Climate change's influence on tornadoes is a mixed bag

http://www.newscientist.com/article/dn24605-climate-changes-influence-on-tornadoes-is-a-mixed-bag.html#.UowEV81wFD8

They are unwelcome at the best of times. But why are tornadoes in the US increasingly popping up at unusual times of the year, and in unusual areas? The question is troubling meteorologists and tornado watchers as citizens in the Midwest count the cost of last weekend's tornado outbreak that killed at least eight people and devastated many towns in 12 states including Illinois, Michigan, Indiana and Kentucky.

The three largest twister outbreaks in the past year have been in November, January and October – outside the usual tornado season, says Jeff Masters, who runs the Weather Underground website.

"These types of storms with that severity are rare in November, as the tornado season is normally in spring," says Martin Hoerling of the US National Oceanic and Atmospheric Agency, adding: "The northerly location in Illinois makes it particularly unusual."

The climate effect

Because severe tornadoes are rare during cold seasons, it is difficult to say anything definitive about how they are being affected by climate change.

Twisters tend to be spawned by warm, moist air in the lower atmosphere, says Hoerling.

You also need a powerful jet stream in the upper atmosphere to draw a tornado upwards. Climate change is leading to more water vapour and warmer temperatures in the lower atmosphere. But it also reduces the intensity of the jet stream.

So climate change is having mixed effects some of which promote and some of which counter tornadoes.

In affected US states, the clear-up continues. The most violent tornadoes, given an EF-4 rating – corresponding to the second strongest tornadoes on the Enhanced Fujita scale used to measure them – hit the Illinois towns of New Minden and Washington, where at least 250 homes have been damaged or destroyed, and about 200 people injured