The full impact of deadly tornadoes that hit the south-central US late last month will not be known until damage surveys are completed, according to catastrophe modeller AIR Worldwide.

The twisters hit at least seven states, with the most damaging in Arkansas. Some areas were hit by hail the size of softballs.

“Powerful winds unseated roofs from buildings, destroyed homes and tossed empty [trucks] nearly 100 feet into the air,” AIR Manager and Principal Scientist Scott Stransky said.

Entire neighbourhoods were levelled in Vilonia, a town of about 4000 people, while in southeast Kansas a grain elevator toppled onto railway tracks and a disaster emergency was declared for Baxter Springs.

At least 30 tornadoes formed, according to preliminary estimates by the National Oceanic and Atmospheric Administration Storm Prediction Centre. At least 30 people died, media reports say.

Persistent cold weather had brought a relatively quiet start to this year’s US tornado season, but Mr Stransky says the long-term average for the storms is relatively constant.

“One thing that may be happening recently is more variability around that long-term mean and also variability in the starting date of the severe thunderstorm season,” he said.

“Even a relatively quiet thunderstorm season does not necessarily translate into low losses. A single outbreak affecting metropolitan areas could cause atypically high losses.”