Raining HEAVILY, isn't it? It would be considered bad English to say raining HEAVY. You can leave out the 'it's' (as in "It's raining heavily, isn't it?" but it should be the adverb heavily. Use the adjective like this: The rain is heavy, isn't it? Get your umbrella! The use of adjectives where adverbs should go is increasing. It is a sad fact that many people use adjectives where adverbs really ought to go. It sounds uneducated, and is not acceptable to many people. But.. ah... times change.....

It's raining heavy, isn't it?