Movie Reality Check
The Day After Tomorrow: Tornadoes in Los Angeles
That's not very normal, according to the above map.
How are they created?
In this specific instance, we are talking about the US.
One of the most important processes to cause a tornado is a supercell. A supercell is most likely to form a tornado under specific conditions. Rising air is also needed, the supercell makes the upward flow of air very strong, because of that, moisture forms at the base of the supercell, and it allows a vortex to form, at this point, it is called a mesocyclone. A wind shear, another necessary part, (rear flank downdraft) goes around the tornado and creates an air temperature difference, the mesocyclone’s lower part becomes thinner and longer, which also increases the wind speed and when it reaches the ground, it connects with the storm which created it and the storm starts spinning with the vortex.
The cold air comes in from the top, Canada, and the warm air comes from the bottom, Mexico. As Los Angeles is located towards the left side, the wind is not quite there as much as it is in tornado alley.
Realistic or not?
The above points prove that the tornado scene in Los Angeles in the day after tomorrow is not very accurate and may have been biased due to hollywood being in Los Angeles, in this location it is almost impossible to have 3 tornadoes going on at once. It's not realistic, but it might not be intended to be realistic, as it is just a movie and the supposed "apocalypse" is occurring. This scene is not realistic at all, as the other scenes in the movie, but it is fun to watch.