JPMorgan Chase & Co. sees the S&P 500 reacting sharply to how Federal Reserve Chair Kevin Warsh communicates Wednesday's rate decision, even though a quarter-point hike is largely priced in. The bank's scenarios range from a 1% gain to a 2% decline, with an unexpected decision to leave rates unchanged potentially causing a 1.25% to 1.75% drop. Polymarket traders assigned an 89% probability to a 25-basis-point hike and 11% to no change, with about $193 million traded on the outcome. JPMorgan says a hold could undermine confidence in the Fed's inflation-fighting resolve and push longer-term Treasury yields higher as investors seek greater compensation for inflation risk. The 10-year Treasury yield is around 5%.