A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft . Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south or southwest side of the main precipitation area.
Note that the rain-free base may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate. See Figs.
HP storm
LP storm
supercell