Extremes in temperature throughout the vast table of the American heartland are making 2008 one of the deadliest years for US tornadoes in recent history.
The supercell thunderstorms that breed twisters have occurred farther north and earlier in the year than is typical, according to some experts. But many are quick to add that this increase in severe weather is not necessarily an indication of permanent climate change.
"These kinds of events wax and wane through time, and I think it is a mistake to definitively come out and say that they are somehow attributed to global climate change," says Laurence Kalkstein, a research professor in the Department of Geography and Regional Studies at the University of Miami, in an e-mailed response to a reporter's question.
Tornado devastation began early this year, with a spate of winter storms that tore through Georgia and Arkansas in early February. The latest round occurred last weekend, when a series of tornadoes ripped through Missouri and other states, resulting in at least 27 fatalities.
According to the National Weather Service, about 100 people have died in tornadoes this year – the highest such grim number since 1998. The worst known year for tornado-related fatalities was 1953, when 519 died.
It is the sheer number of storms, not necessarily their ferocity, that has made 2008 a record year. Through May 11, the United States has experienced between 650 and 700 separate tornadoes, figures Harold Brooks, a research meteorologist at the National Oceanic and Atmospheric Administration's National Severe Storms Laboratory. That would put 2008 right up with 1999, which had 676 tornadoes between January and mid-May – the most on record.
While it's not totally clear what's made 2008 such a stormy year, it is possible, Mr. Brooks says, that the tornadoes are somehow associated with La Niña, the meteorological phenomenon associated with unusually cold ocean temperatures in the eastern equatorial Pacific.
For this and other reasons, the northern half of the US has had a turbulent winter and early spring, and the Gulf of Mexico has been unusually warm, early. This has set the stage for a number of collisions between moist, warm fronts and colder, drier air – the classic condition for producing a tornado.
While tornadoes can occur anywhere in the world, at any time of year, the US has the highest incidence of twisters. This is largely due to the nation's central Tornado Alley. To the west, the long, thick belt of the Rocky Mountains provides an atmospheric buffer that helps tornadoes develop. Then from North Dakota down to Texas, the land is generally flat – an almost perfect arena for twisting winds.
Although it is easy to predict that tornadoes will occur in this belt, it is difficult to say year to year how many tornadoes the spring and summer season will bring. Certain years just seem to have the large-scale atmospheric patterns that produce the deadly storms, Brooks says.
"If you look at Earth from the top of the North Pole, you'll see these waves that circle the planet," he says. "There are years when there are lots of those, and those tend to be active years [for tornadoes]."
The presence of such waves – a sort of ripple pattern in the atmosphere, as Brooks describes it – is indicative of tornado-producing conditions coming together: moisture moving northward from the equator and both cold dry air and strong winds aloft.
"These are all important for the kinds of storms that make tornadoes," he says.
For May 13 and the following days, weather forecasters were predicting a high chance of further tornadoes in areas already hit by deadly storms last weekend.
Experts warn that people in moving vehicles and residents of mobile homes in particular should seek other shelter during damaging storms, since those enclosures are affected disproportionately in tornadoes.