The Tornado Warning Flag
The 'Tornado Warning' flag uses the same colours as the hurricane flag. The background is black and there are two red circles, reflective of the circular motion of a tornado.
Since Hurricanes have their own flag it makes perfect sense that tornadoes get one as well.
Photo Credit: http://taylormarshall.com/2013/05/did-god-send-tornado-problem-of-natural.html