WASHINGTON — Existing-home sales in Frederick County, Md., rose at a year-over-year clip of 57 percent in July, more than tripling the U.S. rate and doubling the Mid-Atlantic region’s rise.

The county’s one-year spike from July 2012 beat the national rise of 17 percent and the regional increase of 24 percent, according to Real Estate Business Intelligence LLC. But Frederick County’s 340 closed sales in July were a 0.9-percent slide from the month prior and fell short of the 6.2-percent national rise over June closings.

“We’ve seen that the surge of pent-up demand, but people are still worried about the economy,” said Frederick County Association of Realtors CEO Steve Jarvis. “It’s not like we have a robust market where everybody wants to move from this place to the next.”

Nationally, July’s seasonally adjusted annual rate of 5.39 million sales, which beat expectations by 5.7 percent, marked the 25th straight month in which year-over-year sales numbers rose, according to the National Association of Realtors.

Housing prices are likely to go up as the market recovers and jobs are added to the economy, said National Association of Realtors Chief Economist Lawrence Yun.

“Mortgage-underwriting standards should normalize over time from current stringent conditions as default rates fall,” he said.

Frederick County’s 887 active listings in July were up 6.9 percent from a year ago. But the county’s 693 detached listings — freestanding residential buildings — constitute just a two-month supply, Jarvis estimated.

“Most people want choices,” Jarvis said. “They want to go to three homes they’d be interested in. All of a sudden, there are not enough attractive properties in whatever price or size class they’re looking at to make an educated choice.”

Jarvis noted that short supply is “an awfully good problem to have,” and he predicted that dwindling listings would encourage once-cautious sellers to enter the market.

Nationally, July listings dipped 5 percent year-over-year to reach 2.3 million homes, a 5.1-month supply.

“Tight inventory in many areas means above-normal price growth for the foreseeable future,” Yun said in a statement.

Frederick County houses sold at a median price of $282,750 in July, a 12-percent jump from 2012 but 3.5 percent lower than June.

“If you have a house of $150,000 or less, it doesn’t last long at all,” Jarvis said. “If you get over $400,000, then it takes the right person, and you might sit on the market a little bit.”

Frederick County’s July closings stayed on the market for an average of 55 days, down 19 percent from a year ago. Thirty percent of homes sold in 10 days or fewer.

“That’s amazing,” Jarvis said. “That’s those right-priced homes. That middle range — $150,000 to $300,000 are really hot.”

July marked the 17th straight month in which national median existing-home prices rose year-over-year, this time reaching $213,500, 14 percent higher than a year ago.

In the Mid-Atlantic region last month, the median home price was $323,000, up almost 8percent from a year ago but down 2.1 percent from June.

July rates for a 30-year, conventional, fixed-rate mortgage rose from 3.35 percent last year to 4.37 percent this year, the highest they’ve been since July 2011, according to Freddie Mac.

“We’ve seen mortgage rates go up almost a full percentage point over the summer,” Jarvis said. “That’s thrown a little bit of caution out there and taken a few people out of the market who might have been willing to do something.”