According to economist Nouriel Roubini, the housing market is in a double dip.

Photo: Oliver Quillia for CNBC Nouriel Roubini

And negative Case-Shiller Home Price numbers out today only confirm that unpleasant truth.

"It's pretty clear the housing market has already double dipped," says Roubini. "And the rate of decline is stronger than in previous months," he said of the new housing data.

Aside from below trend economic growth, there are two factors specific to the housing market that are putting downward pressure on home prices.

The first factor is the expiration of federal home buyer tax credits for first time home buyers.

"If you look at the data, Case Shiller has been falling every month since the tax credit expired in May. Everyone who wanted to buy a home did so by April," Roubini said.

"That tax credit stole demand from the future and its expiration led to another 30% fall in home sales, pushing Case & Shiller lower for the last few months," Roubini wrote in a text message earlier this morning.

The second factor putting downward pressure on home prices is the ongoing chaos with mortgage documentation, and the consequent suspension by banks of mortgage foreclosure proceedings—which has actually worsened the underlying problems in the housing market.

"There has been an effective moratorium on foreclosure," said Roubini.

And the beginning of the end of that moratorium means more housing supply is about to become available on the market.

"The shadow inventory of not-yet-foreclosed homes—due to the moratorium—will surge in the next year," Roubini says.

Both factors, taken in concert, set up a scenario where market fundamentals put downward pressure on prices: "Supply will increase, demand will drop," Roubini said.

The Case Shiller Composite-20 Index, which represents the broadest measure of U.S. home prices in the survey, fell 1 percent on an adjusted basis during the September/October time period, based on data release earlier today.

All 20 Metropolitan Statistical Areas included in the survey showed declines—reflecting a broad based, non-regional erosion of prices in the housing sector.

But Roubini isn't yet predicting a double dip recession for the broader economy.

"The rest of the economy is recovering. Most of the numbers are consistent with a growth rate of 2.7 percent," Roubini said.

But that 2.7 percent growth is still below trend. "So unemployment will likely remain above 9 percent," according to Roubini's analysis.

Roubini adds that there are other ominous economic signs on the horizon including: "The eurozone shock, long-term structural deficits, and state and local governments [operating near] bankruptcy."

And, if homeowners begin walking away from their properties en masse, those negative trends might well pick up steam:

"12 million households are already in negative equity and 8 million more have an LTV btw 95 and 100%. Thus even a 5% fall in home price will push an extra 8 million in negative equity with risk of millions walking away from their home—i.e. jingle mail," Roubini wrote me in a text message earlier today.

It's certainly a sobering scenario to contemplate as we head into the New Year.

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