Weather changes. I grew up in Miami, tornados on land. There were water spouts (small tornadoes) out in the deep bay or ocean, but they did not come to shore then. Recently water spouts have come on land in summer. In the midwest I think they happen during weather changes(betw seasons). Hurricaines safer if U R inland, not near water! I wrked in Iowa and freq Tornado scares?
Go to NOAA website. This should nail it down for you: https://www.ncdc.noaa.gov/climate-information/extreme-events/us-tornado-climatology.
Unpredictable. Our weather pattern is changing and we are having tornadoes when in months when we were not expecting them, such as December. When the conditions are right the chances increase regardless of the month.