Warming to make US conditions more ripe for tornado-making storms, study says

Global warming weakens a key ingredient for tornado-making thunderstorms, prior studies held. True, researchers now say, but the number of days in the US where the right conditions exist will increase.

Conditions that spawn severe thunderstorms – including tornado-makers – across the US are expected to appear more frequently by the end of the century, according to new research.

Until now, studies of the potential impact of global warming on severe and tornadic thunderstorms generally have concluded that a key element needed – a sudden change in wind speed or direction with altitude known as vertical wind shear – would weaken on average as the climate warmed.

This would deprive thunderstorms of the opportunity to grow to the most intense levels, even as three other key ingredients – warm surface temperatures, lots of moisture in the air near the ground, and cold air at higher altitudes – were abundant. It was tough to say whether global warming held the potential to boost the number of severe thunderstorms.

The new research confirms the general pattern of weaker shear over the US, on average, in a warmer world. But when the scientists conducting the modeling study burrowed into the numbers behind the average, they found that as the climate warmed, shear tended to strengthen on days when the other ingredients also were abundant. These other ingredients determine the potential energy available to form a thunderstorm – energy that climate and weather researchers refer to as convective available potential energy, or CAPE.

Thus, while still in the minority in any given year, the number of high-CAPE days during the final years of the 21st century is projected to increase, compared with the conditions during the final 30 years of the 20th century.

"The net effect of warming is to increase the occurrence of days with high CAPE and sufficiently high shear to exceed the severe threshold" for thunderstorms, says Noah Diffenbaugh, a climate scientist and senior fellow at Stanford University's Woods Institute for the Environment, who led the team reporting the results Monday in the journal Proceedings of the National Academy of Sciences.

Weather forecasters define severe thunderstorms as storms that bring winds of at least 58 miles an hour, hail at least one inch across, or tornadoes.

The researchers caution that the study does not try to project the number of severe thunderstorms likely to occur.

"Even today, we often see days when conditions are favorable yet no storms form," or they form over a smaller area than expected, says Robert Trapp, a climate researcher at Purdue University and a member of Dr. Diffenbaugh's research team.

Local factors that trigger or inhibit storm formation occur on spatial scales too small for global climate models to simulate. That's why the researchers focused on broader environmental conditions that are known to favor a buildup of the sprawling, towering thunderheads that mark severe thunderstorms.

The study draws on the results of 10 climate models whose results underpin the next set of reports on climate change from the Intergovernmental Panel on Climate Change. A final summary of the first of three major volumes is slated for release this Friday.

Diffenbaugh and his colleagues analyzed the results for the continental US, with a special focus on the factors contributing to CAPE and shear. The team looked at the daily results, with a special focus on conditions in the late afternoon – when severe thunderstorms are most likely to develop.

Using the most pessimistic scenario for future greenhouse-gas emissions the IPCC is considering for its latest global-warming assessments, the team found that the average number of days conducive to severe thunderstorms increased throughout the year in the eastern US. On a percent basis, winter saw a 50 percent increase in days with severe-thunderstorm potential, while spring sported the largest numerical increase – two additional days by the final 30 years of the 20th century.

Two days out of 365 is a small number, but depending on a storms' path, just one of those days can be disastrous, as Springfield, Mass., learned in 2011 when it was hit by one of six tornadoes that struck New England on June 1.

Indeed, depending on the season, virtually all regions in the lower 48 are projected to see some increase in days with higher storm potential. In the spring, the most pronounced increase comes to the central US, but increases also extend into the upper Midwest and Northeast as well. Summer sees the lowest increases in days with the potential for severe thunderstorms, with some regions actually projected to see decreases, depending on the model.

On high-CAPE days, shear increased at two altitudes important for the evolution of severe thunderstorms. Sufficient shear between the ground and about 20,000 feet sets a thunderstorm cell to spinning, pushing it to supercell status. Under the right conditions, supercells spawn tornadoes. One of those conditions involves vertical wind shear between ground level and about half-a-mile high in a supercell's vicinity.

This lower-level shear in the results could help researchers sort between environments conducive to severe thunderstorms and those that could add a twister to the severe storm. Even then, however, the presence of this low-altitude shear is not a perfect predictor of an impending tornado, Dr. Trapp cautions.

For Harold Brooks, a senior scientist at the National Oceanic and Atmospheric Administration's Severe Storms Laboratory in Norman, Okla., Diffenbaugh's team has taken important strides over previous efforts to estimate the effects global warming could have on severe thunderstorms in the US.

Previous work had tended to rely on one or two models using one greenhouse-gas emissions scenario, he says. And while Diffenbaugh's team uses one scenario, the team's use of output from 10 models, each producing simulations of each day's conditions at different times of day over a 100-year period "is a big deal," Dr. Brooks says. It allows researchers to explore the impact of climate change on severe thunderstorms at unprecedented levels of detail.

The results contain some oddities, he cautions. South Texas shows up as being a potential breeding ground for severe thunderstorms long after historical data show the region's spring season has ended.

Even before this latest study, previous studies also had pointed to south Texas as a hotbed of severe thunderstorm formation at times that didn't match the instrument record.

Looking closely at the details of these previous studies, south Texas's general environmental conditions were willing, but the triggers for initiation were weak, Brooks says.

"The conditions were there, but in some sense the atmosphere was wasting them," he says.