Why are gas prices suddenly heading north?

Gas prices are starting to zoom higher, jumping more than 20 cents overnight in some parts of the country this week.

Residents of Bay City, Michigan, saw gas prices soar to $2.32 a gallon Wednesday from $2.04 the day before, according to GasBuddy.com. Two cities in Ohio -- Cincinnati and Columbus -- saw prices jump 22 cents over the same period.

States that saw dramatic increases overnight include Indiana, Kentucky and Illinois, although Michigan and Ohio were the hardest hit.

The national average for a gallon of gas was $2.15 Thursday, up four cents from the day before and 11 cents in a week. Gas prices had dropped for a record 123 days in a row to $2.03 a gallon on Jan. 26, but have been moving up since then.

Prices generally head north around this time every year. We're at the start of refinery maintenance season, when refineries generally take some of their units offline for inspection, testing and improvements. Refineries also use the time to switch from winter gasoline, which uses more butane, to the more expensive type of fuel used in the summer. The lost production generally sends gas prices up.

Gas prices tend to rise 30 to 50 cents a gallon between early February and the middle of spring, according to AAA.

Gas is also creeping up because crude oil is no longer in the dramatic freefall seen last year. Brent crude, the global benchmark, has climbed to more than $56 a barrel from below $50 in January, while West Texas Intermediate crude in the U.S. has gone from $45 to $50 in the last month.

Don't blame the United Steelworkers strike at nine gas plants for rising gas prices this year. Experts say that the labor issues are not a factor at the pump.

Now, only a third of the filling stations in the country are selling gas for less than $2 a gallon, AAA reports. Last month, it was six in 10 stations. Gas prices move much faster on the way up than down, one petroleum expert told The Canadian Press.

Drivers were bemoaning the price increases Thursday on Twitter. "It was a nice little fling while it lasted," said one user. "Every time I look at the gas prices I want to cry because they keep going up," said another.