Import cargo volumes at major US retail container ports are expected to rise 3.3% in May - but growth could trickle to a standstill by the end of the summer.

The forecast from the monthly Global Port Tracker report released by the National Retail Federation (NRF) and Hackett Associates, "shows that retailers remain cautious, especially when it comes to stocking their inventories," according to Jonathan Gold, NRF vice president for supply chain and customs policy.

"We're looking at barely 1% of year-over-year growth through the early summer, and August and September are expected to be basically flat even though they're supposed to be two of the busiest months of the years."

Cargo import numbers do not correlate directly with retail sales or employment because they count only the number of cargo containers brought into the country, not the value of the merchandise inside them. But the amount of merchandise imported nonetheless provides a rough barometer of retailers' expectations.

US ports followed by Global Port Tracker handled 1.14m Twenty-foot Equivalent Units (TEU) in March. That was down 10.9% from February and 8.6% from March 2012. One TEU is one 20-foot cargo container or its equivalent.

April was estimated at 1.29m TEU, down 1.4% from a year ago. May is forecast at 1.42m TEU, up 3.3% from last year; June at 1.4m TEU, up 1.4%; July at 1.43m TEU, also up 1.4%; August at 1.43m TEU, up 0.1%; and September at 1.41m TEU, unchanged from last year.

The first six months of 2013 are expected to total 7.8m TEU, up 2% from the first half of 2012. The total for 2012 was 15.8m TEU, up 2.9% from 2011.

"We need to see the economy strengthen in the coming quarters before we can begin to see the threat of a further economic downturn dissipating," added Hackett Associates founder Ben Hackett. "Trade will remain at low growth levels until we reach this stage."