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%. Related articlesHow will the S&P 500 react to today's FOMC? JPM lays out the playbookKoch said to explore sale of data center firm Edged for over $15BMarket watchers weigh chances of a surprise Fed rate hike tomorrow