Yesterday was the 70th anniversary of the United States' atomic bomb attack on Hiroshima. It led to the ending of WWII but also created massive carnage and death and was one of only two times an atomic would ever be used in combat (the other time being at Nagasaki, a few days later). So, what if the bomb that hit Hiroshima, code named "Little Boy" hit Downtown Miami today? An interactive application called 'NukeMap by Alex Wallerstein' has a simulation of the carnage. (Public Radio International goes into more detail) With estimated fatalities at 55,400 and injuries at 61,600 (both probably radically low numbers considering at least 70,000 died at Hiroshima), and a mushroom cloud reaching 20,000 feet in height, basically it would be bad. Very, very bad.
If a 15 kilaton A-bomb was dropped in the heart of Miami, the fireball would consume a block of Downtown Miami, although pretty much everyone from the Julia Tuttle Causeway to Brickell Key would die. Third degree burns would extend up to the Omni, along with most residential buildings collapsing. Considering an east-west wind off of the ocean, radiation fallout would extend past Doral and into the Everglades. Of course, the results would be a hell of a lot worse if a modern bomb, like one of the 300 kilaton mothers currently in the U.S. arsenal, were dropped.