Tornado 'second season': new outbreak, old risk

Residents across a huge swath of the South and Midwest are beginning the arduous task of picking up their lives after a bizarre coil of autumn tornadoes ripped across the region, wreaking some of the worst damage in decades. The cluster of storms left dozens dead and hundreds of others missing across at least 13 states. The devastation is raising enduring questions about the effectiveness of local early-warning systems – and is a painful reminder that tornadoes don't just strike in the spring. There's also an often-overlooked second storm season in the fall.

Preparedness, particularly at a time of year that many people don't associate with tornadoes, remains key, according to Daniel McCarthy, warning- coordination meteorologist at the National Weather Service's Storm Prediction Center in Norman, Okla.

"Every year we get an outbreak of some kind in November," typically from the Tennessee Valley to the Southeastern US, says Daniel McCarthy. It's a region that in the past few years has become known as "Dixie Alley."

People in that area "have to be prepared; they need to know where to go for safety," and be as alert to the possibility of fall tornadoes as residents of "Tornado Alley" – the storm-prone Midwest and middle South, from Texas north to Nebraska and Iowa – are to storms in the spring, Mr. McCarthy says.

The number of twisters and the reasons for the high death toll – which at press time stood at 33, with more than 100 injured – may not emerge for a few days. Meteorologists will be reviewing weather-radar data and reports from human "spotters," while local emergency-management officials look at the types and ages of damaged structures.

But they do have a clear idea of the unusual conditions that spawned the storms. A strong storm front running from the Mississippi Valley to the southern plains was moving east. Drawing energy from the warm, moist air that the system was pulling north from the Gulf of Mexico, the storm front intensified as it moved into the Ohio Valley, according to McCarthy. Jet-stream winds at 18,000 feet were clipping along at 110 miles an hour along a line from Arkansas to western New York, while 13,000 feet below, more southerly winds blew at a respectable 62 miles an hour.

Those conditions, dubbed vertical shear, "were the strongest we've seen in many years," McCarthy says. Like a pair of hands rolling Play-Doh, high winds running in different directions at two altitudes set up a broad rolling motion in the layers of air in between that translated into the worst tornadoes since May 1999, when a series of twisters tore through parts of Oklahoma and Kansas, killing 40 people and inflicting $1.2 billion in damage.

Though fall tornadoes are rare in the Midwest, they're not unheard of, according to the National Weather Service, and the Great Lakes' warmth makes weather less predictable. In Ohio, the tornado drew comparisons to a 1999 spring storm in Cincinnati and 1985 twisters that killed 10 and injured 250.

The storms blew in on the tail of Sunday's record highs: Nashville's balmy 81-degree afternoon broke a century-old record.

Typically, Dixie Alley's tornado season runs from the first of October to mid-November. "The abnormality of this particular outbreak was how far north it carried, into Indiana and northwest Ohio," McCarthy says. "Usually second-season tornadoes concentrate no farther north than Tennessee, and mostly in Mississippi and Alabama."

Indeed, during the past few years, the country's second tornado season appears to have intensified. "We started to see it in the late 80s. Northern Alabama had tornado outbreaks on Nov. 15 for three straight years," McCarthy says.

• Material from wire services was used in this report.