Steep holiday discounts and bargain hunting boosted Black Friday sales, but a dour economic outlook leaves analysts skeptical that Americans will be able to sustain their buying frenzy for the duration of the holiday season.

Spending over the four-day Thanksgiving weekend rose 7.2% from a year ago to about $41 billion, according to the National Retail Federation (NRF). However, a shorter holiday shopping season and frugal shoppers could soon drive retail sales back down to their pre-holiday lows.

“Pent-up demand on electronics and clothing, plus unparalleled bargains on this season’s hottest items helped drive shopping all weekend,” said NRF President and CEO Tracy Mullin. “Holiday sales are not expected to continue at this brisk pace, but it is encouraging that Americans seem excited to go shopping again.”

The NRF continues to forecast a mild 2.2% jump in holiday sales, the smallest gain in six years.

Several details in the NRF report underscored the group’s apprehension. For instance, more than half of the 110 million holiday shoppers that braved the elements this weekend went to discount retailers like Wal-Mart Stores Inc. (WMT). Shoppers also took advantage of so-called “door buster” sales that offered products at extraordinary discounts to early morning customers.

More than 23% of the 73.6 million shoppers that hit stores Friday were in the door by 5 a.m. More than half, 57.6%, were at stores by 9 a.m.

Early-bird specials may have provided a nice boost to Black Friday sales, but offering goods at such cheap prices could backfire by marginalizing profits.

“You’re looking at discounts of 50% to 70% off,” Matthew Katz, managing director in the retail practice of advisory and restructuring firm Alix Partners, told the New York Times. “You have to sell two to three times as much to break even.”

Retailers cannot continue to offer shoppers such steep discounts, and consequently, will find it difficult to keep their attention. That is especially true this year, as thrifty consumers have taken a more targeted approach – swooping into stores for the things they want and retreating without taking the time to browse for additional items.

“Shoppers definitely have a mission this year,” Marshal Cohen, chief retail analyst for NPD Group, told CNN. “They are serious about finding the best deals. They are very budget conscious, they’ve done their research and then they’ll go home.”

Also, this year’s holiday shopping season is five days shorter than last year.