When you go on a storm chasing tour you obviously want to get as many great chase days as possible. Many tour operators offer tours from April until July, which is the Tornado Season in the great plains of the USA.
Most often, there is a peak in frequency of severe weather around the last two weeks of May and the first week of June and if you want to maximize your chances of observing tornadoes and severe weather, this is probably the time of the season when you would want to go.
The frequency of tornadoes follow a bell shaped curve that peak through late May and early June but all throughout Tornado Season you will have good chances of seeing severe weather and tornadoes
Do you want to chase in a specific part of the USA?
Typically, the earlier in the season you go on a tour the further down south you are more likely to chase. In April and early May it is quite common to be chasing in Texas and Oklahoma. In late May and early June, chases can be all over the Midwest. Later on in the summer, the severe storms get frequent more and more to the north, such as South Dakota, North Dakota and Wyoming. In July, one could get as far north as Canada on a chase.
Thus, if you want to take the time to see more of Texas and south central USA, then an early tour (April, early May) would be something to look into. If you want to experience the beauty of Wyoming and the Dakotas, for example, then a June tour might be a better option for you.
Read more here about where you are most likely to be chasing.
Lastly, please note that there are no guarantees and no absolute facts about when, where and how storms develop. All information provided here should be considered as rules of thumb rather than facts.