Edmunds forecasts that 1,392,434 new cars and trucks will be sold in the U.S. in September for an estimated seasonally adjusted annual rate of 17.0 million. This reflects a 5.4 percent decrease in sales from August 2018 and an 8.3 percent decrease from September 2017.

"Vehicle replacement demand following Hurricane Harvey bolstered auto sales last September, and Hurricane Florence has had a very limited impact on auto sales this month, which are the primary reasons why we're seeing this year-over-year decline," said Jeremy Acevedo, Edmunds' manager of industry analysis. "With that said, a SAAR of 17 million is certainly not an unhealthy number — September is still shaping up to be a robust month for sales."

Edmunds data reveals that September marks the first time that the SAAR has pushed above 17 million in two months. Experts note that this boost in sales may be attributed to automakers loosening the reins on incentives as they get ready to navigate the model-year selldown.

"Labor Day sales typically help get the month of September off to a solid start," said Acevedo. "Now that we're heading into the fourth quarter and more 2019 model year vehicles are making their way to showrooms, automakers and dealers are likely making more of a concerted effort to address lagging inventories."

Edmunds estimates that retail SAAR will come in at 14.4 million vehicles in September 2018, with fleet transactions accounting for 15.2 percent of total sales. An estimated 3.3 million used vehicles will be sold in September 2018, for a SAAR of 39.6 million (compared to 3.4 million — or a SAAR of 39.7 million — in August).