Is there a time of the year with zero chance of a tornado?

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Dear Tom,
Is there ever a time of the year when there is a zero chance of a tornado in the United States?
— Margaret Robertson, Chicago
Dear Margaret,
There is not. However, the chance of a tornado occurring in the spring or summer is greatly better than the chance in the fall and even more so than in the winter. But tornadoes have been reported in the United States in every part of the year.
About half of all tornadoes form in the late afternoon, between 3 and 7 p.m., with 5 p.m. being the hour of greatest occurrence, but they have occurred at all hours of the 24-hour day.
May is the month of greatest tornado occurrence in the United States, followed by June, April and July. In the winter, tornadoes develop most frequently in the Gulf states, then the maximum occurrence shifts to the southern Plains in the spring, then gradually shifts to the northern Plains in August.

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