The large, violent tornado that touched down near Bennington, Kansas Tuesday hardly moved over the course of an hour – astonishing meteorologists and storm chasers used to twisters plowing ahead at swift forward speeds.
“I have never seen such a stationary tornado, especially not for so long and for such a violent tornado,” said Joshua Wurman, a meteorologist at the Center for Severe Weather Research. “The tornado is moving less than 2 mph for a while, and changes direction, dwelling over the same areas for a long time.”
Aussie Storm Chasers video posted to YouTube by TheSchummy08
An atmospheric log jam set the stage for this anomalous, nearly stationary storm.
At high altitudes (18,000 feet or about 500 mb), a large ridge or bump in the jet stream (river of high altitude winds) was in the process of forming in the northern Plains while expanding into northwest Canada (see animation below). This ridge served as a “block” in the flow, stopping a fledgling weather system in eastern Colorado in its tracks
It was this Colorado weather system, an area of low pressure, that spawned the tornadic storms. The animation below shows how the low pressure system made effectively no forward progress between late Tuesday and late Wednesday.
While the weather system was stuck, all of the ingredients for violent tornado formation were present downwind. In eastern Kansas, the volatile mix converged: a push of warm, humid air from the south, cold air from the northwest, and dry air from the southwest. And up spun the twister that sat and kept spinning, growing over half a mile wide.
Given its non-movement, it’s extraordinarily fortunate the Bennington tornado formed and remained over farmland during its life cycle.
Wurman and his team, who were chasing the storm, clocked a wind gust of 247 mph using their mobile doppler radar unit, known as Doppler on Wheels (DOW).
“There was about a 2.5 km region (1.5 mile diameter) with winds of 100 mph or more,” Wurman said. “If this were moving, say, 2 mph, it would mean 45 minutes of 100 mph winds.”
Wurman said the “core flow” region of the twister – with winds of 150 mph or higher (EF4 to EF5 level, the highest levels on the 0-5 Enhanced Fujita scale) – was almost half a mile wide, and likely remained over the same area 15 minutes.
“15 minutes of winds of 150-247 mph would be pretty horrible,” Wurman said. “Had this occurred over a built up area, there would have been EF5 damage.”
While “high end”, Wurman said the 247 mph reading he and his team clocked Tuesday was exceeded by an EF5 tornado that struck Moore, Okla. of all places on May 3, 1999. Wurman and his team clocked 301 mph winds in that storm, the “highest ever measured wind speed”, he said. (Note: that reading is not considered “official” by the World Meteorological Organization, because it was measured indirectly by radar rather than an anemometer).
Wurman and his team plan to survey the rural region struck by the Bennington tornado to gain a better understanding of the damage inflicted by the storm’s relentless, crushing blow – even if it was primarily to trees.
The preliminary storm survey from the National Weather Service assigned the storm an EF4 rating.
“Damage to structures within the path yielded EF3 damage however supplemental data provided by mobile doppler radar sampled winds suggest that this was a violent [i.e. EF4+] tornado,” the NWS survey said.