Tornadoes form by 2 type of air the cold and the warm air. The cold air push down the warmer air the warmer air is lighter than the cold air and if they come to get they will rises validly as the warmer air move upward it spin around and form a tornadoes.
First find a room without window that don't have window near by than you want to crouch down on your knees and protect your head with your hand and wait until the tornadoes pass your house.