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U.S. home values continue to appreciate, according to a new study released today, but the rate of appreciation is slowing.

The Zillow Home Value Index rose to $157,600 as of the end of the first quarter, up 5.1 percent from a year ago and up 0.5 percent from the fourth quarter of 2012.

The index is the median value for a given geographic area, on a given day, and includes the value of all single family residences, condos and coops, whether or not they sold within the period. It is seasonally adjusted.

U.S. home values in March rose for the 16th consecutive month-although the rate of appreciation slowed for the second consecutive month. Home value appreciation in the first quarter was 0.5 percent, versus 2.1 percent for the quarter before.

Historically, housing markets enjoy value appreciation of about 3 percent a year, according to Zillow's chief economist, Dr. Stan Humphries. Zillow compiles and analyses real estate data from some 350 U.S. markets.

Markets that have seen the greatest appreciation in home values include five metros that showed year-to-year appreciation of more than 20 percent: Phoenix (up 24 percent), Las Vegas (up 23.3), San Jose (up 22.1), San Francisco (21.4) and Sacramento (20.1).

Markets that saw a decrease in appreciation include New York (down 0.3 percent) and Chicago (down 1.4 percent, the biggest drop of any in the study). St. Louis and Charlotte also saw modest drops.

Those drops, Humpries tells ABC News, can be attributed in part to the fact that Chicago and New York are in states where foreclosures require judicial review-a time-consuming process that, he says, "impairs recovery."

Over the past year, says Humphries, the national housing market has rebounded strongly, even though "the sometimes dramatic run-ups during these months were never expected to be sustainable. Recent slowdowns are indicative of a market that is slowly finding its natural level."

Looking forward, he says, he expects annual value appreciation to continue to slow, as more inventory comes up for sale. But pockets of very rapid appreciation, he says, will remain "a troubling sign of volatility and a potential future headache, as affordability is compromised and homes begin to look much more expensive to average buyers."

Already, he tells ABC News, "San Jose and San Francisco are starting to look expensive again. Prices there are back above their peak levels. Palo Alto and Cupertino are at or near peak levels again."

Las Vegas and Phoenix, by contrast, remain "extraordinarily affordable," he tells ABC, still down about 60 percent from their peak.

He says the two have similar "demand profiles," with both markets having a large number of retirees and second-home buyers. Both are priced low enough to attract speculators-not buyers looking to "flip" homes, but buyers wanting to enjoy positive cash flow by buying depressed properties for rental income. With rents trending up 5 percent year to year, he says, buyers in these two markets "are chasing opportunity."