For continental U.S. the answer is never. Since the earth's rotation axis is tilted 23.5 degrees with respect to its orbital motion around the su, one would have to be less than 23.5 degrees above or below the equator to have the sun pass direclty overhead
This is true.
The definition of megalopolis is that it is a large urban area, huge and densely populated. St. Petersburg is definitely one, however, Miami is relatively small compared to other megalopolises so it might not fall within that category.