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.

The bank’s economist Michael Feroli expects the Fed to leave rates on hold, "with at least 2 hawkish dissents," including from Hammack and Logan.

The firm's Market Intelligence desk said the chance of a hike is lower than the market implies, at about 30%, citing U.S. GDP growth near trend with upside risks and inflation that, while elevated, does not appear "at-risk of an upside explosion."

It added that if the Fed were going to hike, June looked more appropriate, when CPI was above the fed funds rate.

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. A 50-basis-point hike and an outright cut were each assigned just 1%.

The firm noted options expiring July 29 are pricing a move of roughly 0.8%, based on July 28 prices, below the roughly 1.1% typically priced into recent CPI events.