Rocky September is ahead, warns BlackRock strategist

It is a common expression on Wall Street: "Sell in May and go away." But BlackRock Chief Investment Strategist Russ Koesterich says that this year, the month for investors to avoid might be September.

On Tuesday's "Futures Now," he presented four big reasons why next month could bring a great deal of volatility.

Reason one: The calendar screams "Sell!"

First, Koesterich says that when you buy in September, seasonal trends are against you.

"We don't pay a lot of attention to the calendar, but it is worth it in September," he said. "September is the only month of the year that, statistically, has a very significant bias. And unfortunately, that bias is negative."

(Read more: Three reasons the market is peaking: Doug Kass)

While the market rose in September 2012, in general, the market has only risen in September about 40 percent of the time, according to Koesterich. Compare that to the 60 percent of the time that the market tends to rise in the other 11 months.

Still, Rich Ilczyszyn of iiTrader says that buying in September is generally a good call. "The market tends to drift higher toward the end of the year," he explained.

Reason two: Fed anxiety will increase

When asked for the most important day for the rest of the year most traders point to Sept. 18. That's when the Federal Open Market Committee will release its next statement, which will be followed by a press conference. On that day, the big question of whether the Fed will begin "tapering" down its asset purchases in September will finally be answered.

That gives Koesterich another reason to be concerned. "In the U.S., we have the Fed taper, and the volatility likely to surround that announcement," he said.

And this strategist believes that tapering is somewhat of a foregone conclusion. "The Fed is likely to start in the fall, and the only question is how much?" Kosterich said. "Are we going to see a more aggressive taper, or a taper-light? In my mind, that's the question that investors are going to start to focus on."

(Read more: Marc Faber: Look out! A 1987-style crash is coming)

Caution Tape
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Reason three: Europe could be a source of volatility

Another thing that might hurt in September? Europe.

The German national elections will be held on Sept. 22, and Chancellor Angela Merkel is seeking a third term. And as HSBC economist Stefan Schilbe wrote in a recent note, "The outcome is eagerly awaited, not least because euro zone decision-making has effectively been on hold."

The main opponent of Merkel and her Christian Democratic Union party is Peer Steinbrück, the candidate from the Social Democratic Party. And though Merkel leads the polls by a comfortable margin, it is conceivable that Merkel would be forced to form a coalition with the more liberal Social Democratic Party.

This could improve the cohesion of the euro zone. As Jim O'Neill, the ex-chairman of Goldman Sachs Asset Management, noted in the London Evening Standard, "A German government involving a left-of-centre group would probably be more inclined to be lenient on fiscal challenges everywhere, as well as perhaps more supportive of things like genuine euro-denominated bonds."

(Read more: Siegel: Keep buying—you 'can't lose')

Reason four: The U.S. budget debate will heat up

Koesterich's fourth reason for September volatility comes from D.C. In September. "We also have the budget battle, which was not solved, so much as it was put off," he said.

The government's fiscal year ends Sept. 30, which is when the continuing resolution that funds the government expires. That means that unless another budget is approved, there will be a government shutdown—an event that some say is increasingly conceivable.

And regardless of whether or not a shutdown actually come to pass, the potential brinkmanship leading up to the deadline could be very hazardous for the market.

—By CNBC's Alex Rosenberg. Follow him on Twitter: @CNBCAlex.

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