You can get tornadoes any time of the year, however there is an increase during tornado season. Tornado season April, May and June. The peak being mid May, well in theory at least. The last few years have not entirely fallen in line with those expectations. The tour groups generally go from the last week in April through to first week in June. During the spring, warm moist air starts to invade the plains. This clashes with the cold dry air coming down from the north. The result is huge monster storms which are capable of producing tornadoes.