Mesocyclones do not always produce tornadoes. About 1,700 mesocyclones strike the United States every year, but only half produce tornadoes. If the cyclone runs out of wet, warm surface air, it dies out. If it does not run out of this fuel, however, the rotating cloud stretches toward the ground and may become a giant tornado. Observers have found that tornadoes are likely to form when the clouds look yellow or green, or when the air is muggy and still after rain and hail has stopped.
Tornadoes appear on the right rear side of supercells, taking place where a stream of cool air from the downdraft twirls into the main warm air updraft.
Mesocyclones can also drop many tornadoes at once, which spin around the mother funnel like spokes around a central wheel. On February 19,1884, one such storm brought 60 tornadoes in the same day.
Forces of Nature: ThinkQuest 2000 (Team #C003603)
http://library.thinkquest.org/C003603/english/tornadoes/themesocyclone.shtml
The mesocyclone is the "mother of tornadoes." It is a spine of wind that gradually narrows and spins more and more fiercely vertically through the supercells. A wall of clouds swirls with it, making it visible to the human eye as a huge, black cloud bulging down from the supercell thunderclouds. It can be anywhere from half a mile to six miles wide (almost the height of Mt. Everest!).