Links
Government

Storm Prediction Center
Storm Prediction Center - Warning Coordination Meteorologist (WCM)
National Climatic Data Center
National Weather Service
Louisville, KY National Weather Service Office - Tornadoes of Central Kentucky and South Central Indiana

Other sites

Twister History - A standalone application for visualizing tornado locations and statistics
April 3, 1974 - Tornado Super Outbreak of April 3, 1974