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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.

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