Rainbow and Lightning.

From the website:

The clash of weather was seen above the affluent city of Fort Smith, in the Southern state Arkansas.

One onlooker said: "It was awe inspiring. The lightning made a huge rumbling sound and when you looked up there was also this incredible rainbow forming on the horizon."

The intracloud lightning, known as an anvil crawler, is the most common form of lightning, with the electrical charge contained within a single cumulonimbus cloud.

Link - via Cellar Image of the Day and Boing Boing