'Mothership' supercell rips through US in volatile tornado season

A storm supercell dubbed the "mothership" has ripped through the US in one of the most prolific tornado seasons in years.

The last two weeks have seen a volatile weather mix of warm moist air from the Southeast and persistent cold from the Rockies clash and stall over the Midwest, including Kansas and Nebraska.

On Monday, the US tied its current record of 11 consecutive days with at least eight tornadoes confirmed on each of those days, said Patrick Marsh, warning co-ordination meteorologist for the federal Storm Prediction Center.

The previous 11-day stretch of at least eight tornadoes per day ended on June 7, 1980.

"We're getting big counts on a lot of these days and that is certainly unusual," Marsh said.

Viewers were stunned in Oklahoma when TV network News 9 aired footage of water vapour apparently being sucked from trees towards a tornado.

"It's coming out of the trees," one viewer exclaimed on Twitter after a storm in Guthrie, Oklahoma.

Weatherman David Payne appeared to be gobsmacked when he saw the footage being aired before him.

Meanwhile stormchasers were having a field day with pictures of a supercell near Imperial, Nebraska dubbed the "Imperial Mothership".

Stormchaser Freddy McKinney posted a video describing it as "the most amazing storm I have ever seen" with "insane motion".

"Storm structure doesn't get any better than this and this was easily a top 3 storm chase for me," he wrote.

Fairfax photographer Nick Moir said the storm is being called the "most beautiful cell of the decade."

The US National Weather Service has received 934 tornado reports so far this year, up from the yearly average of 743 observed tornadoes.

More than 500 of those reports came in the last 30 days. The actual number is likely lower, however, because some of the reports probably come from different witnesses who spot the same twister.

The U.S. has experienced a lull in the number of tornadoes since 2012, with tornado counts tracking at or below average each year and meteorologists still working to figure out why.

"A lot of people are trying to answer that, but there's no definitive answer," Marsh said.

The recent surge in tornado activity over the past two weeks was driven by high pressure over the Southeast and an unusually cold trough over the Rockies that forced warm, moist air into the central U.S., sparking repeated severe thunderstorms and periodic tornadoes.

"Neither one of these large systems -the high over the Southeast or the trough over the Rockies- are showing signs of moving," Marsh said.

"It's a little unusual for them to be so entrenched this late in the season." Those conditions are ripe for the kind of tornadoes that have swept across the Midwest in the last two weeks, said Cathy Zapotocny, a meteorologist for the weather service in Valley, Nebraska. Zapotocny said the unstable atmosphere helped fuel many of the severe winter storms and subsequent flooding that ravaged Nebraska, Iowa and Missouri earlier this year.

"We've been stuck in this pattern since February," she said. Zapotocny said the number of tornadoes this year was "basically normal" until the surge this week.

May is typically the month with the highest incidence of tornadoes, usually in the Plains and Midwestern states collectively known as Tornado Alley, where most of this year's twisters have hit.

So far this year, 38 people have died in 10 tornadoes in the United States, including a combined seven within the last week in Iowa, Missouri, Oklahoma and Ohio.

The relative quiet in recent years followed the massive tornado that killed 161 people and injured more than 1,100 in Joplin, Missouri, in 2011. The EF5 storm packed winds in excess of 321km/h and was on the ground for more than 35km.

Scientists also say climate change is responsible for more intense and more frequent extreme weather such as storms, droughts, floods and fires, but without extensive study they cannot directly link a single weather event to the changing climate.