I am hoping to get some information about the top 50 fastest growing cities in the U.S. during the past year.
The closest thing we have to this is a set of tables giving the top ten fastest growing cities and metropolitan areas in the decade between the two most recent census reports.
The degree of detail you're looking for is harder to pin down. Your best bet is to search through the U.S. Census Bureau website; whether or not they can answer that specific question, they have a wealth of relevant statistics. (And if they don't have the answer, it's a safe bet that nobody does.)
You might also be interested in our section about U.S. Cities.