When a cold front meets a warm front, the cold front typically slides under the warm front as both try to continue along their original paths. The cold air condenses the moisture in the warm front, forming clouds that often produce rain.
Sometimes a large cold front slides over a smaller warm front, creating instability in the air as warm air tries to rise, and cold air tries to fall. Rising warm air often causes thunder and lightning, including so-called dry lighting. If the warm air is humid, this may trigger a thunderstorm or supercell, a very powerful thunderstorm. If conditions are turbulent enough in the thunderstorm, cold air downdrafts become powerful enough to form a tornado.
Cold and warm fronts are caused when a large mass of air hovers over a large unvariegated land or water area, such as plains, deserts or lakes. If there is little or no wind and the air moves slowly, the air mass picks up the characteristics of the land. If it's a warm lake, the air mass grows humid and warm, and if it's a cold snowy plain, the air mass becomes cold and dry. For this reason, supercells and tornadoes are more common over the American Midwest, where warm, humid air flowing north from the Gulf of Mexico often comes in contact with faster-moving cold air dropping south from Canada.