Import cargo volume at the nation’s major retail container ports is expected to increase 3.6% this month over the same time last year as retailers begin to bring in merchandise for the holiday season, according to the monthly Global Port Tracker report released by the National Retail Federation and consulting firm Hackett Associates. Imports for the year are expected to be up 4.2% over 2014.

“Consumers might be out buying back-to-school supplies but toys and sweaters are starting to show up on the docks,” says Jonathn Gold, NRF’s vice president for supply chain and customs policy. “There are still some lingering congestion issues, but retailers are working with their supply chain partners to make sure all of that merchandise flows smoothly to store shelves.”

Ports covered by Global Port Tracker handled 1.57 million twenty-foot equivalent units (TEUs) in June, the latest month for which after-the-fact numbers are available. That was down 2.5% from an unusually busy May but up 6.2% from June 2014. One TEU is one 20-foot-long cargo container or its equivalent.

July was estimated at 1.59 million TEUs, up 6% from 2014. August is forecast at 1.57 million TEUs, up 3.6%; September at 1.59 million TEUs, down 0.1%; October at 1.58 million TEUs, up 1.2%; November at 1.45 million TEUs, up 4.5%, and December at 1.4 million TEUs, down 2.8%.

Those numbers would bring 2015 to a total of 18 million TEUs, up 4.2% from last year. The first half of 2015 totaled 8.9 million TEUs, up 6.5% over the same period last year.

Some retailers are paying less to transport their merchandise this year, thanks to the use of more large-capacity ships by ocean carriers. Ben Hackett, founder of Hackett Associates, says the increased capacity has driven down rates, but the relief could be short-lived because some lines have already canceled voyages to counteract the trend.

“We are seeing complete chaos on the high seas in terms of the amount of capacity available and the level of spot freight rates,” Hackett says. “One has to wonder why carriers cannot match supply to demand. The end result will likely be a highly volatile situation of freight rates moving up and down.”

Global Port Tracker 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.