American Heritage Dictionary via GuruNet:
"In the upper southern United States the words powerful and mighty are intensives used frequently like the adverb very: Your boy's grown powerful big. The new baby is mighty purty. [Sounds like Nebraska talk to me. ;)] Powerful is used as an adjective in some expressions: The storm did a powerful lot of harm. ... Colloquial English is always on the lookout for ways to make language more vivid with new intensives. We think of the upper southern part of the United States as linguistically conservative, but in fact it has preserved uses of power, powerful, and mighty that were innovative in their time."
"In the upper southern United States the words powerful and mighty are intensives used frequently like the adverb very: Your boy's grown powerful big. The new baby is mighty purty. [Sounds like Nebraska talk to me. ;)] Powerful is used as an adjective in some expressions: The storm did a powerful lot of harm. ... Colloquial English is always on the lookout for ways to make language more vivid with new intensives. We think of the upper southern part of the United States as linguistically conservative, but in fact it has preserved uses of power, powerful, and mighty that were innovative in their time."