The retail industry was hobbled by a sluggish back-to-school season. And now, it doesn't have a whole lot to look forward to in the normally robust holiday shopping season of November and December either, says an early forecast by ShopperTrak, which measures store traffic at over 60,000 locations internationally.

Retail sales this holiday season will rise only about 2.4 percent from 2012, the report predicts, less than the 3 percent rise the previous year, or the 4 percent rise the year before that.

The deceleration is major cause for concern: Retailers can make up to 40 percent of their yearly revenue in just those two holiday shopping months.

To make matters worse, retailers are still feeling the effects of a weak back-to-school season that led retail giants like Macy's and Kohl's to cut their profit outlooks for the year, and even caused Walmart to miss per-share earnings expectations. Sales of clothing and accessories increased by just 3.8 percent in the first six months of 2013 compared to those months in 2012. For comparison, car sales shot up 9.5 percent.

That gap may be part of what's driving the middling retail trend. "Pent-up demand for cars and home improvement needs may have drained away discretionary dollars that might otherwise have been spent on clothing stores and restaurants," said Shelly Banjo in The Wall Street Journal last month.

But other pressures are contributing to the sluggish growth, too. Consumers might be feeling the pressure of "a host of macroeconomic and political issues," like the budget fight in Washington, and uncertainty about Syria, says Suzanne Kapner in The Wall Street Journal.

She also suggests a simpler cause:

Adding to the malaise is a lack of hot new fashion trends like colored jeans that were a must-have item last year. As a result many shoppers have delayed purchases, behavior that was in evidence during the lackluster back-to-school shopping period. [The Wall Street Journal]

Whatever the cause, shoppers are being a lot more cautious about how they spend. People are spending more time researching purchases before they buy anything, Bill Martin, founder of ShopperTrak told Daily Finance. And it doesn't help that the window between Black Friday — the biggest, most horrifying shopping day of the year — and Christmas is seven days shorter this year than it was last year.

For now, the retail industry awaits another holiday forecast by the National Retail Federation, the country's biggest retail trade group. Perhaps it can take solace in the fact that ShopperTrak low-balled 2011's holiday season by a full 1 percent.