It is true that they can and do happen every month of the calendar year. In modern history (1950-2018), about 10 percent of all violent tornadoes have happened in March. It’s the fourth most likely month for them. The usual suspects of April (30 percent), May (24 percent), and June (15 percent) fill out the top spots.
When it comes to where the first violent tornado of the year tends to occur, Mississippi leads the way historically with nine occurrences. Missouri (8), Oklahoma (7), Texas (7), and Kansas (6) round out the top five. Alabama runs neck-and-neck with Oklahoma when it comes to the most killer violent tornadoes on record.
On the flip side of the season, the final violent tornado of the year has struck Iowa more than any other state, six times. The top five here also includes Kansas (5), Texas (5), Wisconsin (5), and Illinois (4). Four other states also have four: La., Mo., Miss., and N.D.
These days, your typical violent tornado season is a bit less than 5 months long, or about a week to two less than 150 days.