Vortex Flow

My son wanted to know why tornados go in the same direction as hurricanes (counter-clockwise, in the northern hemisphere). I thought it had something to do with vorticity cascading down from higher scales, but some poking around online suggests this is not considered the explanation any more. The technical meteorological papers on the subject are really hard to read though. Presumably, there must be some asymmetry tied to the Coriolis force somewhere, if only because of the overall direction of the air masses.

The library copy of "Vortex Flow in Nature" has come back in, and maybe it will help me sort this out.