The Hows and Whys of a Possibly Record-Breaking Tornado Month

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The South is reeling from what could be one of the deadliest tornado systems in U.S. history. Yesterday storms and tornadoes ripped through Alabama, Mississippi, Tennessee, Georgia and Virginia, killing as many as 200 people, and potentially far more. At least 139 separate tornadoes were reported yesterday. That number is almost certain to rise, and just as certainly, April 2011 will do down as record-breaking month for tornadoes, even worse than April 1954, when an estimated 407 tornadoes struck. Yesterday’s system could be the deadliest since the one that struck April 3, 1974, which killed 315 people.

But those statistics don’t convey the sheer terror and destruction wrought by these storms. This eyewitness video from CBS, taken in Alabama, comes close:

When a weather event this extraordinary happens, we all want to know the answer to one question: what’s causing it? The answer, most immediately, is weather. Andrew Freedman of the Washington Post‘s great Capital Weather blog, explains the conditions behind monster tornadoes:

In order for tornadoes to form, several factors have to combine in just the right way. These ingredients include: a warm and humid atmosphere, strong jet stream winds, and atmospheric wind shear (winds that vary with speed and/or direction with height), as well as a mechanism to ignite this volatile mixture of ingredients – such as a cold front.

As Jason discussed Friday, an unusually powerful jet stream has steered a cascade of frontal systems across the midsection of the country. “There’s been a ton of systems coming through, that’s I think one of the biggest single things” leading to the recent severe weather outbreaks, says Harold Brooks, a research meteorologist at the National Severe Storms Laboratory in Norman, Oklahoma.

But that’s just the immediate cause. What we really want to know is: did climate change play a role in the monster tornadoes of April, and will a warming world see more destructive cyclones like these? (I know this because my editor, as he’s wont to do after major weather events, walked down to my office and asked me, “Bryan, does climate change play a role in this?”)

And the answer is… well, I actually already answered this in another post just 10 days ago, which if nothing else shows just how out-of-control this month has been for tornadoes. Scientists really don’t know. It’s true that the average number of April tornadoes has steadily increased from 74 a year in the 1950s to 163 a year in the 2000s. But most of that increase, as A.G. Sulzberger reports in the New York Times, comes from the least powerful tornadoes, the ones that touch down briefly without causing much damage. Those are exactly the kind of tornadoes that would have been missed by meteorologists in the days before the Weather Channel and Doppler radar—scientists today would almost never miss an actual tornado touchdown, no matter how brief or weak. That makes it very difficult for researchers to even be sure that the actual number of tornadoes is on the rise, let alone, if they are, what might be causing it. The number of severe tornadoes per year has actually been dropping over time.

It is true, however, that as the climate warms, more moisture will evaporate into the atmosphere. Warmer temperatures and more moisture will give storm systems that much more energy to play with, like adding nitroglycerin to the atmosphere. This month’s possibly record-breaking tornadoes are due in part to an unusually warm Gulf of Mexico, where as Freedman reports, water surface temperatures are 1 to 2.5 C above the norm. The Gulf feeds moisture northward to storm systems as they move across the country, and that warm moist air from the south meeting cool, dry air from the Plains often results in some powerful weather. But at the same time, other studies have forecast that warmer temperatures will reduce the wind shear necessary to turn a routine thunderstorm into a powerful system that can give birth to tornadoes. So in a hotter world we could see more frequent destructive thunderstorms, but fewer tornadoes—although some researchers think we could still end up with both.

What does it all mean? For one thing, we should remember that tornadoes—and other severe weather systems—have always been with us, and almost certainly will be, whatever happens to greenhouse gas emissions and temperatures in the years to come. But we can keep people from being killed by those storms, through better forecasting, better building and better emergency preparation. Indeed, we’ve actually been doing better over the years—National Oceanic and Atmospheric Administration statistics show that the number of Americans killed by tornadoes killed per capita has steadily declined since the 1920s:

It’s very likely that a warmer world will see more severe weather, and certainly a more populated world will mean more people at risk from those events. That just increases the need to invest in disaster preparation and response—with vital agencies like the National Weather Service and NOAA—facing drastic budget cuts. In the wake of a black day for Americans in the South, that’s simply idiotic.

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