Why does the most powerful financial institution on the planet expect another 15% market decline?

The big banks can be considered some of the most powerful institutions on the planet, influencing world affairs and the lives of millions of people without exaggeration. And their opinions carry enormous weight. The biggest of them now comes with a warning that there will be a dramatic market downturn and a new bottom.

Last month, the S&P 500 index hovered mostly around 4,100 points, without a significant breakout in either direction. But J.P. Morgan $JPM 's chief strategist Jason Hunter thinks change is coming. And not in a positive direction.

The S&P 500 has been moving in a fairly modest range

"We continue to expect a sell-off from this area to retest intermediate-term support at the December low of 3764, and we expect an eventual pullback to key levels near 3500 by early summer," Hunter said recently.

"We believe this will eventually set a cycle bottom, especially if this weak performance is accompanied by an impulsive steepening of the yield curve. Until such a setup emerges, we…