Corrected entry: At the part when they are driving in the car (near the climax scene of the big tornado) the weather radio in the car is issuing alerts about the big "F5" tornado on the ground nearby. That is a huge mistake, because there is no way to know that until a survey is done of the tornado damage afterwards. Only then can a tornado be called an "F5" or "F3".
Correction: Announcer was Gary England; who is a very well known meteorologist out of Oklahoma.
Corrected entry: The volume of Jo and Bill's voices changes in the scene where he tries to stop picking up the Dorothy sensors that a tornado has already destroyed. When they're in the truck, they're loud probably because their dialogue was recorded in a studio later. When they're outside their voices aren't as loud because they were in the location.
Correction: It's common in movies for audio - especially dialogue - to be boosted or lowered depending on the needs of the scene. (Ex. In this case, the voice-over audio being boosted while they're in the car so the audience can hear them when they wouldn't otherwise be able to - we're basically hearing the conversation in the car from outside.) A filmmaking tactic such as this can't really be counted as a mistake because it's a common and necessary requirement in filmmaking in order to convey the dialogue, and is used countless times in just about every film. Nothing about it is technically a "mistake" per se.
Correction: People's voices changing volume isn't really a mistake because it can easily just be that they stopped speaking as loudly for any reason.
Correction: More of a character mistake than a movie mistake. The radio anouncer probably knew it was a huge twister and incorrectly pronounced it an "F5" before it could be correctly classified later.
Mad Ade