When storm clouds produce lightning and thunder, energy changes to energy and energy.

When storm clouds produce lightening and thunder, then, potential energy changes into kinectic energy.

The clouds become negatively charged, and the ground is positively charged during a storm. So there creates a potential difference between the charged clouds and the ground. And when this becomes huge so much that it over comes the electrical insulation cover of air, the electrons jump to the ground. i. e. the potential energy changes into kinetic energy.