How FOMC Decisions Impact S&P 500 Volatility
Eight days a year, a scheduled announcement compresses weeks of uncertainty into ninety minutes. Volatility models that spread risk evenly across the calendar miss this entirely.
Scheduled risk is different from ambient risk
Most volatility arrives unpredictably. FOMC decisions do not: the dates are published a year ahead, and every participant knows precisely when the information will land.
That predictability changes the shape of risk rather than its size. In the sessions before a meeting, realized volatility often falls as positioning pauses, while implied volatility for options spanning the meeting stays elevated. Risk is not absent — it is being stored up for a specific hour.
Why the decision itself is rarely the surprise
By the time a meeting arrives, the policy decision is usually well anticipated. Rate futures and prediction markets typically assign high probability to the outcome, so the announcement confirms rather than reveals.
The genuine surprises come from elsewhere: the wording changes in the statement, the updated economic projections and rate path, and the press conference. Markets frequently trade quietly through the decision and then move sharply half an hour later — which is why single-day volatility around FOMC is often back-loaded.
What it does to the probability distribution
A meeting inside your horizon makes the distribution of outcomes lumpier. Instead of a smooth accumulation of small daily moves, you get many ordinary sessions plus one with a much wider range.
This breaks the smoothness assumption behind square-root-of-time scaling. A model that spreads volatility evenly will understate the risk on the meeting day and overstate it on the surrounding days, even if the total across the window is about right.
Practical implications
Check whether an FOMC date falls inside the horizon you are modelling. If it does, treat a smooth probability cone as a floor on uncertainty rather than a fair estimate.
Reading rate-decision odds alongside a volatility model is also more informative than either alone: the odds tell you what outcome is expected, while the volatility estimate tells you how much room the market is leaving for being wrong about it.
Check the modelled range for the S&P 500 across horizons that span the next policy meeting — and see the confidence score behind it.
Explore the live probability cone →Quick answers
Why does volatility fall before an FOMC meeting?
Positioning tends to pause ahead of a known catalyst, so realized volatility declines while implied volatility for options spanning the meeting stays elevated. Risk is deferred, not removed.
Is the rate decision itself usually a surprise?
Rarely. The decision is typically well anticipated by rate futures and prediction markets. Surprises more often come from statement wording, updated projections, or the press conference.
How should a probability model handle FOMC dates?
Smooth square-root-of-time scaling understates risk on the meeting day and overstates it on surrounding days. Treat the resulting range as a floor when a meeting falls inside the horizon.