The National Retail Federation has modified its retail sales forecast for 2015 in response to unexpected slow growth during the first half of the year.

NRF forecasted in February that retail sales would grow 4.1 percent in 2015 over 2014, but today’s revision lowers that forecast to 3.5 percent.

“A confluence of events, including treacherous weather throughout the United States through most of the winter, issues at the West Coast ports, a stronger U.S. dollar, weak foreign growth and declines in energy sector investments all significantly and negatively impacted retail sales so far this year and thus have changed how future sales will shape up for the rest of 2015,” says Jack Kleinhenz, chief economist, NRF. “Additionally, household spending patterns appear to have shifted purchases toward services and away from goods, though this may be transitory. Additionally, a deflationary retail environment has been especially challenging for retailers’ bottom lines.”

Despite these obstacles, NRF does expect sales to steadily increase through the remainder of the year. NRF calculated that sales grew 2.9 percent during the first half of 2015 and are expected to grow at a more positive pace of 3.7 percent over the next five months. This estimate includes general retail sales and non-store sales, excluding automobiles, gas stations and restaurants. Revised non-store sales are now expected to grow between 6 and 8 percent, still within the 7 to 10 percent range originally forecast.

“Despite all of these hurdles, we are optimistic that consumer spending during the second half of the year will benefit from recent improvements in the housing and labor markets along with lower energy costs, and believe consumer confidence will grow enough to bolster retail purchases for the year,” says Matthew Shay, president and chief executive officer, NRF.