Tampa Bay home sales rose 17 percent in August compared to the same month a year ago, but slackened unexpectedly from their relatively brisk pace in July.

The sales retreat caught real estate agents off guard. For most of the past several years, sales held steady between those two summer months. Economists had predicted an upturn based on the soon-to-expire $8,000 first-time home buyer tax credit.

"It's real unpredictable. It's just a weird market," said Lisa Mann, a Hillsborough Realtor who has noticed that banks take as long as six months to approve sales of distressed homes, even with the looming Nov. 30 tax credit expiration.

Bay area Realtors reported 2,370 sales in August, up from 2,029 a year earlier. The median sales price declined 17 percent in August to $144,600, compared with $173,900 a year earlier.

Those totals were compiled from Pinellas, Pasco and Hillsborough counties. Hernando County Realtors didn't file their report on time and were left out of the calculation. Including Hernando, Tampa Bay home sales in July were 2,822.

The same trend emerged nationally. The National Association of Realtors said sales fell to a seasonally adjusted annual rate of 5.1 million in August, 2.7 percent below July's 5.24 million.

National home sales had been expected to rise to an annual pace of 5.35 million, according to economists surveyed by Thomson Reuters.

Lawrence Yun, chief economist of the national Realtor group, suggested the sales drop reflected tougher appraisal standards that took effect May 1. As foreclosures flood the market, depressing prices, appraisals sometimes fail to match the previously agreed loan amount.

In Florida, home sales increased 28 percent year over year, but also gave back ground from July to August. Gainesville and Pensacola, both late arrivals to the housing slump, were the only Florida metro areas where home sales fell year over year.

Realtors are pressuring Congress to expand and extend the tax credit, but lawmakers haven't committed to propping up the housing market further. Across the nation, first-time buyers bought almost a third of homes in August. Together with investors snapping up foreclosures, they have provided most of the momentum in the market this year.

If buyers see clear evidence of stable prices, the housing market recovery can be self-sustaining, Yun said.

In the Tampa Bay area, prices have ricocheted in a narrow range since February, leading many to predict home values have already bottomed.

Low mortgage rates have helped. Interest rates in Florida have dropped below 5 percent once again after flirting with 5.5 percent in late spring.

Paul Dales, U.S. economist with Capital Economics, urged people not to get too depressed over August's dip.

"We suspect it is just a temporary blip in the improving trend," he said.

Information form Times wires was used in this report.