The average US price of a gallon of gasoline has ticked up over the past few days, hitting $3.68/gallon on May 24 in advance of the Memorial Day start of the summer driving season, the Energy Information Administration reports.

In the Midwest, the increase has been particularly acute. Midwest – or Petroleum Administration for Defense District (PADD) 2 – gasoline prices have shot upward since mid-April, from $0.09 per gallon below the national average on April 15 to $0.15/gallon higher than the national average on May 20, according to the EIA. Minnesota has seen the sharpest rise – the price of a regular gallon of gasoline rose by a whopping $0.81/gallon between April 15 and May 20, to $4.29/gallon, which was $0.62/gallon above the national average.

“Leading up to this year’s holiday, high gasoline prices in the Midwest largely reflect supply constraints stemming form refinery outages and lower-than-normal gasoline inventories”, the EIA said. Planned maintenance at Midwest refineries is taking some gasoline volumes offline, and gasoline inventories in the region have fallen off sharply over the past several weeks.

The Midwest can bring in supplies from other parts of the country, but deliveries take time. “While resupply from the US Gulf Coast is available via pipeline, transit time to the upper Midwest can be as long as three weeks,” the EIA said.