In winter an artic air mass (very cold and dry air) can move over the ocean, picking up some warmth and moisture from the warmer ocean and becoming a maritime polar air mass (mP)- one that is still fairly cold but contains moisture (I hope this helps you!)
Typically peak tornado months are from April to June and thus most the 45% of storms form in this period. As compared to the rest of th months onwards i.e from June to august it decreased from 45 to 34 % and so on.
