"With the holidays nearly here, retailers are making sure their shelves are well-stocked," says Jonathan Gold, NRF vice president for supply chain and customs policy. "Cargo is continuing to move through the ports, but the government shutdown has left some agencies short-handed, so NRF will monitor the situation closely as the holidays approach."

U.S. Customs and Border Protection has furloughed 6,000 workers because of the government shutdown that began last week, but Acting Commissioner Thomas Winkowski said the impact at the docks should be "minimal" since ports will remain open, with inspectors continuing to work and process cargo. But other government agencies that have a role in clearing cargo at the ports have not remained as staffed as CBP, leaving retailers cautious.

The forecast comes as NRF is predicting that this year's holiday sales will grow 3.9 percent over last year to a total of $602.1bn. Cargo import numbers do not correlate directly with sales because they count only the number of cargo containers, not the value of the merchandise inside them.

August, September and October are the months when most of the holiday season's merchandise is brought into the country. The 4.42 million cargo containers expected for those months combined is a 5.9percent increase over last year and accounts for 25.6 percent of all retail imports for the entire year.

U.S. ports followed by Global Port Tracker handled 1.48 million twenty-foot equivalent units in August, the latest month for which after-the-fact numbers are available. That was a 2.5-percent increase over July and up 3.8 percent from August 2012. One TEU is one 20-foot cargo container or its equivalent.

September was estimated at 1.47 million TEU, up 4.9 percent from last year. October is forecast at 1.46 million TEU, up 9.1 percent; November at 1.33 million TEU, up 3.4 percent; and December at 1.31 million TEU, up 1.8 percent. January 2014 is forecast at 1.35 million TEU, up 2.9 percent from January 2013, and February at 1.18 million TEU, down 8.1 percent from last year.

The total for 2013 is forecast at 16.3 million TEU, up 2.7 percent from 2012's 15.8 million TEU. The first six months of 2013 totaled 7.8 million TEU, up 1.2 percent from the first half of 2012.

Despite the current increases, container traffic growth overall has been slow this year, and the reduced demand for shipping capacity has ocean carriers cutting the number of vessels on the water and taking other steps, Hackett Associates founder Ben Hackett said.

"The supply-and-demand balance dictates pricing," Hackett said. "This has left the carriers to find ways to cut costs as a means to better financial results. Using larger ships is one solution, and larger alliances as a means to managing capacity is another.

Global Port Tracker, which is produced for NRF by the consulting firm Hackett Associates, covers the U.S. ports of Los Angeles/Long Beach, Oakland, Seattle and Tacoma on the West Coast; New York/New Jersey, Hampton Roads, Charleston, Savannah, Port Everglades and Miami on the East Coast; and Houston on the Gulf Coast.

Source: National Retail Federation