According to the movie's opening on screen text, "The story you are about to see is based on factual events."
As the two best friends, Glen and Antonio are driving to their hiking destination in California's Sierra Nevada mountains, it can be heard over the car radio that, "This is now officially Los Angeles' second wettest rainfall season since records began in 1877 and the wettest in 121 years. The most rain to fall in a rain season was 38.18 inches during the 1883-1884 season."
These weather related facts are all true, leading up to the basis of the film's primary concept.
In a ploy to continually manipulate the audience with ambiguity, writer
Dan Hefner even goes so far as to use the actor's real first names as the characters portrayed in the film. Also note, the film's cleverly vague tagline, which also alludes to it, "It could happen... and it did."