Import volumes at major US retail container ports are expected to drop 8.4% in February - a higher rate than first predicted - as the shipping cycle reaches its slowest month of the year.

But the respite is likely to be followed by a "busy season" as retailers bring in their spring merchandise.

The forecast from the monthly Global Port Tracker report released by the National Retail Federation (NRF) and Hackett Associates, comes as NRF said it expects US retail sales to grow 4.1% this year.

"Ports and distribution centres are getting the break they deserve after the busy holiday season, but it won't last long," said NRF vice president for supply chain and customs policy, Jonathan Gold.

"Retailers will be moving spring merchandise toward their shelves in just a few weeks, and early numbers point to a busy season ahead."

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. The amount of merchandise imported nonetheless provides a rough barometer of retailers' expectations.

US ports followed by Global Port Tracker handled 1.3m Twenty-Foot Equivalent Units (TEU) in December, the latest month for which after-the-fact numbers are available. That was down 3.3% from November as the holiday season came to an end, but up 0.6% from December 2012.

The December numbers brought 2013 to a total of 16.2m TEU, up 2.3% from the prior year's 15.8m TEU. One TEU is one 20-foot cargo container or its equivalent.

January was estimated at 1.37m TEU, up 4.5% from the same month last year. February, historically the slowest month of the year, was previously forecast to be down 7.5% to 1.18m TEU, but is now expected decline 8.4% to 1.17m TEU.

March is forecast at 1.29m TEU, up 13.7% from last year; April at 1.39m TEU, up 6.9%; May at 1.45m TEU, up 4.2%; and June at 1.43m TEU, up 5.6%. These numbers would total 8.1m TEU for the first half of the year, up 4.3% over last year.

"On the consumer side, there is continued hesitancy in spending as net disposable income remains virtually flat," warned Hackett Associates founder Ben Hackett. "As a result, the inventory-to-sales ratio remains stubbornly high."