Building a Monster Tornado

BY Jenny Marder  May 22, 2013 at 2:53 PM EDT

Homes, trees and cars damaged by Monday’s tornado are seen by early morning light near Telephone Road and SW 4th Street in Moore, Okla., on Tuesday, May 21. Photo by Steve Sisney, The Oklahoman / NewsOK.com.

On Tuesday, the violent tornado that cut through Moore, Okla., chewing up and spitting out houses and trees, destroying neighborhoods and flattening two elementary schools, was upgraded to an EF-5 on the Fujita scale — that’s the highest score possible.

About one of every 1,200 tornadoes gets that top-of-the-scale rating by the National Weather Service, according to Robert Henson, a meteorologist and science writer with the National Center for Atmospheric Research. This was the first in nearly two years to receive that rating, with winds estimated between 200 and 210 mph.

A tornado is a violently rotating column of air that needs several ingredients to take shape: a strong jet stream at about 30,000 feet, warm, moist air closer to the ground and upward-moving winds called shear that change direction with height.

In Oklahoma on Monday, the jet stream winds were strong, racing out from the Rocky Mountains at about 100 miles per hour. And the weather, at 84 degrees with 69 percent humidity, was warm and muggy.

“When I saw how warm and humid it was getting, I said ‘Boy, it’s going to be bad,’” said Henry Margusity, a senior meteorologist with Accuweather.

The strong jet stream winds act like a vacuum cleaner, he explained, pulling the low-altitude humid air high into the atmosphere, creating what’s known as an “updraft.”

“You start sucking that warm, humid air up in the sky and you create thunderstorms,” Margusity said. “When there’s wind shear present, tornadoes follow right behind.”

A special kind of thunderstorm called a “supercell thunderstorm” produces tornadoes. The wind shear creates a horizontal spinning effect that veers from a southeast to a southwesterly direction, increasing in speed as it rises. The updraft tilts that spin upward into a vertical wind funnel and the warm surface air fuels the spin like wood fuels a fire.

Bob Henson from the National Center for Atmospheric Research explains how tornadoes are formed in this interview with Jeffrey Brown, which aired on the NewsHour on May 21.

While many think of a tornado in terms of the iconic cone coming out from a cloud, rotation can occur for miles up into the atmosphere, Margusity said. The funnel at the bottom is just a small piece of the overall rotation within the thunderstorm.

The tornado that struck Moore, Okla., on Monday was tremendous, more than a mile wide at some points, and its path carved through 17 miles for 40 minutes. And according to this Associated Press report, the energy that the tornado unleashed over Moore “dwarfed the power of the atomic bomb that leveled Hiroshima.”

“We don’t understand every aspect of what makes a tornado so massive,” said Bill Bunting of NOAA’s National Severe Storm Prediction Center. “It needs an unstable atmosphere, it needs a wind profile that changes in speed and wind direction and it needs even stronger levels of wind shear down in lower levels of the atmosphere.”

A tornado’s size has to do with the amount of winds and dust rotating at the surface and the extent of heat and humidity, Margusity said.

“It was the first tornado of the day, and it had all the fuel to work with,” Margusity said. “It was able to grab all this fuel and produce a massive tornado.”

NASA captured several images of the storm from space with its Aqua satellite, and NOAA monitored the storm’s movement and developments with its GOES-13 satellite. This satellite data helps weather forecasters track the storm.

This is an image from NOAA’s GOES-13 satellite of the storm system that generated the tornado in Moore, Okla. The image was captured at 3:55 p.m. EST. The line of thunderstorms that created the tornado is seen in the south central United States and resembles an exclamation mark. The tornado was generated near the bottom of that line of clouds. Image by NASA/NOAA GOES Project, Dennis Chesters.