Tax-credit withdrawal syndrome continued to dog the housing market in September as price declines accelerated from August, according to the S&P/Case-Shiller Home Price Indices released Tuesday.

The 10-City Index dropped 0.5% in September from August, the 20-City declined 0.7%, leaving the former with a 1.6% year-over-year gain and the latter with a 0.6% rise. As recently as May, those annual gains were 5.4% and 4.6%, respectively.

Since the market peak in spring, 2006, the 10-City is now down 28.7% and the 20-City down 28.6%.

The U.S. National Home Price Index, which covers all nine census divisions, fell 1.5% during the third quarter of 2010 versus the same quarter a year earlier. Prices nationally were back to mid-2003 levels in September.

"Another weak report; weaker than last month," said David M. Blitzer, chairman of the index committee at S&P. "Overall, there are few, if any, good numbers in this month¹s data."

Prices were down in all but two markets, Las Vegas (+0.1%) and Washington, D.C. (+0.3%) on a month-to-month basis. The biggest losers were Cleveland, down 3% on the month and 1.9% during the previous 12 months; Minneapolis, down 2.1% and 1.2%, respectively; Portland, down 1.9% and 3.6%; Dallas, down 1.6% and 2.6%; Phoenix, down 1.5% and 1.9%; and Chicago, down 1.5% and 5.6%.Among others, Boston dropped 1.3% but remained 0.4% ahead of September, 2009; Detroit (with an index approaching 70, which means homes there have lost 30% of their values in 2000), fell another 1.3% and 3%, respectively; Miami declined 1.2% and 2.7%, respectively. Atlanta, Charlotte, Denver, and San Diego all dropped 1% from August, while, year-over-year, Atlanta was down 3.1%, Charlotte down 3.7%, Denver down 1.6% and San Diego up 5%. Tampa hit a new index low of 136.45 with a 0.8% drop month-to-month and a 4.3% decline year-over-year.

There are now only four markets in the 10- and 20-City Indices that are showing year-over-year gains, with San Francisco now in the top spot with a rise of 5.5%, San Diego of 5%, Washington of 4.5%, Los Angeles of 4.4% and Boston of 0.4%. New York, with a 0.3% month-to-month drop, was 0.1% off September, 2009. All other markets have losses of more than 1% from last September.

"While some of the bad numbers may reflect the end of the government's tax incentive for first-time homebuyers, there are other problems weighing on the housing market," said Blitzer. "The national economy is certainly the number one issue for housing. Additionally, there is a large supply of houses on the market and further, hidden, supply due to delinquent mortgages, pending foreclosures or vacant homes. New construction is running at less than half the pace needed to meet normal demand, so a sustained recovery could be a ways off."

Blitzer also predicted that "many analysts will argue that a double dip will be confirmed before Spring."