"The actual number is unknown, because it is likely that (throughout the course of history) many tornadoes were either not reported or erroneously categorized.
NCDC has developed an interactive online severe weather database which you can use to search your state and/or county for tornado segments and other severe weather reports.
Jeff Evans has broken down the SPC tornado data into table of tornadoes by state for three decades ending in 2009.
NOTE: NCDC tornado data is not for whole tornadoes, but for county-segments; and there are still some incorrect county codings or other errors as documented by Doug Speheger of NWS Norman."
- Interactive Online Severe Weather Database (Database), National Climatic Data Center
Search by county and date for any type of severe weather event including tornadoes.
- U.S. Tornado Climatology (Map/Webpage), National Oceanic and Atmospheric Administration
U.S. map of the average annual number of tornadoes for each state and links to monthly tornado occurance maps
- U.S. Annual Tornado Maps (1952-2011) (Map/Webpage), National Oceanic and Atmospheric Administration
U.S. maps of the number, location, and category of recorded tornadoes each year from 1952 to 2011. Note that the number of recorded tornadoes has increased substantially since the 1950s, largely due to higher populations, better detection technology, and increased reporting.