Re 68 (myself)-- of course, some tornados are not from supercells — or are not formed by the exact same
mechanism; generally what is needed is an updraft over a region of
partly horizontal shear such
as that associated with a gust front — it could be a gust front from another storm... there are landspouts, gustnados, etc... these tend to be weaker then the supercell tornados,
as far
as I know (And I'm not entirely sure what defines a «landspout»; for that matter, if a supercell tornado moves over water, is it then a waterspout or is it still a supercell tornado... anyone?)