While talk around the Federal Reserve's ultra-easy monetary policy has focused on tapering off its quantitative easing program, a weakening economy will likely keep the central bank in the game and could even mean more aggressive measures.

The Fed has been pumping in $85 billion worth of liquidity each month in hopes of lifting asset prices and boosting economic growth.

But even as stock prices soar to new highs, economic gains have remained elusive, and expectations are growing that the spring-into-summer swoon the U.S. has seen in the past is reappearing.