JPMorgan has outlined its S&P 500 scenarios ahead of the upcoming Federal Reserve rate decision. The firm's base case, with a 50% probability, is a hawkish hold on rates, which it expects would leave the S&P 500 roughly flat to down 0.5%.

A dovish hold, assigned a 28% probability, is considered the most bullish outcome, potentially lifting the index up to 1%. Conversely, a 25 basis point rate hike, with a 20% probability, could send the S&P 500 down 1.5% to 2%, with the Nasdaq likely underperforming.