(Portland, OR) -- Gas prices are starting their seasonal increase. Triple-A reports refinery problems in the Midwest are causing prices to rise and refineries are starting to produce more expensive summer blends of fuel that reduce pollution. The national average price for a gallon of gas increased five cents over the last week to 3-28. Oregon's average increased a penny to 3-60. Portland has the highest average at 3-73 while Pendleton has the lowest average at 3-42. The price of diesel in Oregon increased two cents to 4-14 a gallon.