Investment bank JPMorgan Chase (JPM) Friday downgraded its expectations that housing prices will improve. The researchers now say their base home price forecast now shows at peak-to-trough a 34% decline for the Standard & Poor’s/Case-Shiller national index. That marks an additional 3% to 4% drop from fourth quarter to a bottom by the first half of 2011. “This is the first downgrade to our forecasts in the past 10 months, driven by bigger-than-expected price declines in recent months and increasing uncertainty around the supply-demand imbalance,” said analysts from the JPMorgan U.S. Fixed Income Strategy division. The revision indicates that after some gains in housing, the market may double dip (click chart below). The researchers add that home prices are expected to continue a downward trend in the spring, but they do expect to see moderate improvements in the summer, leaving overall home prices down 2% to 3% in 2011. The researchers say recent changes to the National Association of Realtors‘ home sales data may overstate actual home sales. NAR is expected to revise its figures, and JPMorgan analysts will adjust their forecasts accordingly. The glut of housing supply, mixed with tighter lending criteria mean that home prices will likely not begin to improve until more jobs are created, the research indicates. Write to Jacob Gaffney. Follow him on Twitter @JacobGaffney.