The bond market is warning that trouble could be on the horizon, either from an economic slowdown or an eventual recession. The yield curve, a set of interest rates watched closely by bond market pros, has gotten to its flattest level since before the financial crisis. The spread between 2-year note yields and 10-year yields this week reached near the lows, at about 0.75, it has been since before the financial crisis. "It certainly is giving you some sort of signal in here. The signals are when the yield curve flattens, it tells you that inflation is not a problem and the Fed is doing something at the front end," said David Ader, Informa Financial Intelligence chief macro strategist. "Historically, it signals a slowdown or recession." But with the Federal Reserve set to raise interest rates in December, and uncertainty about there are also other concerns in the market, including that a new Fed head could be more hawkish and set the Fed on a more rapid rate-hiking course. "It's also telling you there could be a policy error in the Fed's hiking particularly if they accelerate it," said Ader. Bank of America Merrill Lynch's monthly fund manager is a central bank policy misstep.

Some strategists say ignore it at your own peril, but others point to the fact that stocks can still rally when longer-duration interest rates are low but the short-term rate is rising. The fear is that a flattening yield curve could lead to an inversion, meaning the short-end rate would actually go higher than the longer-end yield. That is typically viewed as a recession signal, and the flattening curve is a warning of that. "Typically you eventually get to a much flatter yield that could lead to a recession," said Peter Boockvar, chief market analyst with the Lindsey Group. "Right now it's hard to get to inversion because of how actually low short-term interest rates are. You don't need to get to the inversion this time." Jeff Gundlach, CEO of DoubleLine, weighed in on Twitter, pointing out that stock market bulls point to low rates as a positive, yet rates are climbing. The was at a new nine-year high Wednesday, touching 1.57 percent, while the was at 2.34 percent. Tweet Strategists say years of quantitative easing by global central banks and extreme low interest rates cast doubt on some of the conventional wisdom about bond behavior. For instance, the Fed is also slowing down its purchases of Treasurys, mostly at the short end, and that could be influencing the behavior of the curve. "I think it's the expectation that further Fed tightening, whether it's on the short end or it's the quantitative tightening, is eventually going to slow the U.S. economy, and that's what the yield curve is saying, while the stock market is drunk on hopes for tax reform," Boockvar said. At the same time, expectations for a Fed rate hike at its December meeting continue to rise.

Dallas Fed President Rob Kaplan said the low rate of the 10-year may not be because of easy financial conditions. "That may be a sign of worry about future growth," Kaplan told reporters after participating on a panel with New York Fed President William Dudley about regional economic trends. Source: Strategas Research Todd Sohn, technical analyst at Strategas, looked at the behavior of the stock market during periods of flattening yield curves, and he found that until the curve actually inverted, stocks performed very well. In some cases, it took awhile for stocks to react when the curve inverted. "It's on our mind," he said. "But until you get the inversion I don't think we should put too much weight on it. Equity performance is still positive." Sohn said as the curve flattened between August 1977 and August 1978, for example, the gained 7 percent. But after the curve inverted in August 1978, the S&P corrected, falling about 14 percent from September to mid-November. As the curve flattened between July 1988 and January 1989, the S&P was up 9 percent. But Sohn said after the curve inverted in January 1989, the S&P went uninterrupted until October 1989, when it corrected about 10 percent through February 1990. Then it saw a 20 percent correction from July 1990 to October 1990. "The curve inversion in June 1998 saw a sharp 19 percent S&P correction from mid-July 1998 and the end of August 1998… before the race higher into the March 2000 peak," he noted. Just ahead of the financial crisis, the curve inverted in January 2006. There was a shallow 8 percent correction from May to June 2006, and stocks moved higher until October 2007. "It's very case-by-case but curve inversion does typically lead to some form of a correction," Sohn noted. "We're not there yet but just something worth keeping in mind."

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