First and foremost,
Mike - supercells are a plenty in the United States given the shear, moisture and instability profiles that exist. The judgement there as to a favourable season is based on the level of tornadic activity. Once the conditions become ideal, I think you can guarantee a good dose of supercells during the months of March April especially May and June. It begins to ease of in July and August and mainly can be expected in the northern states during this period. In some years but not as reliable but still can be lethal is the second season during October and November mostly across the southern states.
If you are heading to the US with an expectation - forget it. I guess the dream is to see the tornado and excellent structure. The best I can advise is to look at past events as well as comprehensive reports I have written in Storm News during the May - June period since 2001.
One thing is for certain - it does not matter where you have been in this country. Storms here simply are a far cry from what is observed in Tornado Alley. You want structure, supercells, lightning, heavy rain violent giant hailstones - it occurs at higher frequency. Scary if you lived there imo.
Regards,
Jimmy Deguara