Boosted by continuing sales growth, August is expected to be the busiest month on record for imports at the nation’s major retail container ports and 2017 is on track to set a new annual high, according to the monthly Global Port Tracker report released Friday by the trade group the National Retail Federation (NRF) and the consulting firm Hackett Associates.

“Retailers are selling more and that means they need to import more,” said Jonathan Gold, NRF vice president for supply chain and customs policy said. “With sales showing year-over-year increases almost every month for a long time now, retail supply chains are working hard to keep up.”

Ports covered by Global Port Tracker handled 1.69 million 20-foot equivalent units (TEUs) in June, the latest month for which after-the-fact numbers are available. That is down 2% from May but up 7.5% from June 2016. July was estimated at 1.72 million TEUs, up 5.6% from the same time last year. One TEU is one 20-foot-long cargo container or its equivalent.

August is forecast at 1.75 million TEUs, up 2.1% from last year. That would be the highest monthly volume recorded since NRF began tracking imports in 2000, topping the 1.73 million TEUs seen in March 2015. The 1.7 million-plus numbers seen in May and July and now expected for August and October would represent four of the six busiest months in the report’s history.

September is forecast at 1.67 million TEUs, up 4.7% from last year; October at 1.72 million TEUs, up 3%; November at 1.62 million TEUs, down 1.4%, and December at 1.59 million TEUs, up 1.5%.

Those numbers would bring 2017 to a total of 19.7 million TEUs, topping last year’s previous record of 18.8 million TEUs by 4.9%. That compares with 2016’s 3.1% increase over 2015. While July numbers are not yet final, the first half of 2017 tentatively totaled 9.7 million TEUs, up 7.4% from the same period in 2016.

The import numbers come as retail continues a long-term pattern of increased sales. Total retail sales have grown year-over-year every month since November 2009, and retail sales as calculated by NRF (excluding automobiles, gasoline stations and restaurants) have increased year-over-year in all but three months since the beginning of 2010. Retail employment, despite recent short-term fluctuations, has increased by 1.5 million jobs during the same period.

NRF has forecast that 2017 retail sales (excluding automobiles, gasoline and restaurants) will increase between 3.7 and 4.2% over 2016, driven by job and income growth coupled with low debt.

Cargo volume does not correlate directly with sales because only the number of containers is counted, not the value of the cargo inside, but nonetheless provides a barometer of retailers’ expectations, according to the group.

Hackett Associates Founder Ben Hackett noted that U.S. gross domestic product grew at an annual rate of 2.6% in the second quarter of this year, more than double the 1.2% level seen in the first quarter.

“This relatively strong growth underlies the robust level of imports we have forecast and witnessed,” Hackett said.

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.