The market turmoil continued Tuesday as weak data out of China pushed all major U.S. indices down more than 2 percent. The S&P 500, and have now fallen a respective 6.5, 9.5 and 1 percent year to date, and according to one renown technician, the move may have signaled the end of one of the longest-running bull markets in history.

Looking at a chart of the S&P 500, Louise Yamada noted that momentum has been declining for four months, which by her work, is a "classic" sell signal.

"This is suggesting to me that we are looking at a bear market," said Yamada said Tuesday on CNBC's "Futures Now." Yamada noted that the last two times the market saw a similar shift in momentum were in January 2008 and June 2000.

At this point, the S&P 500 is already 10 percent from its May all-time high, but for Yamada, it's about to get a lot worse. "We could certainly see the S&P test its 2009 uptrend at 1,800," said the founder of Louise Yamada Technical Research Advisors.

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"If the 1,800 level were to be breached, I think we could go all the way back toward 1,600 which is the breakout point through which the market moved in 2013," she continued. "In a normal technical concept, it's nothing more than a pullback to the breakout, but it would be a 24 percent decline [from the peak], which would hurt if people didn't protect themselves."

Yamada doesn't see the sharp selling coming in one swift move. Instead, she believes the market could see bounces and the overall decline will happen over the course of a number of weeks to months.

In the meantime, Yamada has a simple message for investors: Tread carefully.

"Lighten positions in stocks that are beginning to become fragile."