NEW YORK (CNNMoney.com) -- The onset of cooler temperatures, a retreat in gas prices and exciting fall merchandise in stores combined to help pump up sales at department stores and specialty clothing retailers last month.

Analysts on average expect that total September same-store sales rose 3.8 percent, slightly below the 3.9 percent gain in the same month a year ago, according to Thomson Financial.

The research firm said 70 percent of the retailers it tracks beat expectations, while 28 percent had missed.

Kohl's, Nordstrom and American Eagle Outfitters were a few of the merchants that trotted out stellar September results at their stores open at least a year - a key retail measure known as same-store sales.

Richard Hastings, senior retail analyst with Bernard Sands, wrote in a note that specialty apparel sales were probably boosted in September by the combination of "weather, better pricing and great store displays, and by the huge dip in gas prices."

"Not only are we seeing solid comparable sales numbers, but the leaders like J.C. Penney, Kohl's and American Eagle are still growing margins and earnings," Hastings said.

Department store chain Kohl's (Charts) said sales surged 16.3 percent last month. Sales at high-end seller Nordstrom (Charts) increased a strong 13.4 percent.

Penney's (Charts) sales soared 8.7 percent in September. As a result, the retailer upped its third-quarter profit guidance to $1.11 per share, an increase of 4 cents from its previous guidance. Analysts expect the company to earn $1.09 for the quarter, according to First Call.

Teen apparel seller American Eagle Outfitters (Charts) posted a 19 percent increase in its sales last month.

American Eagle also boosted its third-quarter profit guidance, citing its better than expected sales performance in the month.

Limited Brands (Charts), which operates Victoria's Secret and Bath & Body Works chains, said sales jumped 12 percent at its stores.

Ken Perkins, president of research firm Retail Metrics, wrote in a note that a solid jobs market, favorable weather conditions and declining energy prices all were favorable catalysts that put consumers in a mood to shop.

However, not everybody had reason to cheer.

Sales at No. 1 apparel seller Gap Inc. (Charts) fell 3 percent last month despite the company's aggressive new autumn marketing push, which includes the Audrey Hepburn skinny black pant campaign.

Goth- and music-inspired clothing and accessories seller Hot Topic (Charts) posted a larger than expected 7.3 percent decline in its September same-store sales and warned that third-quarter profits would be at the low end of its previous forecast.

And Wal-Mart (Charts), the world's largest retailer, said sales rose a weaker than expected 1.3 percent. Wal-Mart's results were at the low end of its forecast for a 1 to 3 percent increase in September sales.

Total sales for the five-week period ending September 29 rose 11.8 percent to $31.2 billion.

The retailer said consumables continued to outperform other product categories during the month. However, the company said apparel sales "did not meet expectations for the month."

Wal-Mart said it's "optimistic" about the sales of its holiday-related merchandise like toys and electronics.

The upcoming November-December shopping period is a critical time for retailers since it can account for as much as 50 percent of their annual profits and sales.

For October, Wal-Mart expects same-store sales to increase between 2 to 4 percent.

The cooling housing market is still a cloud hanging over consumer spending, but Hastings said he believed the current boost from lower gas prices "is enough to offset the impact of weakness in the housing sector."

"Under these conditions, we do expect the total holiday sales growth rate to dip slightly compared to the prior two years," he said.

For its part, the National Retail Federation (NRF), the industry's largest trade group, expects holiday sales to increase 5 percent, or slower than last year's 6.1 percent increase.

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