Chief Market Strategist at Carson Research, Ryan Detrick, is advising S&P 500 investors to prepare for seasonal headwinds, cautioning that markets should brace for August and September, as they are historically the "worst two months" of the year.

The warning follows a rare back-to-back monthly decline for the index in June and July.

Historical Red Flags After Summer Drop The S&P 500 ended July down 0.1% after dropping 1.1% in June, snapping an 11-year July win streak.

According to Carson Investment Research data, consecutive losses in June and July are a "dubious distinction" shared by infamous market years like 1974, 1990, 2001, 2002, and 2008.

"Prepare to hear a LOT about this from the bears, but down in June and July has been weak for the rest of the year historically," Detrick noted.