By Dirk van Dijk of

NEW YORK ( TheStreet) -- In September, home prices continued to slip, and the declines were very widespread. The Case-Schiller Composite 10 City index (C-10) fell 0.67% on a seasonally adjusted basis, and is up just 1.52% from a year ago. The broader Composite 20 City index (which includes the cities in the C-10) fell by 0.80% on the month and is up 0.55% from a year ago.

In August, the year-over-year gains were 2.50% for the C-10 and 1.61% for the C-20, so it looks like the year-over-year gains are rolling over. Of the 20 cities, only one (Washington DC, and it was only up 0.05%) posted a gain on the month, while 19 saw prices fall. Year over year, five metro areas saw gains and 15 suffered losses. In August, there were also 19 down and just one up. It thus looks like a new downtrend in housing prices is under way.

Consider Seasonal Adjustments to Prices

There is a seasonal pattern to home prices, and thus it is better to look at the seasonally adjusted numbers than the unadjusted numbers. Most of the press makes the mistake of focusing on the unadjusted numbers.

While the 0.55% rise in the C-20 year over year in isolation is not the end of the world, it hardly makes up for the damage that was done in the popping of the housing bubble, and it is also unlikely to last. From the April 2006 peak of the housing market, the C-10 is down 29.83%, while the C-20 is off by 29.56%.

The Case-Schiller data is the gold standard for housing price information, but it comes with a very significant lag. This is September data we are talking about, after all, and it is actually a three-month moving average, so it still includes data from July and August.

Existing home sales have been weak since the home buyer tax credit expired. In the process, the inventory-to-sales ratio has been extremely high, at 10 months, although that is down from the June peak of 12.5 months. That is what we saw during the implosion of housing prices that took place in 2007 or 2008. Housing prices are going to fall again in the coming months.