Severe Weather Climatology

Spring is fully in gear, and that means the atmosphere is warming after the chill of winter. But it also means severe weather season is approaching its annual peak.

Driven by the seasonal rise in temperatures and humidity, severe thunderstorms will continue to pop up, bringing the dangers of damaging winds, hail, and tornadoes. And one study from researchers at Stanford and Purdue Universities suggests that the environment that produces severe thunderstorms will be more common in a warming world.

Any of these individual threats can be deadly, but tornadoes are the most frightening. This week marks the anniversary of the April 27, 2011 super outbreak. In Alabama alone, 62 tornadoes were verified that day, including a catastrophic EF4 that ravaged Birmingham and Tuscaloosa, killing 65.No one knows exactly what this tornado season will hold, but a team at Columbia University, for the first time, has published a tornado outlook for this severe weather season. The team indicates there is a 60% chance the season will bring an average number of tornadoes and a 30% chance the number will be below average. And there is still much to learn, because at the local level, we don’t fully understand why some rotating thunderstorms (aka mesocyclones) produce tornadoes and some don’t.We also don’t know how climate change will affect tornado frequency or intensity in the future. It’s reasonable to think it might: the parent thunderstorms that spawn tornadoes feed off of atmospheric heat and moisture, and we should expect more of both as the planet warms under its growing blanket of greenhouse gases.

But tornado formation is a complex process. In addition to the fuel provided by the warm and humid air, wind shear is needed to generate spin. Plus, a trigger is needed to start the convective process, which is often provided by a cold front or a dry line. How those latter two factors will evolve in a warming world is still unclear.

What scientists do know is that the number of days with at least one tornado happening in the continental U.S. is decreasing — but that we’re seeing more tornadoes per day when they do happen. Further complicating matters is that tornado detection has improved over the past few decades. Harold Brooks, of NOAA’s National Severe Storm Laboratory, emphasizes that “climate models indicate an increase in this variability in the future.”