A wildfire in southwest France grew so powerful that its towering smoke column became a thunderstorm, generating lightning that struck the ground and ignited new fires beyond the original blaze, French officials said. The so-called pyrocumulonimbus — which France’s national firefighters federation said had never before been seen in the country — also generated violent, erratic winds that could drive the flames in new directions. The inferno was no longer simply being propelled by the weather around it. It had begun creating its own.