Tampa Bay home prices rose 1.4 percent from June to July, reflecting a broad national upturn in housing values over the summer, according to the S&P Case-Shiller home price index.

The question now is whether the modest monthly price increase is sustainable. Already, Tampa Bay Realtors are reporting a sales slowdown in August and September after a fairly active July. Case-Shiller's numbers lag Realtors' by a month.

"These figures continue to support an indication of stabilization in national real estate values," said David Blitzer, chairman of the index committee at Standard & Poor's. "But we do need to be cautious in coming months to assess whether the housing market will weather the expiration of the federal first-time buyer's tax credit in November, anticipated higher unemployment rates and a possible increase in foreclosures."

Tampa Bay home values have plummeted 40 percent from their peak in July 2006. From July 2008 to July 2009, home prices dropped 18.4 percent locally, Case-Shiller said.

Among the 20 cities on the index, those with the biggest annual housing plunges were clustered in the Sun Belt and industrial Midwest: Las Vegas, Phoenix, Detroit, Miami and Tampa.

The Tampa Bay area's 1.4 percent month-to-month home price increase still lagged the 1.6 percent national average increase. From June to July, Las Vegas and Seattle suffered the only price declines.

Craig Beggins, a Realtor whose brokerage dominates southeast Hillsborough County, said home sales peaked in July, slipped a bit in August and fell off a small cliff in September.

His agents sold 110 homes as of Monday, 44 percent below the 197 they sold in July. That could foreshadow signs of weakness in upcoming Case-Shiller reports.

Case-Shiller gets kudos for accuracy because it compares repeat sales of individual homes in a market.

"Usually October is the dropoff," Beggins said. "We're trying to figure out what's causing it. Is it back to school? Is it swine flu? We're seeing a lack of customers."