On the evening of Tuesday, September 25th (6:30 PM), a cold front was dropping south through southern Wisconsin and was being reinforced by a shot of cooler air coming in off of Lake Michigan. See the image below. As it neared the radar here at the NWS Milwaukee/Sullivan office in southeast Jefferson county, the dBZ level, or intensity of the radar returns, increased significantly. In fact, the portion of the front east of the radar was showing returns of 53dBZ! These types of returns are common in some thunderstorms. Our radar's software even flagged it as a "new [thunderstorm] cell." But, in this case, there wasn't a cloud in the sky!
So, why the high radar returns? Typically, these strong returns are the result of bugs, dust and maybe birds collecting within the convergent zone of the two opposing wind regimes. In addition, the airmass discontinuity across this boundary (warmer, drier air to the south, cooler and more humid to the north,) creates a reflective surface for radar energy to bounce off of. Someone on our Facebook page said, "... This is seen as "tropospheric ducting" and is used regularly on the VHF / UHF amateur radio bands." I did notice quite a bit of corn and soybean harvesting on the way into the office. This was just a few hours before this image was captured. The amount of dust generated was impressive, especially within the soybean fields. Even corn fields were generating excessive dust, likely related to the drought conditions this summer. These especially high dBZs are most likely from that enormous amount of dust "piling" up in the convergent wind zone. But, I admit, this is pure conjecture on my part.
Our WSR-88D radar often picks up the arrival of cold fronts and lake breezes. In this case, the radar returns were quite a bit higher than what we usually see.
The image below is just a zoomed in picture of the first image: