Twenty tornadoes reported across the Southern Plains but were less destructive than warnings predicted.

Residents of the Southern Plains of the US are breathing a sigh of relief after the much-publicised tornado threat turned out to be less severe than feared.

On Monday, the government’s Storm Prediction Center (SPC) raised the threat of tornadoes to high risk, the most serious of SPC’s five risk categories, as a rare combination of a summer-like air mass and a winter-like upper-level low combined.

This was expected to lead to an unusually intense, widespread and prolonged severe weather outbreak.

Oklahoma residents were particularly nervous because Monday was the sixth anniversary of a massive tornado in Moore that killed 24 people.

Ahead of the storms, some schools in the state, including Oklahoma City and Norman, cancelled classes, while schools in western Texas sent students home.

Tinker Air Force Base near Oklahoma City moved several planes to other military installations and state workers were sent home early.

The severe weather spawned 20 tornadoes, hail as large as 10 centimetres in diameter and winds gusting as high as 143 kilometres per hour.

The storms damaged houses, destroyed barns, brought down trees and power lines – triggering numerous power cuts – but this wasn’t as bad as feared.

National Weather Service meteorologist John Pike said a layer of relatively warm air aloft developed late on Monday afternoon over central Oklahoma and capped the development of thunderstorms.

One of the tornadoes struck the town of Mangum.

Glynadee Edwards, the Greer County emergency management director, said some homes incurred roof damage and the high school’s barn was destroyed but the livestock survived.

“The pigs are walking around wondering what happened to their house,” she said.

More severe thunderstorms are expected on Tuesday evening, with the Storm Prediction Center warning that the threat is greatest over Missouri and northern Arkansas.