Doppler radar, installed in the early and mid-90's across the United States, was a huge step forward in the field of tornado warnings. Doppler radar senses the winds within a thunderstorm to detect rotation as in the Oklahoma example below. The problem is that the radar detects the rotation aloft and there are times when the rotation does not reach the ground. So, meteorologists are not sure whether a tornado is on the ground or not.
Dual polarization radar has a product called "correlation coefficient." Precipitation (rain, hail, snow) have relatively high signal correlations as compared to debris (wood, metal).