Storm Prediction Center:
http://www.spc.noaa.gov/products/index.html

Tornado Project - Tons of historical tornado info:
http://www.tornadoproject.com/

Info on severe weather safety and preparedness:
https://www.weather.gov/safety/

“Severe Weather 101 — Tornadoes”:
https://www.nssl.noaa.gov/education/svrwx101/tornadoes/

NOAA Environmental Visualization lab severe weather event satellite images):
http://www.nnvl.noaa.gov/MediaHome.php?MediaTypeID=1&CategoryID=36