On Thursday, August 19, 2010, a cold front moved south down Lake Michigan and into southeast Wisconsin. We often refer to this as a "back door" cold front since it moves in from the east and north instead of from the more typical north and west direction. This cold front was essentially a strong lake breeze - seen periodically through the Summer months along the Wisconsin Lake Michigan shoreline.
What was unique about this front/lake breeze was how it showed up so clearly on our WSR-88D doppler weather radar (see the animation below.) There are a number of reasons for this. One is due to the convergence of air along the boundary. Note the arrows above. Dust, bugs and other aerosols collect along the boundary and act as a surface for the radar energy to reflect/scatter off of. Secondly, there was a good density discontinuity in the airmass across the cold front or lake breeze. Cooler and a bit drier air was flowing in from the east, while a warmer and more humid airmass was flowing north into Wisconsin. This density boundary will cause radar energy to reflect back toward the radar.
There were some cumulus clouds that developed along the cold front/lake breeze, but they were a minor contributing factor to the strong radar returns. See the visible satellite imagery below. The strong radar returns continued for a few more hours, even after the cumulus clouds dissapated.