Fundstrat Global Advisors' Tom Lee, one of the most consistently bullish and correct strategists during this seven-year bull market, is frightened about the next 30 days.

"August scares us," the co-founder of Fundstrat told CNBC. "A few recent developments suggest August is likely to be a down month. We are not changing our views and we remain buyers of weakness," Lee wrote in a recent note to clients.

"However, for our tactically minded clients, it may make sense to fade strength and be prepared to add to weakness in August."

Lee cited two economic statistics.

• Lee observes that going back to 2009, the S&P 500 has fallen an average of 6 percent during the month of August.
• The odds of a slip could be even greater this year, given that "the bond market has become a lot more volatile than equities, and whenever this happens, 68 percent of the time, the stock market falls in the following month."

This adds up to a "pretty scary" outlook for next month, Lee said. However, after an August slip, Lee expects a fall rebound.

Lee says that once the S&P falls to 2,100, "we think you should buy it," in anticipation of a substantial rally to Lee's 2,325 year-end price target. Near midday Wednesday, it was at 2,163.25.

He said long-term investors "shouldn't do anything, because a 2 to 3 percent sell-off isn't enough to warrant a big shift in a portfolio. But 2 to 3 percent for an active manager is relative performance."

Lee isn't alone in his reservations about the market.

Doug Kass, president of Seabreeze Partners Management Inc., doesn't trust U.S. stocks at all.

"Aggressive liquidity continues to buoy stocks. The money is coming from a combination of the world's central banks and historically large share buybacks, coupled with quant strategies that ignore fundamentals at the expense of share-price action and risk-based allocation," he wrote for Real Clear Markets.

"To me, the market could be a "trap door" that's getting ready to open. Old correlations between stocks and oil prices or currencies are shifting and losing their impact in what many of the bulls see as a new market paradigm."

(Newsmax wire services contributed to this report).

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