Official Storm Data Publication: Monthly reports summarizing storm events and unusual weather phenomena in the United States. NOAA and educational/academic users have free access to this site, but other users will need to pay a fee in order to obtain Storm Data Publications.
Storm Event Database: A searchable database compiled from the Storm Data Publication. Search by state for documented tornadoes, flash floods and other storm events. This site is free to all users.
Tornadoes: Maps and reports on tornadoes that have affected the United States
Monthly Tornado Statistics: Find out how many tornadoes have occurred so far this year, and how this year compares to past years. Information at this site is updated daily.