/k/ here, now, what we are looking for in the nuke is an aerial detonation above the tornado, this will stop the tornado from throwing the nuke off course. We don't need a high yield, so a 5 ton yield would be enough to see results.
Most likely, due to the air pressure and direction, it would flatten out the tornado and spread it out, making it more like a dwarf tornado, this could make it more destructive if it survives, but most likely it will die off quickly. Though as it whips around before it dies off, it will destroy everything that gets caught in remaining blast of the tornado, powerful enough to rip trees out by the roots. The power of the nuke won't cause any major damage depending on the altitude of the blast, but that could also assist in the destruction of nearby homes and land.
Would it be stupid? Possibly. Would it be awesome? Definitely. Would it be magical? Obviously.