Shoppers hit the malls for back-to-school shopping in August and handed retailers a healthy boost despite continued worries about the job market and a sluggish economy.

Nordstrom led the way with a 21 percent sales jump, nearly double what Wall Street expected. The Seattle-based retailer said a later start to its anniversary sale pushed some promotions into August, helping results.

For the combined July and August period, sales at Nordstrom stores open more than a year rose 8.4 percent.

Major chain stores posted a 3.6 percent sales increase in August compared with the same month a year earlier, outpacing analysts' expectations of a 2 percent rise, according to Thomson Reuters' tally of 16 retailers.

"Consumer spending increased for the first time in three months," said Chris Christopher, an economist at IHS Global Insight, "providing evidence that after hunkering down for two consecutive months there are signs of life on the consumer front."

Issaquah-based Costco Wholesale reported a 6 percent sales gain, beating Wall Street's expectations of 4.5 percent growth.

Lynnwood-based clothing chain Zumiez posted a 3.7 percent increase.

Off-price retailers Ross Stores and TJX Cos. both reported a strong 8 percent gain. Limited Brands, parent company of Victoria's Secret and Bath & Body Works, also saw sales rise 8 percent.

Other retailers posted weak results. Struggling teen clothier Wet Seal, which recently ousted its chief executive and is facing a discrimination lawsuit, said sales plummeted 18.3 percent.

August has traditionally been the core of the back-to-school shopping period, the second-most-important time of the year for retailers (after the winter holidays), accounting for more than 10 percent of the industry's annual sales. But this year, many parents and kids say they are waiting until after school starts to shop in order to pick up the best deals and buy the right trends.

Results are based on sales at stores open at least a year, known as same-store sales and considered an important measure of a retailer's health because it excludes the effect of stores' openings and closings.

Seattle Times business reporter Amy Martinez contributed to this report.