Like everyone else said, it is ground clutter, present on ALL radar images I've ever seen. In order to see trace precipitation in a large region they need to be highly sensitive. Turn up the sensitivity (gain) and you get more and more ground clutter. You can never tell exactly what is reflecting the radar back - it could be a bird, a building, sprinklers, a moving semi, etc. They have a choice, limited radar range with limited clutter - or large radar range with lots of clutter. If there was a sudden change, then most likely they desided to change the radar range.
Also, cities are hotter than their surroundings. Cars burn fuel, houses have heaters running, bakeries have large ovens, the blacktop pavement absorbs more sunlight than dirt does, etc. All energy from electricity used, coal burned, gas burned, etc ends up as heat. Most of this heat enters the atmosphere. Thus the temperature in big cities is always 2°F to 3°F hotter than the surrounding areas. This temperature difference can affect the way clouds behave. Look at lots of radar pictures and you will often see clouds form right downwind of all big cities. This answers your other concern about clouds behaving differently over the city.
