An F6 tornado is a storm with winds above 318 mph. It often is referred to as an inconceivable tornado because an F6 has never been recorded as of 2014, according to About.com.
Tornadoes are categorized by the Enhanced Fujita Scale, also known as the F-Scale. The F-Scale goes from F0 to F5, which is a storm with winds of 261 to 318 mph. Because the scale stops at F5, the Storm Prediction Center notes that no storm receives an official F6 designation, even if the winds register stronger than 318 mph. Tornadoes are rated based on calculations of wind and damage.