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Analysis of radar imagery and damage photographs indicates that a microburst likely occurred across parts of Schiller Park , Franklin Park, Northlake and Leyden Township around 5 pm Tuesday afternoon.  Numerous large trees and power poles were damaged, some falling on cars and buildings. A section of a large billboard was blown into a vehicle on the Tri State Tollway, near milepost 38 near the O’Hare Oasis.

 

 

 

The majority of damage reports received by the National Weather Service, were along an approximately 1 mile wide, 4 miles long, west to east swath, from northern Northlake and unincorporated Leyden Township , east across much of southern Franklin Park , to River Grove.

 

Microburst winds are produced by acceleration of downdraft winds in a thunderstorm, which then spread out along the ground as damaging straight line winds along the direction of storm motion, which is usually aligned with stronger mid level atmospheric winds.

 

The radar image below shows Doppler velocity from the O’Hare Terminal Doppler Weather Radar (TDWR), which is located near La Grange . The green colors on the Doppler velocity image indicate an inbound (toward the radar) component of around 45 knots (50-55 mph).  Note that this velocity maxima is depicted south of the main area of reported damage. This is likely because the radar is seeing only the divergent spreading of the microburst winds along the southern flank of the main wind maxima. The strongest winds, likely in excess of 60 mph, are actually blowing west to east just to the north of the depicted wind maxima. However, Doppler velocity is only a representation of the component of motion toward or away (along the beam) from the radar. Thus, wind blowing perpendicular to the radar beam is not accurately detected.

 

The next radar depiction is a radar reflectivity image also taken from the O’Hare TDWR at the same time (2201 UTC, or 501 pm CDT) as the velocity image shown earlier. Note that the location of the wind velocity maxima is collocated with a strong radar reflectivity maxima, which indicates the heavy precipitation core of the thunderstorm.  These signatures, along with the path and type of damage strongly suggest downburst winds caused the damage seen across the Franklin Park and Northlake areas.

 

 

A typical wet microburst sounding, from Wakimoto 1985, shows conditions that are favorable for the development of wet microbursts from thunderstorm downdrafts. Note the dry mid level air (above about 550 mb, around 18,000 feet) above the nearly saturated low levels.

 

Compare this to the 1800 UTC (1 pm CDT) radiosonde observation from WFO DVN just west of the forecast area that afternoon. Also note the mid level westerly winds of 30 knots, which could be convectively transported and accelerated to the surface by strong thunderstorm downdrafts.

 

References:

Wakimoto, R.M., 1985: Forecasting dry microburst activity over the high plains. Mon. Wea. Rev., 113, 1131-1143.

 

M. Ratzer, 07/11/07

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