Chicago gas prices are among the highest in the country, jumping to nearly $4.20 a gallon on Monday, according to petroleum analysts. Average gas prices in the U.S. have increased 18 cents over the past two weeks.

Gregg Laskoski, a senior petroleum analyst with the gas tracking website Gas Buddy, said the drought and its effect on corn supply is a major issue.

“The drought that is perhaps one of the worst in the last 50 years is certainly having a horrible impact on corn supply,” he said. “That in turn does indeed affect ethanol because corn is a major factor in ethanol production, so that’s pushing ethanol prices higher.”

The Lundberg Survey of fuel prices puts the average price for a gallon of regular at $3.69, about eight cents higher than the same time last year.