The Sisternado is a supernatural, and unusual form of weather that the Loud sisters have the power to form.
The Sisternado is formed whenever Lincoln's sisters have an equal set of emotions together. They all summon miniature tornadoes, and combine them into one tornado that is small enough to fit inside the house.
The Sisternado has strong levels of wind in a matter of rotation. The faces of Lincoln's sisters are visible on its surface.
The Sisternado's first appearance was in the episode "Heavy Meddle". Lincoln had made several weather reports to the viewers about his state in trying to avoid letting his sisters find out that he was being picked on by somebody at school so they wouldn't meddle. It seemed sunny and clear at first, but as soon as they found out, Lincoln made an update that a storm would be approaching. It was then that the sisters found out that Lincoln's bully was a girl and assumed that her picking on him was a sign of romance. They all got so excited that they all got together and Lincoln reported that they had formed the legendary Sisternado. It would not cease until the sisters' wish for their brother to kiss his bully would come to fruition. Lincoln had gotten caught in the eye of the "Her-ricane" and sought shelter in the bathroom, trying to deny their beliefs that the bully wanted to be his girlfriend, but soon came to concede to their beliefs. He faced the Sisternado and stopped it by telling his sisters that he is ready to kiss his bully, and they had all turned back to normal.
It is unknown if Lincoln's sisters really did form a tornado, or if it was part of his imagination.