The Storm is a freak electrical storm which damaged much of Bridgton, Maine: in 1980 in the novella, and 2007 in the film. In the film, it was also (according to a newspaper) the greatest electrical storm in recorded Maine history. It is believed to be affiliated with the Mist.
Some fans have speculated that the Arrowhead Project caused the Storm as well as the Mist. Others believe the Storm is what caused the Arrowhead Project to go awry in the first place when it damaged equipment at the base where the project was taking place. In the original script for the film adaption, the latter was revealed to be true.