spread damage across the city.

A storm survey of this damage path was completed on Tuesday by NWS meteorologists. The damage path started across a wide swath of western Griffith toward Highland with mainly 3 to 6 inch diameter tree branches down. A few large trees were noted down along and just east of Grace St in Highland. This damage was not highly localized and probably just caused by straightline winds at this point.

The path continued eastward along and just north of Ridge Rd to the busy intersection of Cline Ave and Ridge Rd. It was at this intersection that the more notable and convergent damage began at Griffith Park Plaza mall. Here, windows were blown out of a row of stores and there was a portion of a roof blown off of a vacant store. At this point, it was thought that it may have been localized strong winds since most of the debris was blown in one direction and none of the trees had any leaves or branches plucked from it. The damage done to this mall was given an EF1 rating with winds of around 90 to 100 mph. But as the survey team went around to the backside of the mall, it became apparent that this was damage from a tornado.

The damage path then continued along a continuous line to the east behind the mall. Two parked semi-trailers on the back side of the mall were shifted eastward while a third was completely knocked over. The area to the east of this in the subdivision along and north of 37^th Ave near Lafayette Ave and Rensselaer Ave sustained some of the strongest wind damage of note along the damage path. The damage path was approximately the length of two houses or about 30 yards wide. Two houses on Lafayette Ave had total roof loss with another house on Rensselaer Ave also having a roof collapse and also a garage wall pushed inward. This damage was given an EF2 rating with winds of around 120 mph.

With the fast moving progression eastward of the line of storms, it was determined that although all the damage was in one direction, a fast moving tornado would likely have scattered all the debris downstream as it raced eastward. The damage path is also highly consistent with the rotation path observed on radar.

Other significant damage that occurred was at the Habitat for Humanity storage building at Colfax St and Ridge Rd. Here, a cinder block storage building completely toppled to the ground. Although the building was knocked over, it appears as though the main cause of it faltering was due to a large tree that fell on top of it. At a nearby gas station within feet of this building, no damage occurred and people actually witnessed the tornado descend on the storage shelter. This damage was given an EF1 rating with the large tree being uprooted with winds of around 90 mph.

The damage path continued eastward 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 tree branches were still noted to be down further east from here, but this was likely more so due to the line of storms rolling through.

Tornado rated EF2 (Time - 828 PM) with a damage 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 12:21 pm 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.

Additional stories....

1. Tornado destroys homes
Published: 04:00 AM Aug 5,2008

By GITTE LAASBY Post-Tribune staff writer

The storm that ravished Griffith and other parts of Northwest Indiana Monday night was a Category 2 tornado, the National Weather Service confirmed Tuesday afternoon.

2. Path of destruction
Published: 04:00 AM Aug 6,2008

By Gitte Laasby Post-Tribune staff writer

The tornado that slammed into Griffith Monday was rated an EF2, the strongest in more than a decade, according to the National Weather Service.

3. Quickly for Porter County Neighbors 4/20
Published: 04:00 AM Apr 20,2007
Portage will never be smart enough to go smoke-free. They put our elementary children in the cafeteria for tornado drills. If Rep. Ed Soliday was not a shoo-in for '08, he certainly is now. Great strategy, guys. The Democrats who made the decisio...

4. NIPSCO still working to restore power
Published: 04:00 AM Aug 6,2008

Post-Tribune staff report

Thousands of NIPSCO customers in Lake and Porter counties remain without power as crews continue to reconnect those affected by Monday's tornado and severe storms. Porter County power is expected to be fully restored today, while many Lake County homes may have to wait till Thursday, NIPSCO said. Watch the Post-Tribune for continuing storm coverage.

5. Power outages expected through Saturday
Published: 12:00 AM Aug 7,2008

By DIANE KRIEGER SPIVAK Post-Tribune staff writer

Twenty thousand Northwest Indiana residents and businesses remained without power late Wednesday, two days after a tornado and severe thunderstorms slammed the region, snapping trees, destroying homes and leaving one motorist dead. Most of the outages ...

6. Conditions perfect for microbursts, expert says
Published: 12:00 AM Aug 7,2008

By John Byrne Post-Tribune staff writer

VALPARAISO -- Northwest Indiana's resident tornado expert was vacationing in sunny Wisconsin when a twister struck the region this week. Valparaiso University meteorology professor Bart Wolf said he watched with a mixture of awe and envy as storm tower...



Fifth Tornado from Monday Confirmed - Orland Park

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 135^th 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 135^th 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, 92^nd Ave., Elm Street, and 137^th and 138^th Streets. Trees fell on rooftops, shingles were blown off a shed, there was damage to eves on homes, and there was porch and eve damage due to falling trees and branches. Power lines were downed due to falling tree branches.

Additional but more sporadic primarily tree damage extended along the same line to the southeast crossing 140^th Street near Clearview Drive and along 144^th Street between 87^th and 84^th 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 in the area observed on the evening of the 4^th.

Tornado rated EF0 (Time - 800 PM CDT) with a path 0.25 miles long and 30 yards wide.

MARITIME NOTE

Try and have a better weekend!
RS