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 in 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.

Here's a glance at the numbers: Download Caseshillerjuly09

The cities with the biggest annual housing plunges were clustered in the Sun Belt and industrial Midwest: Las Vegas, Phoenix, Detroit and Miami.

I talked to Craig Beggins, a big time Realtor whose brokerage dominates southeast Hillsborough County. He said sales peaked in July, slipped a bit in August, and fell off a mini cliff in September.

"Usually October is the drop off," 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."

Prices may continue to improve modestly if foreclosures make up a smaller proportion of sale here on out. Removing distressed properties from the mix could inject a little helium into housing values. That's the theory anyway.