Ecommerce spending in the US grew 13% year-over-year in the third quarter to $47.5 billion, a softening from Q2 when the growth rate hit 15%, according to comScore’s State of the US Online Retail Economy report.

“There was a bit of a dip in consumer sentiment,” commented Andrew Lipsman, VP of industry analysis at comScore. “The bottom hasn’t fallen out or anything too bad, but there’s a lot of exogenous forces that maybe are making people feel a little less positive about spending.”

Rewinding to 2008 and 2009, there was a lot of softness as growth rates dipped into the negatives or flatlined at zero percent. In the past few years, the growth rate picked back up and reached the mid-teens.

“This year has been kind of an interesting year in ecommerce,” Lipsman said. “I’ve probably never seen a year with such mixed trends where there are some really positive indicators and then some things that are also a bit concerning.”

Seasonality plays a powerful part, as well, with fluxing growth rates in the summer due to uplifts in the housing and auto industries. Consumers may shell out more cash on higher ticket items, which has a negative impact on discretionary items and subsequently compresses spending thereafter.

This year, the no. 1 concern is the “buzzsaw of a holiday season” as short as any since 2002, Lipsman noted. “There are only 26 shopping days between Thanksgiving and Christmas and that absolutely can have an impact. My sense of what we’ll see in the fourth quarter is that it’s going to provide a little bit of a drag on the growth rate.”

Despite the shorter shopping season, mobile commerce should surge. Experiencing a growth rate of 26% year over year in Q3, with smartphone and tablet-transacted spend reaching $5.8 billion, Lipsman estimated mcommerce could hit the $10 billion-mark in Q4 given the fact that the category hit $7 billion last year in the same quarter.

Mobile spending has consistently hit the 25-30% growth mark each quarter. As an emerging channel, consumer interest is demanding investment and the dollars are following.

“I might say, why not give consumers an incentive, discount or deal if they download your app?” Lipsman advised marketers. “Apps are scarce real estate. … Maybe incentivize consumers if they complete a transaction on a device or through your app. I think mobile accounts for more than 50% of time spent in the retail category.”

ComScore’s ecommerce spending analysis is based on anonymously aggregated Internet user data on more than 1 million US consumers who have opted in for participation in comScore’s panel.