Fresh Snow and Temperature Outlook Ahead of Christmas Day

If you are looking for fresh snow by Christmas morning or are curious about potential travel disruptions, the best chances for at least 1" of new snowfall early this week exist across the mountainous West, Great Lakes, and Northeast. Otherwise, temperatures this last full week of December will average above normal for much of the lower 48 states. Read More >

June 18th Bow Echoes

Panoramic shot of shelf cloud as it was crossing into northwest IL.
(Click on picture for amazing time lapse video of approaching shelf)

On June 18^th, 2010 a double shot of severe weather affected much of Northern Illinois and Northwest Indiana area producing severe damaging winds. The first storm developed over Eastern Nebraska early in the morning, and worked its way east, entering Illinois by mid afternoon. The second line of thunderstorms developed over western Iowa around this time and strengthened as it made its way towards the Chicago Metro area by late Friday Evening.

Figure 1: Radar imagery (click for loop) of the first line of storms over Indiana with the second line entering Northwest Illinois

Meteorological Setup

During the morning of June 18^th, a frontal boundary had set up across Southern Nebraska and Iowa (Figure 2) with 500mb shortwave troughs propagating over a flat ridge. A storm system that dropped considerable precipitation across Northern Illinois and Northwest Indiana the night before as well as moderate southwesterly flow ahead of the system, which pulled up warm, moist air, helped to prime the atmosphere with warm temperatures and high dewpoints (figure 3) that continued to feed and strengthen the system as it headed towards Northeast Illinois and Northwest Indiana.

Figure 2: Surface Analysis during the late morning of June 18^th. The first storm has already developed and is moving over Iowa.

Figure 3: Equivalent Potential Temperature (shaded green) is a measure of the temperature and moisture in the atmosphere. An axis of high Equivalent Potential Temperature values extends north over Illinois. This axis is where we can expect convection to increase in strength.

Instability, which is a measure of an air parcels ability to rise and accelerate through the atmosphere creating strong updrafts, was very strong during the day Friday. With temperatures soaring into the upper 80s to lower 90s, and dewpoint values in the 70s, MLCAPE values (a measure of instability) rocketed to around 3000 J/kg by midday (Figure 4).

Figure 4: MUCAPE Values of 3000 J/Kg are seen over Central and Northern Illinois.

The system evolved into what is known as a Bow Echo (Figure 5) as it headed east. The Bow Echo, aptly named for the shape it takes on, is known to be a producer of strong and damaging winds. The strongest winds are typically seen at the apex of the bow. Bow echoes form as cold dry air from a thunderstorm downdraft accelerates downward and surges ahead along the line of thunderstorms. The morning sounding from Davenport, IA (Figure 6) shows dry midlevels and steep lapse rates (temperatures cool quickly with height), two ingredients for the development of strong downdrafts. Wind reports in excess of 70 MPH had already been reported upstream over Iowa, and as the storm raced east over the Chicago forecast area, reports quickly came in of winds in excess of 60 and even 70 MPH with trees and power lines down. (Pictures of the damage can be seen below).

Figure 5: Radar Reflectivity of the first Bow Echo as it approached the Chicago Metro Area

Figure 6: Davenport, IA sounding from the morning showing dry midlevels and steep lapse rates.

Damage from the storms was widespread. Trees were reported down across a large swath of Northern Illinois and Northwest Indiana. Half a million residents lost power during the storms. Windows were reported blown out in some of the high-rises of downtown Chicago. A summary of impacts from the storm can be seen plotted on Figure 7.

Figure 7: A map showing the location of the damaging wind and large hail reports from across the area June 18th.

Pictures of the Storm and Storm Damage


Shelf Cloud in Dixon (courtesy of Paul Downes)	Shelf Cloud near Rockford (courtesy of Jim Zandonai)	Shelf Cloud in Naperville (courtesy of Paul Downes)

Shelf Cloud near Franklin Grove(courtesy of Lee Co. EM)	Shelf Cloud north of Rockford (courtesy of Dan Neal)	Shelf Cloud near Franklin Grove(courtesy of Lee Co. EM)

Damage in Westmont (courtesy of Matt Fischer)	Damage in Oak Park (courtesy of Sam Augustyn)	Damage in Rochelle (courtesy of Mike Katz)

Damage in Whiting IN (courtesy of Carolyn Szepanski)	Damage in Whiting IN (courtesy of Carolyn Szepanski)	Damage near Franklin Grove(courtesy of Lee Co. EM)

Damage in Willowbrook (courtesy of Adam Lucio)	Damage in Whiting IN (courtesy of Carolyn Szepanski)	Damage on NW Side of Chicago (courtesy of Tommy Nava)

Damage near Utica (courtesy of Scott Anderson)	Damage near Utica (courtesy of Scott Anderson)	Damage near Utica (courtesy of Scott Anderson)

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Fresh Snow and Temperature Outlook Ahead of Christmas Day

June 18th, 2010 Bow Echoes

June 18th Bow Echoes

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