Strong F-150 sales have meant great profits for Ford in recent months. But if a top analyst is correct, September's numbers might not look so good. Photo credit: Ford Motor Co.

Auto sales in the U.S. have been rising steadily for over a year now. That has been a good sign for a recovering economy, and great news for automakers like Ford (NYSE:F), which has banked huge profits on very strong U.S. sales.

But now, one analyst thinks that this growth streak might be in trouble. He's expecting U.S. new-vehicle sales to decline when this month's totals are reported next week -- and he might be right.

In this video, Fool contributor John Rosevear explains what's behind the prediction, and why Ford might be one of the very few automakers to post a gain when this rough month ends.