It not very often that the weather radar displays show so neatly the action of a low pressure system, a cyclone, rotating over North America. The lower atmosphere is a constant swirl of high and lows as they move around the hemisphere under a huge array of influences. The basic counterclockwise rotation of a low pressure system in the northern hemisphere is a result of the rotation of the earth and the drag of the atmosphere. This display of a such a well defined cell occurs in the lower 50,000 feet of the atmosphere, and is artificially animated by the time-lapse traces of the NEXRAD radar pattern, compiled from a host of NWS radar sites. You can clearly see the comma shape, and if you click on the image to bring up the animation, you can see the distinct rotation [Ed. note: this system is now mostly dissipated and animations will show current patterns]
The radar is visualizing water density consistent with rainstorms. The long comma tail is the confluence of the cold frontal air being dragged down from the north by the low, against the warm moist air being dragged up from the south. This instability is swirled into the centre of the low by its rotation, forming rain bands, as the low pressure cell itself slowly moves eastward.
The same system as seen as a visible cloud satellite view.