(CNN) -- Gas prices jumped 18 cents over the past two weeks, the biggest jump of the year, according to a survey published Sunday.
The average cost of a gallon of regular is $3.69 nationwide, the Lundberg Survey found.
That's nearly 8 cents higher than a year ago, but still 28 cents lower than it was in April of this year.
"Higher crude oil prices, higher corn-based ethanol prices, plus a spate of refinery and pipeline problems around the country combined" are behind the spike, said publisher Trilby Lundberg.
The U.S. drought is pushing up prices for all corn-based products.
It's impossible to predict what will happen with gas prices in the near future, Lundberg said. "The refinery and pipeline issues that have added to the price rise are certainly temporary in nature. But the other two -- the ethanol price rises and the higher crude oil prices -- aren't so easily dismissed as immediately temporary."
But, Lundberg said, she doubts prices will jump this quickly in the near future because many of the refinery issues are being resolved.
The Lundberg Survey tabulates prices at thousands of gas stations nationwide.
The city with the highest average in the latest survey was Chicago, at $4.23.
The lowest average price was in Tucson, Arizona, at $3.18.
Here are average prices in some other cities:
- Boston, Massachusetts - $3.74
- Charleston, South Carolina - $3.43
- Atlanta, Georgia - $3.64
- Indianapolis, Indiana - $3.84
- Denver, Colorado - $3.45
- Tulsa, Oklahoma - $3.54
- Portland, Oregon - $3.74
- San Francisco, California - $4.06