National Weather Service Weather Forecast Office

   Scattered storms sprang up on Friday afternoon across Chicagoland and to the southwest along the Illinois River Valley.  The warm lower atmosphere was favorable for quickly developing updrafts to eventual thunderstorms.  The environment was also conducive for these thunderstorms to readily produce strong downdrafts, due to high cloud bases and a somewhat dry lower atmosphere allowing for ample evaporational cooling.  The cooler dense air would sink and spread out quickly due to having a greater density than the warm ambient air.  With little wind shear and slow storm motions, these outflows tended to spread out in circular fashions around the storms on Friday.  Non-meteorological radar scatterers such as dust particulates and insects, and at times condensed water droplets, can sometimes collect on the leading edge of these cooler dense outflows.  When this occurs it allows for them to be detected on radar.  Such was the case on Friday. 

    Check out the 0.5° KLOT reflectivity loop to the lower left and note the arcs and circular areas around storms.  These are rain-cooled outflows.   Some of these boundaries served as foci for updrafts to new thunderstorms.

   Meteorologists can detect an atmosphere capable of producing gusty thunderstorm winds per analysis of upper air soundings and computer model output, and from there relate to conceptual models.  One of the variables than can summarize the lower atmosphere's state to produce gusty thunderstorm winds is downdraft CAPE (DCAPE). A diagnostic model analysis of DCAPE from early Friday afternoon can be seen below.  For more on an explanation and example of DCAPE and how it ties to gusty thunderstorm winds, see here.