Import cargo volume at the nation’s major retail container ports should see a small but significant increase this month as merchants stock up for the back-to-school season, then see a larger wave in late summer and fall for the holiday shopping season, according to the monthly Global Port Tracker report released July 12 by the National Retail Federation and the consulting firm Hackett Associates.

Ports covered by Global Port Tracker handled 1.63 million 20-foot equivalent units (TEUs) in May, the latest month for which after-the-fact numbers are available. That is up 12.8% from April and up 1.1% from May 2015. (One TEU is a single 20-foot-long cargo container or its equivalent.)

Helping to fuel growth is a recent surge in U.S. retail sales, which rose 0.5% in May from the month before following a 1.3% jump in April. Retail sales are important for trucking for several reasons. It drives about two-thirds of all U.S. economic activity, and retail freight spends at least part of its life on trucks before making it to store shelves. Figures for June are due out next week.

“Back-to-school and the holidays are the two biggest shopping seasons of the year for retailers, and these numbers reflect that,” says NRF Vice President for Supply Chain and Customs Policy Jonathan Gold. “After a year of difficult comparisons in the wake of the West Coast ports slowdown, we’re finally starting to see normal trends. Some numbers are still down from last year, but the pattern of building up toward the big seasons has returned.”

June import cargo volume is forecast to be at 1.56 million TEUs, down 0.5% from the same month last year, but July is projected at 1.64 million TEUs, up 1.4% from last year. Looking later into the year, August is expected to see 1.65 million TEUs, down 2%; September at 1.58 million TEUs, down 2.6%; October at 1.62 million TEUs, up 4.4%, and November at 1.52 million TEU, up 2.8%.

Even though volume will be lower than the same month last year, August is expected to be the peak shipping month of the year, according to the NSF.

The first half of 2016 is expected to total 8.99 million TEUs, up 1.5% from the same period in 2015. Total volume for 2015 was 18.2 million TEUs, up 5.4% from 2014.

“Trade is holding on to a small margin of growth, but this growth comes in the face of some adverse statistics as well as positive ones,” said Hackett Associates Founder Ben Hackett. “The good news is that retail sales have remained positive as the consumer continues to cautiously spend. The hope is that this spending will continue.”

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.