Investing.com -- JPMorgan told clients in a note that today's Federal Reserve decision is the most important catalyst during regular trading hours, mapping out how the S&P 500 could move across a range of outcomes. In its scenario analysis, JPMorgan assigned a 50% probability to a hawkish hold, its base case, in which the S&P 500 lands anywhere from up 25 basis points to down 50 basis points. A dovish hold, given a 28% chance, is "the best outcome for stocks," lifting the index 50 basis points to 1%. On the hawkish side, JPMorgan put a 20% probability on a 25-basis-point hike, which it said would send the S&P 500 down 1.5% to 2%, with the Nasdaq 100 falling harder. Related articlesHow will the S&P 500 react to today's FOMC?