U.S. retailers reported mixed August sales results after Hurricane Irene drove away business at some chains and boosted it at others.

The final tally on Thursday, based on reports from 23 retailers, showed that sales at stores open at least a year rose 4.4 percent in August, just shy of the 4.6 percent rise analysts expected.

The results raise the possibility that certain back-to-school sales are lost for good.

Chains were evenly split between those that beat expectations and those that missed. For a graphic on same-store sales, click http://r.reuters.com/hep53s

You can't be over-exuberant about it, but I think you're seeing steady progress, said Barbara Kahn, a marketing professor at the University of Pennsylvania's Wharton School.

Discount stores such as BJ's Wholesale Club Inc and Target Corp saw a lift in sales ahead of Hurricane Irene, as shoppers along the Atlantic coast stocked up on necessities like batteries, flashlights and bottled water.

BJ's said same-store sales, including those of gasoline, rose 11.5 percent, blowing past the analysts' forecast of 7.8 percent, according to Thomson Reuters.

Target beat analysts' expectations of a 3.5 percent gain with a 4.1 percent increase. The company said the pre-storm rush lifted same-store sales in August by about 0.5 percentage points and would reduce them in September by a little less than that. It sees September same-store sales up at a low to mid-single-digit rate.

While the pace of the economic recovery is uneven and uncertain, Target had solid results in the back-to-school and back-to-college categories, said CEO Gregg Steinhafel.

The discounter benefited from moves such as putting dorm-sized refrigerators next to clothing in some stores.

Costco Wholesale Corp , the largest U.S. warehouse club and the biggest retailer to report monthly same-store sales, posted a higher-than-expected 11 percent rise late on Wednesday.

August was a period like many others for Costco throughout the economic recovery, showcasing an upper middle-income consumer continuing to be fully engaged in the process of saving where applicable, Wall Street Strategies analyst Brian Sozzi wrote in a research note.

Costco also said legendary co-founder Jim Sinegal plans to step down as CEO in January.

The Standard & Poor's Retail Index <.RLX> was up 0.1 percent in late morning trading, in line with the wider S&P 500 <.SPX>.

STORM RAINS ON BACK-TO-SCHOOL

August is the peak of back-to-school shopping, the second-most important period for U.S. retailers after the year-end holiday season. Sales in recent months have held up despite weak economic indicators, offering hope for August and the rest of the year, until the hurricane disrupted shopping along the East Coast during a key weekend.

Macy's Inc , which closed more than 100 stores for all or part of last Saturday due to Irene, said the storm shaved about 1.5 percentage points from its August same-store sales. Still, it posted a 5 percent gain, topping the analysts' estimate of 4.5 percent.

We expect the hurricane's effect on sales will be substantially offset as we move through September and the third quarter, said Macy's Chief Executive Officer Terry Lundgren.

J.C. Penney Co Inc and TJX Cos Inc both missed August same-store sales estimates and blamed the storm.

TJX, the operator of the off-price TJ Maxx chain, reported a 1 percent same-store sales gain, below the 2 percent forecast. Following Irene, it said business rebounded solidly early in the September reporting period, leaving it comfortable with its quarterly sales and earnings forecasts.

Kohl's Corp , whose sales also came in lower than expected, blamed weak traffic and its stores and said it would focus more on pricing this fall to reverse this trend.

The monthly sales tally speaks to the strength of consumer spending, which accounts for roughly 70 percent of U.S. economic activity. Still, major retailers that saw brisk storm-related business, such as grocers, Home Depot Inc and Wal-Mart Stores Inc , do not report monthly results.

The storm's full sales impact will not be known until September's monthly report, since many retailers' August reporting periods ended on Saturday.

(Additional reporting by Jessica Wohl in Chicago, Phil Wahba in New York and Nivedita Bhattacharjee in Bangalore; Editing by Lisa Von Ahn)