False radar returns on weather radars (such as the ones tonight) are typically produced by stationary objects on the ground during times of strong atmospheric temperature inversions. This is most typical during the overnight hours in an area of high pressure. The strong temperature inversion causes the radar beam to be ducted or refracted to the ground causing it to receive returns from ground-based targets. This phenomenon is commonly know as anomalous propagation or AP.
Below is an example of AP from the night of January 23rd-24th, 2006.