The unrelenting blast furnace gripping the metro area is pushing roads to the breaking point.

MnDOT says it has responded to nearly 45 road explosions in the Twin Cities area since 90-degree temperatures arrived June 3, and with only a slight cool-down in the immediate forecast — 88 degrees on Saturday — the agency is bracing for more.

"It is a traffic emergency," said Anne Meyer, a MnDOT spokeswoman. "If drivers see one, they should call 911."

Road blowouts, as they also are called, happen during periods of extreme heat as pavement overheats and pops. And with eight straight days of temperatures over 90 degrees as of Thursday, the conditions have been ideal.

The Twin Cities is experiencing the third-longest streak of temperatures at 90 degrees or higher in history. Such a streak has happened six times, most recently from July 9-17 in 2006, according to the National Weather Service. The longest stretch was 14 days during the Dust Bowl era, from July 5-18, 1936, the weather service said.

As pavement warms, it expands. MnDOT cuts grooves in concrete to give it room to expand and contract, but buckles occur when the pavement expands beyond the space allowed, causing it to pop up, Meyer said.

Crews normally assigned to painting projects and lawn mowing are reassigned to fix pavement malfunctions. "We get out there pretty quickly," Meyer said.

Most often, road blowouts occur with weak sections of concrete, Meyer said. But newer pavement can buckle, too, as it did Wednesday afternoon on Hwy. 610 near County Road 81 in Maple Grove. The road was closed for a few hours, interrupting rush hour traffic.

Numerous road buckles have happened in greater Minnesota, too, Meyer said. On Wednesday, a lane on Interstate 90 near Eyota in southeastern Minnesota was closed after pavement buckled, MnDOT said. Buckles are slow in forming, sort of like potholes. They fan out, and as road temperatures exceed the ambient temperature and vehicles drive over the compromised pavement, it "reaches its final breaking point," Meyer said.

"They are not predictable," she said.

Though less likely to happen on asphalt roads, blacktop is not immune. While the material is more flexible, extreme heat can cause a bump similar to a frost heave, Meyer said.

Both road explosions and bumps in the pavement can cause damage to vehicles. Drivers encountering one should slow down and change lanes if possible, Meyer said.

"Motorists need to be alert," she said.

Tim Harlow • 612-673-7768