It's a stunning track record for U.S. stocks. The S&P 500 Index (SPX) reached 24 new record highs and rose nearly 9 percent this year through the open of this week's trading while the Dow Jones Industrial Average (DJIA​) and Nasdaq Composite (IXIC​) also shattered records.

6 Warning Signs

But stocks may face major turmoil in the second half of 2017, Barron's reports. As evidence that "growth might be scarce," Barron's cites a combination of dipping commodities prices, declining bond yields, crowded investing in big tech stocks, rallies in emerging markets and the skyrocketing price of Bitcoin. Bank of America Merrill Lynch, a division of Bank of America Corp. (BAC), also sees a worrisome "big top" in the market this fall, as both liquidity and profits peak, according to the Financial Times. Also, the S&P 500 in the second quarter is expected to get the vast majority of its earnings gains from energy stocks, which are beginning to feel the pain of falling oil prices.

Adding to the concerns of cautious investors is the fact that the market has shot upward since the election without any deep, sustained reversals. The biggest decline in the S&P 500 so far this year was a 2.8% drop, the second-smallest first-half retreat in 89 years, according to Bespoke Investment Group, which calculates the historical average as 11.2%, per Barron's.

Popular technology shares that have led the market upward may be among the most vulnerable, including FANG stocks Inc. (AMZN), Facebook Inc. (FB), Alphabet Inc. (GOOGL​) and Netflix Inc. (NFLX​).

Slowing GDP Growth

Tumbling oil prices are, of course, bad news for energy companies and energy stocks, but they are good news for consumers and for companies using energy inputs. So far, at least, the declining price of oil is attributable to a supply glut, Barron's notes. (For more, see also: Why Oil May Plunge to $30 a Barrel.)

However, the prices of other industrial commodities, such as iron ore, also are falling, Barron's adds. These may be signals of weakening demand. Bank of America Merrill Lynch cut their 2018 growth forecast for U.S. GDP from 2.5% to 2.1%, per the June edition of their U.S. Economic Weekly report. The firm cites fading hopes for tax reform and fiscal stimulus as the major reason for their lowered outlook.

Inflationary expectations are falling, which can be a plus for stocks, Barron's says. However, yields on the 10-year U.S. Treasury Note are sinking, flattening the yield curve. This also may signal weakening economic growth in 2018, per observations from Thomas Lee of Fundstrat Global Advisors, cited by Barron's.

At JPMorgan Chase & Co. (JPM), Marko Kolanovic, the global head of macro quantitative and derivatives research, is quoted by the FT as saying that  "the current macro environment does not warrant all-time low volatility." Also, Barron's quotes him commenting that "While we don't know when the next recession will happen, every Fed rate hike is bringing us closer to it." Moreover, Kolanovic notes that, as the Fed and other central banks reduce their balance sheets, a major prop for the prices of financial assets will be withdrawn, and this is likely to produce market turmoil and increased volatility, per the FT. (For more, see also: Bill Gross: QE is "Financial Methadone.")