NEW YORK (AP) — June sales heated up for stores.

Costco Corp., Fred's Inc. and Stein Mart Inc. were among the retailers that reported their strongest sales gains since January, as shoppers, enticed by warm weather and an improving economy, took advantage of summer discounts.

Revenue at stores opened at least a year — an industry measure of a store's health — rose 3.9 percent in June compared with the same month a year ago, according to a preliminary tally of 12 retailers by the International Council of Shopping Centers. The mall trade group had expected an increase of 3 to 3.5 percent.

The data, released Thursday, offers positive signs for the back-to-school season, which is the second-biggest shopping period behind the winter holidays. June is when stores clear out summer merchandise to make room for goods for fall, so brisk sales mean that stores likely won't be stuck with piles of summer shorts and T-shirts that they need to get rid of.

June's performance was its best performance since January's 4.5 percent gain and showed a gradual improvement since early this spring. The tally was up 3.4 percent in May and 3 percent in April.

Hot weather was a big factor in June that helped shoppers get into the mood of buying tank tops and shorts. But stores also benefited against an improving economic backdrop. The economy gained a robust 195,000 jobs in June. Employers have added an average 202,000 jobs for the past six months, up from 180,000 in the previous six. The housing market is also gaining strength. And consumer confidence in June is at the highest level since January 2008, according to the Conference Board.

"The reports are encouraging," said Michael P. Niemira, chief economist at the International Council of Shopping Centers. "We had seen consumers pull back a littler earlier this year, but now there's a willingness to spend. It adds to the flavor of the other economic data out there that looks better."

Stein Mart, a department store chain that sells lower priced merchandise, said June revenue at stores open at least a year climbed 6.5 percent, driven by strong sales of women's clothing. analysts had expected a 4 percent increase

Stein Mart had the biggest gains in women's casual sportswear and boutique items. Weaker areas were men's sportswear, women's accessories and plus-size clothing.

Costco also was among the retailers that posted a revenue increase in June. The wholesale-club operator said revenue at stores open at least a year climbed 6 percent, topping Wall Street's expectations. Analysts surveyed by Thomson Reuters predicted a 5.4 percent increase in the figure.

While big chains such as Wal-Mart Stores, Target Corp. and Macy's Inc. no longer report monthly revenue, the stores that do offer economists a snapshot of consumer spending habits. In total, the retailers that report monthly data represent about 6 percent of the $2.4 trillion in U.S. retail industry sales.