California gas prices, which have been at unusually low levels for weeks, are about to head north, according to national gas price tracker GasBuddy.com.

Southern California motorists could see at-the-pump costs spike 20 to 35 cents over the next week as the region switches from winter gasoline to the pricier spring/summer blend, GasBuddy said Thursday. Northern California will make the transition in about a month.

The average retail price of gas in the Sacramento area on Thursday was $2.12 a gallon, down nearly 30 cents from last month and about 71 cents below the average a year ago.