Often considered an afterthought compared to the juggernauts of economic indicators — such as theConsumer Price Index(CPI) inflation andjob market report—retail sales datararely moves the needle for Federal Reserve policymakers.

But this Tuesday, the August retail sales numbers could mean something as they land just before the Fed's two-dayFederal Open Market Committee(FOMC) meeting, where a pivotal interest rate decision is on deck for Wednesday.

A rate cut on Wednesday is already a done deal; the real uncertainty is how big that will be. The markets are currently placing their bets on a...