Factual error: When they are at Aunt Meg's the first time, they get word that a tornado has been spotted and somehow they already know its rating. Tornadoes get their ratings from the amount of damage they do. This is determined after the tornado is gone.
Other mistake: When the twister hits the drive in theater, the sirens are wailing, but nobody reacts. Only when Jo screams at the waitresses they start panicking and run. The movie is set in Oklahoma, smack in the middle of tornado alley. The people there know what the sirens mean, they sure don't need no out-of-town big-shots to tell them to get under cover.
Doc
Suggested correction: Sirens go off so many times that true Okies tend to ignore them. We might get nervous if the cable goes out.
Do true Okies also run screaming the second an out-of-town big-shot yells at them? Sorry but that argument just doesn't compute. People ignoring fair warning simply isn't a thing in the movie. The main plotline is people not getting warnings soon enough.
DocStupidity: In the opening, it's surprising that Jo and her family have to actually go outside and run even a short distance to get to their cellar. Farm houses, especially in tornado prone areas like rural Oklahoma, would typically be built overtop of the cellar, which would be accessible from inside the house. Furthermore, Jo's cellar door seems to be secured with what seems to be little more than a latch much like those used for restroom stalls! Not very practical for a door intended to protect the interior from outdoor elements.
Suggested correction: Their storm cellar is made of an old septic tank, which is very common to do. It's half buried into the ground so it provides maximum protection, unlike under the house because an F5 tornado would simply rip the entire house away, removing all the protection the cellar would have had. Also the door has several latches, not just one.
lionheadA cellar under the house would likewise be buried into the ground, often more than halfway, so it would offer just as much protection, if not more. Looking at the scene again, the door has two latches, both the same size and woefully insufficient.
Plot hole: During the scene when Jo introduced the first Dorothy to Bill for the first time, you can see the blurry figure of Dusty and Dr Melissa holding hands in the background, which is super weird given she was annoyed by his presence at that point.
Suggested correction: Dusty is leading her to Dorothy, to let her see what they are working on. Obviously she doesn't mind being led there by him. He is eccentric and direct and she isn't used to that, but that doesn't mean she thinks he's annoying. Not even a character mistake.
lionhead
Suggested correction: The scale back then was based on the size of the tornado, it's only more recently it is based on damage. So during the time of the movie, the scale was being used correctly for size not damage.
The Fujita scale was introduced in 1971 and was in use during the 90's when this film came out. The Fujita scale measured the damage caused by a tornado to man-made structures after ground or aerial surveys, it was not a measurement of tornado size (an F5 tornado is a tornado that's rated on the Fujita scale). It is true the Fujita scale was replaced by the enhanced Fujita scale in 2007, but that was only to align the ratings to the damage better, it did not change rating tornadoes from size to destructive powers.
Bishop73