Federal Reserve Board Chairwoman Janet Yellen speaks in Washington.

There's speculation this week the Fed may not raise interest rates until later this year or early next year due to concerns about a weak economy, but two veteran market strategists are sticking with their forecasts.

"One of the biggest risks for the markets is if economic weakness outside of the U.S. begins to meaningfully undermine U.S. growth. However, we don't expect this to happen. We still believe the first Fed tightening will occur in June of this year," Erik Ristuben, chief investment strategist at Russell Investments, told CNBC's "Power Lunch" on Friday.