Edmunds, the leading car shopping and information platform, forecasts that 1,121,932 new cars and trucks will be sold in the U.S. in January for an estimated seasonally adjusted annual rate (SAAR) of 16.7 million. This reflects a 29.8 percent decrease in sales from December 2017 and a 1.4 percent decrease from January 2017.

"In January, automakers are expected to pull the reins in on the more generous incentive programs that we saw at the end of 2017," said Jessica Caldwell, executive director of industry analysis at Edmunds. "However, it's typical to see a slowdown at dealerships in January following the high-selling holiday months. This isn't necessarily a solid indicator of the direction that the year is headed in terms of overall sales."

Sluggish January sales are also expected as a result of severe winter weather in many parts of the country.

"The bomb cyclone that tore through the East Coast at the beginning of the month certainly didn't help an already slower sales month," said Caldwell.

Edmunds estimates that retail SAAR will come in at 13.4 million vehicles in January 2018, with fleet transactions accounting for 19.5 percent of total sales. An estimated 3.1 million used vehicles will be sold in January 2018, for a SAAR of 38.9 million (compared to 2.7 million — or a SAAR of 38.7 million — in December).