Risk assets may face stormier conditions if the Federal Reserve cuts interest rates, as expected, on Sept. 17. That's the message from futures tied to the VIX index, a measure of expectations of volatility in the S&P 500 over the next 30 days.

The index, also called Wall Street's fear gauge, is calculated in real time from prices of options on the S&P 500, and reflects how much investors expect the market to swing, with higher values indicating greater levels of uncertainty.

The spread between the October VIX futures contract (the next-month contract) and the September contract (the front-month contract), has widened to 2.2%, an extreme level by historical standards, according to data source TradingView. The September contract expires the same day as the Fed meeting.

Meanwhile, the fron

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