(From wiikipedia) Smooth Channel Lightning are positive cloud to ground lightning strikes where the forward stroke originates from the ground upwards to the cloud. The smooth channel is in the lower section of the lightning channel but should branch higher up (not visible as the "branching" is inside the cloud).  Large supercells generate tremendous areas of positively-charged cloud material (thick anvil) and wind shear prevents excessive negative strokes as with "normal" thunderstorms. Downdrafts, such as the forward flank downdraft (FFD), bring the positively charged cloud material closer to the ground, where such lightning occurs.