An area the size of a city is too small for any comprehensive tornado statistics. Miami itself has been hit by tornadoes as strong as F1 while areas not far from the city have had tornadoes up to F3 strength. This means that while strong tornadoes have not struck the city in recent history, it can still happen. As with all areas, most tornadoes in Florida are weak with strong ones occurring less frequently.
Tornadoes happen in Miami for the same reason they happen anywhere else. See the related question for how tornadoes form in general.
Tornadoes do not occur in Miami every year. Since 1950 Miami has had 27 tornadoes, which suggests an average of a little less than 1 tornado every 2 years.
Big tornadoes are usually strong, but not always. A large, poorly organized tornado is not likely to be very strong. Some tornadoes even weaken as they expand.
In the United States, strong tornadoes, counted as those rated F2 or higher, account for about 11% of all tornadoes.
Tornadoes are sometimes divided into "weak" tornadoes "strong" and "violent" tornadoes. Weak tornadoes are those rated EF0 and EF1. Most tornadoes are weak. Strong tornadoes are those rated EF2 and EF3. Violent tornadoes are those rated EF4 and EF5. They are the rarest of tornadoes, only about 1% of tornadoes are this strong.
By how strong the tornado is. Even though strong tornadoes tend to be larger how strong a tornado is does not determine its size. Relatively weak tornadoes have been very large and extremely strong tornadoes have been relatively small.
About 20% of tornadoes are rated as strong (EF2 or stronger).
This actually did happen at least once. On May 12, 1997 Miami, Florida was hit by an F1 tornado. There was moderate damage to trees and buildings. Fortunately strong tornadoes are rare in Florida.
Yes. Maryland has had tornadoes as strong as F4.
Most of the tornadoes in California are weak , though a few strong ones have occurred, some as strong as F3.
Yes, there are tornadoes in California as strong as F3.
Yes. New Jersey has had tornadoes as strong as F3.