The approaching summer months and international issues are factoring into the higher gas prices that drivers will have to pay when filling up their tanks this weekend.

Pennsylvania residents were paying an average of $2.58 per gallon yesterday, up 9 cents from last week's average of $2.49, said Jason Toews, co-founder of the Web site www.pennsylvaniagasprices.com.

Toews added that prices are expected to continue rising through the spring and summer.

"We're expecting significant increases in prices, possibly over $3 per gallon by summer," he said.

Despite the increase in gas prices in the area that has occurred over the past week, a manager of one local gas station said he has not seen sales go down.

"We don't usually see an effect on sales when the prices go up," said Jason McFadden, a manager at Graham's Exxon, 815 S. Allen St.

Pennsylvania AAA Federation Executive Director Ted Leonard said there are internal and external factors causing the current increase in gas prices. He added that it is common for gas prices to increase at this time of year.

"We're approaching the traditional beginning of travel season -- Memorial Day -- when travel tends to increase," he said.

Jonathan Mathews, assistant professor of earth and mineral sciences, also cited the time of year as a cause for the increase.

"Summer is the time when we drive the most and use the most gasoline, so prices will typically be higher," he said.

Aside from the rise in travel, gasoline prices annually increase this time of year because of a national requirement for the June 1 switchover to "summer gasoline," which has a lower pressure to prevent evaporation in the heat. The change forces gas companies to stop using some refineries, Leonard said.

Leonard and Mathews added that a price drop in the U.S. gasoline stocks and an increase in crude oil prices to $66 a barrel factor into the change in gas prices.

International issues are also playing a role in the price jump.

"There are currently three oil-supplying countries causing international uncertainty," Mathews said.

He said the escalation of conflict over Iran's nuclear policies, tensions between the United States and Venezuela, and the "increasing of demonstrations and violence over control of oil revenues in Nigeria" are all global factors leading to the rising prices.