Analysis of radar imagery and damage photographs indicates that a microburst likely occurred across parts of
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
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
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
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.
Wakimoto, R.M., 1985: Forecasting dry microburst activity over the high plains. Mon. Wea. Rev., 113, 1131-1143.
M. Ratzer, 07/11/07