Upper middle-class shoppers spooked by a whipsawing stock market and shoppers waiting till the last minute for the best deals could result in the weakest U.S. holiday sales season for retailers since the recession, AlixPartners said. This forecasted increase is on the low end of holiday sales performance since 2010, AlixPartners said in an advanced copy of a report given to Reuters. Holiday sales have averaged 3.9 percent in the last five years, the firm said.