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(CNN Business) —  

Californians are paying more than $4 for a gallon of gas, and prices are creeping up in the rest of the nation, too. The rising costs could start to have ripple effects.

The average price of a gallon of gas in the United States is now nearly $2.88, according to AAA. That’s about four cents higher than it was Monday.

Costs have been climbing quickly. The average price of a gallon across the nation has jumped between five and 25 cents each week over the past two months, said AAA spokesperson Jeanette Casselano.

Consumers could start driving less, rethinking large purchases and changing their summer travel plans should the national average price near $3.25 per gallon, Casselano said. While the national average probably won’t hit $4, Casselano said $3.25 is possible.

There are a couple of factors contributing to the increase. Prices are usually higher during the warmer months because refineries switch to a more expensive, environmentally friendly fuel blend that is mandated by the EPA at this time of year. Gas prices also typically spike between five and 10 cents every spring as refineries perform routine maintenance to prepare for increased demand in the summer.

But there has been other, unplanned maintenance this year that has caused some refineries in California to slow production, contributing to the rising costs. One California refinery also cut production to rebuild after a fire, said Patrick DeHaan, head of petroleum analysis at GasBuddy.

As a result, the nation’s current supply of gas is down 30% from what it was during the same time in 2018, DeHaan said.

Crude oil prices are also likely playing a role. Those prices have been rising, and climbed further this week after the US announced this week that it would end a sanctions waiver program for several countries that buy oil from Iran.

California and the surrounding states will see the biggest increases, mainly because of the refinery issues. But DeHaan said the rest of the country will continue to see rising costs, too.

DeHaan said he expected that gas prices across the country will peak in mid-May, and then decline between five and 15 cents in June. And even though prices at the pump spiked the most in California, DeHaan said that also means they’ll see the most relief.

“Especially in California, there has been some level of sticker shock,” DeHaan said. “It would not surprise me if some of the pictures of people getting out and dusting off their bicycle are accurate.”