Here's an image of a small squall line moving through south western connecicut associated with a cold front on Monday Nov 17th 2014.
Below is the storm velocity image. That shows the winds moving towards the radar station (green) and away from the radar station (red). You can actually see te squall line! Pretty cool!
Velocity images, like the one above are awesome used to detect rotation in tornadic thunderstorms. When the red and green are touching or wrapped around eag other, it signifies winds rotating. That's a little weather knowledge for you!