WASHINGTON (AP) — Unusually warm temperatures and a storm system moving east linked to the La Nina weather pattern helped create the conditions necessary for the monstrous tornadoes and severe weather that tore through parts of the the Midwest and South.

Tornadoes in December are unusual, but not unheard of. Scientists say figuring out how climate change is affecting the frequency of tornadoes is complicated and their understanding is still evolving.

Still, they say the atmospheric conditions that give rise to such outbreaks — including rising temperatures, moisture, and wind shear — are intensifying as the planet warms.