Why are there two different time periods?
Tornado data exist back to 1916, however it is important to note that
a national reporting system was not in place until after 1953.
The early, pre 1953 data are included here since they exist, but caution
should be used in comparing the pre 1953 data to the post 1953 data.
Was tornado forecasting once banned in the U. S.? Yes. Before 1950, at various stages of development of the Weather Bureau, the use of the word "tornado" in forecasts was at times strongly discouraged and at other times forbidden, because of a fear that predicting tornadoes may cause panic. This was in an era when very little was known about tornadoes compared to today, by both scientists and the public at large. Tornadoes were, for most, dark and mysterious menaces of unfathomable power, fast-striking monsters from the sky capable of sudden and unpredictable acts of death and devastation. As the weather patterns which led to major tornado events became better documented and researched, the mystery behind predicting them began to clear--a process which still is far from complete, of course. In 1950, the Weather Bureau revoked the ban (PDF) on mentioning tornadoes in forecasts. This paragraph is from the On-line Tornado FAQ page at SPC.
Some related interesting links: