"For Sale" signs may seem like an eyesore to neighbors on any given local street, but the lack of them is a much bigger problem.

Just 1.82 million homes were listed for sale in December, according to the National Association of Realtors. That is a 22 percent drop from a year ago and the lowest supply since May of 2005, when words like "boom" and "bubble" followed the word "housing." At the current sales pace it would take just 4.4 months to sell those homes.

"The greatest concern in the market is the inventory situation," said Lawrence Yun, chief economist for the NAR. "Even if we see an increase in the Spring and Summer, if home sales hold at the [current] level or even a 5 to 6-month supply, price increases are guaranteed. We don't want to see rapid appreciation in prices faster than income."

The reasons for the low supply are varied, and the low numbers are in fact feeding on themselves. If potential buyers can't find something to their liking, they will probably not list their homes for sale.

There are also still 10.7 million borrowers who owe more on their mortgages than their homes are worth, according to the latest report from CoreLogic. An additional 2.3 million have less than five percent equity in their homes, referred to as near-negative equity. Most of these homeowners are stuck in place, unable to sell unless they can afford to pay in to their mortgages. As for new supply, even though builders are increasing starts, they are still not even at half the pace they were at the height of the housing boom.

As a result, home prices are now rising more and faster than most analysts predicted due to this short supply, up 7.4 percent year-over-year in November, according to CoreLogic. They are especially surging in some of the hardest hit markets from the housing crash, where large-scale investors are swarming with cash in hand. In Phoenix, home values jumped nearly 32 percent from a year ago in November and are now at the highest level since October of 2008 according to DataQuick. While still 39 percent off their boom-high in June of 2006, they are now up 41.5 percent from the bottom, and there is not much on the market.

Healthy housing market gains are historically driven by increasing employment and income, not by lack of supply; the latter leads to price bubbles. First-time home buyers, who generally account for 40 percent of the home-buying market or higher are still under-represented at just 30 percent, according to the Realtors. This is due to tighter credit conditions in the mortgage market and now decreasing affordability.

December's disappointing drop in home sales, month-to-month is a clear warning for the housing recovery going forward. Rising home prices are not the sole measure of a healthy market. Supply and demand need to fall closer in line, and a robust economic recovery should be driving both home sales and prices.

Source CNBC —By CNBC's Diana Olick;

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