There have been nearly 1,200 tornadoes in the U.S. so far this year. That's nearly twice the usual number of twisters, and it comes as something of a surprise to the scientists who study them. It turns out there is still no good way to predict tornado outbreaks more than a couple of days ahead of time.
Every year, government meteorologists tell the public about how many hurricanes to expect during the Atlantic season. But the government doesn't do that with tornadoes.
"I would say we have no skill in predicting whether the coming season will be good or bad, depending on your perspective, for tornadoes," says Howie Bluestein, a meteorologist at the University of Oklahoma.
The Right Conditions
He says that even in the midst of a very busy year like this one, there's no way to know what's likely to happen even a few weeks from now.
"This could be it for the rest of the season. Or it could continue to be crazy," Bluestein says. "We absolutely don't know. Our predictability is not that good."
He says one reason is that a tornado season can change dramatically in just a few days if the conditions are right. The right conditions usually involve severe thunderstorms in an area where warm and cold air are colliding. Near Joplin yesterday, the conditions also included a type of thunderstorm called a supercell, which has a powerful, rotating updraft.
This year, conditions like that produced more than 300 tornadoes in just three days at the end of April. That's a fairly typical length of time for an outbreak, according to Bluestein.
"Oftentimes an extreme event will occur, it will last for a short period of time and that will be a spike," he says.
Those spikes are hard to predict and they aren't necessarily part of a larger, seasonal trend. There's also no solid evidence that tornadoes have been influenced by climate change.
Hurricanes: A More Predictable Hazard
In some ways, tornadoes are a lot like hurricanes, which are easier to predict. Both are violent, rotating storms that can produce winds of more than 100 miles per hour. But Greg Jenkins, an associate professor of physics and astronomy at Howard University, says that's where the similarities end.
"The time scales are different, the space scales are different and the sources are different," says Jenkins.
He says tornadoes depend on weather conditions in a relatively small area east of the Rocky Mountains.
"Hurricanes are coming generally from West Africa," he explains. "There's a disturbance that's crossing the western part of Africa it goes out over the ocean and then maybe one out of ten maybe become hurricanes."
As a result, hurricanes are affected by patterns that cover a big chunk of the globe. For example: sea surface temperatures in the Atlantic, or winds shaped by El Nino or La Nina conditions thousands of miles away. Because these patterns occur on such a large scale they tend to change slowly — over months or years.
"Tornadoes are much smaller in scale," Jenkins says. "Often one sees a pattern that might persist for several days or a week, lots of warm air streaming up from the Gulf of Mexico and then we get this big outbreak of tornadic thunderstorms."
Getting Better At Warnings
Those patterns only last for a few days, so meteorologists' predictions are limited to the same short time span. Jenkins says another factor is that even a big tornado like the one that hit Joplin is rarely more than a mile across.
"The hurricane can stretch out for three, four, 500-thousand miles in the case of really big ones like Rita, some years ago," he says.
Rita struck the Texas and Louisiana coasts in 2005 and caused more than eleven billion dollars in damage.
Despite the difficulties making long term tornado predictions, forecasters have become much better at warning people what's going to happen in the next couple of days. At least 24 hours before the tornado struck Joplin on Sunday, a map produced by the U.S. Storm Prediction Center in Norman, Oklahoma, showed a red warning zone in an area that included Joplin. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.