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SAN DIEGO — Gasoline prices have spiked in California, soaring well above what most Americans are paying at the pump. In some locations, Californians are paying $5 for a gallon of gas.

A number of refinery outages tightened gas supply in the market. The average price of regular gas in California rose to $4.18 a gallon, the highest level since May 13, 2014, according to the Oil Price Information Service, which gathers data for the AAA.

California’s gas prices are the most expensive in the United States: The national average is currently $2.65 a gallon.

Most motorists around the country are noticing gas prices declining or stabilizing, which is normal for autumn. Gas prices typically drop after the busy summer driving season, AAA said. But that trend hasn’t taken hold on the West Coast this fall.

“We’re surprised just because it was unexpected, given that prices typically have declined,” said Devin Gladden, spokesperson for AAA. “That just speaks to the unprecedented nature of how many refineries were down and how tight supply was that ultimately gave rise to the price increase.”

In San Diego, a run of 19 increases in 21 days that pushed the average price of a gallon of self-serve regular gasoline in San Diego County to its highest amount since July 20, 2015, ended Tuesday with a decrease of one-tenth of a cent to $4.205.

Most refineries perform planned maintenance in the fall or winter, which usually does not affect gas prices because the refineries can work around the repairs. They may buy gasoline ahead of time knowing that their facilities will be producing less of it, for example.

But over the past two weeks, several refineries that supply the West Coast with gasoline had unexpected outages at the same time, which further hampered California’s ability to produce fuels.

At most, seven of the region’s 25 refineries were either down or at lower production capacity, Gladden said.

“To see several refineries have unexpected outages at the same time, it’s not normal,” said Patrick DeHaan, head of petroleum analysis at Gas Buddy.

More than other states, California gas prices can sometimes skyrocket, because the state mandates cleaner gas with fewer emissions. That makes California gas cost more to refine, because it’s a special oxygenated blend that meets the state’s strict air-quality rules. Refineries have to use a specialized process, and only a few refineries are able to make California-approved gas, which makes it harder for the state to import oil.

However, the price increases are slowing as refineries work to resume production capacity. Assuming no additional outages, DeHaan expects California pump prices to stabilize toward the end of the week.