Following the midterm elections, investors have pivoted their focus to Thursday's October consumer price index report, which could dramatically shake markets depending on the data, according to JPMorgan.

The bank's trading desk ran a scenario analysis of where it expects the S&P 500 to trade depending on the CPI report. Economists surveyed by Bloomberg expect it to show a year-over-year headline inflation increase of 7.9%, which would be a decline from September's 8.2% reading.

JPMorgan also sees a 7.9% reading as the most likely scenario, ascribing a 40% probability for the outcome, and that would be good news for investors as the bank expects the S&P 500 to rise as much as 1.5%. 

"I think bonds, and thus stocks, take this as a small positive since it meets expectations and does not reprice yields higher," JPMorgan's Ellen Wang and Andrew Tyler wrote. Any improvement in inflation from last month would give investors some confidence that the Fed may be closer to a slowdown in its rate-hike cycle.

But things would get dicey quickly if October's CPI report comes in above expectations, with the bank expecting a 30% chance that headline CPI prints 8.1% to 8.3%. If that scenario plays out tomorrow, the S&P 500 could plunge between 2% and 3%.

The least likely scenario happening, according to JPMorgan, is a huge beat or miss in the October CPI report. 

"Seeing a stepdown in inflation of this magnitude likely pulls the 10-Year yield below 4% (currently 4.158%) and triggers a sharp rally in stocks. This may also reset the yield curve lower with terminal rate expectations falling under 5%," JPMorgan said about its most-bullish scenario.

The October report "could be as much [as] a 'game changer' as 'hot' August CPI derailed S&P 500," he said.