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."