Housing markets clustered in California, New Jersey and Illinois are facing the greatest risk of downturns while less vulnerable markets are spread throughout the South, Midwest and Northeast, according to a recent report from real estate data provider ATTOM.

ATTOM weighed gaps in home affordability, underwater mortgages, foreclosures and unemployment for its Special Housing Risk Report.

"Some parts of the country continue to pop up on the radar as places to watch for signs of housing market drop-offs based on key quarterly measures," ATTOM CEO Rob Barber said. "Once again, it is important to stress that getting onto the most vulnerable list doesn't signal an imminent crash for any local market. It just means that they have greater potential tripwires that could lead to a decline. Those remain areas to watch, especially given the overall varied trends in the market."

The metropolitan areas surrounding Chicago, New York City and Central California are the most vulnerable. The 50 most at-risk counties included three in New York City, six in New York City suburbs, seven in the Chicago metropolitan area and five in central California.

At least 5% of residential mortgages were underwater in the third quarter in 30 of the 50 most-at-risk counties. Nationwide, 5.2% of mortgages fell into that category, with homeowners owing more on their mortgages than the estimated value of their properties. Counties with the highest underwater rates were Web County (Laredo), Texas at 56.6%; Tangipahoa Parish, Louisiana east of Baton Rouge, 24.3%; Philadelphia County, Pennsylvania, 17.4%; St. Clair County, Illinois, outside of St. Louis, 15.3%; and Peoria County, Illinois, 14.3%.

The highest foreclosure rates among the top 50 counties were in the New Jersey counties of Cumberland, Warren, Sussex, Gloucester and Camden.

The unemployment rate in August was at least 5% in 35 of the most at-risk counties while nationwide it was 3.9%. The highest rates were in Merced County, California at 8.9%; Kern County, California, 8%; Cumberland County, New Jersey, 7.3%; Bronx County, New York, 7.2%; and Madera County, California, 7%.

The one measure among the four key factors that varied little between the most- and least-at-risk counties was home affordability. Major ownership costs such as mortgage payments, property taxes and insurance on median-priced single-family homes and condos consumed more than one-third of average wages in 38 of the 50 counties considered most vulnerable.

The highest percentages were in Kings County, New York at 109.9% of average local wages needed for major homeownership costs; Riverside County, California, 71.8%; El Dorado County, California, 70.1%; Bergen County, New Jersey, 68.8%; and Richmond County, New York, 68%.