Major American retail container ports are expected to see a 3.2-percent jump in imports this December over the same time period last year, a report shows. The increase comes as retail stores bring in the last of the merchandise for the holiday season, according to the monthly Global Port Tracker report released today by the National Retail Federation (NRF) and Hackett Associates.

Ports covered by Global Port Tracker handled 1.67 million twenty-foot equivalent units (TEUs) in October, the latest month for which after-the-fact numbers are available. That was up 4.6 percent from September and up 7.4 percent from October 2015. November was estimated at 1.53 million TEU, up 3.6 percent from last year, and December is forecast at 1.48 million TEU, up 3.2 percent.

The numbers reinforce NRF's forecast of $655.8 billion in holiday sales, which would mark a 3.6-percent increase over last year. Cargo volume does not correlate directly to retail sales because it tracks the pure number of containers, not the value of the cargo inside. But the trends tend to move in parallel.

"There's still shopping to be done, and retailers are making sure the gifts that need to be under a tree are waiting on the shelves," NRF Vice President of Supply Chain and Customs Policy Jonathan Gold said in a release. "Imports are up a healthy amount over this time last year, and that's a good sign for holiday sales and the economy."

Cargo volume for the full year of 2016 is expected to total 18.6 million TEU, up 2 percent from last year. Total volume for 2015 was 18.2 million TEU, up 5.4 percent from 2014.

With cargo growth at covered U.S. ports for the year coming in at only 2 percent, Hackett Associates founder Ben Hackett said a trend of imports exceeding growth of gross domestic product appears to have ended. "This is a new phenomenon," Hackett said. "It was not long ago when industry leaders were doing their forecasts based on trade growth outpacing GDP by a ratio of more than 2 to 1. Those days are gone."