Edmunds, the leading car shopping and information platform, forecasts that 1,435,375 new cars and trucks will be sold in the U.S. in September for an estimated seasonally adjusted annual rate of 17.5 million. This is the highest SAAR so far in 2017; the previous highs were set in January and February, which both had a SAAR of 17.3 million. This rate reflects a 2.9 percent decrease in sales from August 2017 and a 0.4 percent increase from September 2016.

Edmunds analysts say this slight year-over-year increase is due in part to automakers pumping up the deals over the Labor Day holiday; typically, auto sales over Labor Day weekend are 33 percent higher than the average first weekend of a month.

"Labor Day weekend got September auto sales off to a strong start," said Jessica Caldwell, Edmunds executive director of industry analysis. "Automakers are finally starting to dial up the incentives to clear excess inventory, which we anticipate will continue through the rest of the year."

September sales are also expected to get a slight lift from buyers who needed to replace vehicles that were destroyed in the recent hurricanes.

"We anticipate that the recovery from the recent hurricanes will give vehicle sales an incremental boost in September, and will likely continue to slightly lift the market in the months to come," Caldwell said. "When you have hundreds of thousands of people affected by an event of this magnitude, not everyone will hit the market at once."

Edmunds estimates that retail SAAR will come in at 15.0 million vehicles in September 2017, with fleet transactions accounting for 14.0 percent of total sales. An estimated 3.19 million used vehicles will be sold in September 2017, for a SAAR of 38.8 million (compared to 3.33 million — or a SAAR of 39.0 million — in August).