The slide in oil prices over the last month will probably bring $2 gasoline back to filling stations across the United States later this year, AAA said Friday.

U.S. crude prices have fallen 20 percent over the past month amid a glut of shale oil. Pump prices are already at the lowest seasonal level since 2009, averaging $2.66 a gallon, and they’re in store for another “dramatic” decline in September when refineries switch to making a cheaper winter gasoline blend, the nation’s biggest motoring club said in an emailed statement.

The average U.S. gasoline price last fell below $2 a gallon in 2009. The country was less than four cents shy of slipping under the threshold in January before an oil rally and a spate of refinery outages cut the decline short. A handful of filling stations in the U.S. are already selling gasoline for less than $2, and the southeastern and central U.S. will probably be home to the most places with $2 gasoline by winter, AAA said.

“The recent price declines are hopefully just a precursor of much bigger savings to come at the pump,” Michael Green, an AAA spokesman in Washington, said by e-mail. “We could see many parts of the country make another run towards $2 per gallon by the end of the year if everything keeps running smoothly.”