Investors are winding down another volatile week with U.S. stocks are pace to wrap a losing week. Federal Reserve Chairman Jerome Powell fired a warning over tariffs, on Thursday President Trump clapped back calling for Powell to cut rates.

Pointing to data showing run-ups in foreign purchases of U.S. stocks before the 1987 crash, the dot-com bubble in 2000, and the 2008 financial crisis, Ed Yardeni wrote in his eponymous research newsletter, "Their buying has a record of being a contrary indicator. They tend to be big buyers right before bear markets."

Investors who may have been hoping the Federal Reserve is poised to come to the aid of tariff-rattled markets took away an unwelcome message from the central bank's chief on Wednesday: the Fed may be in a bind.