Explanation

• It is raining heavily
• ***
• The above sentence presents a continuous sentence
• (subject)+ is /am / are + (first form +ing ) + (rest of the sentence)
• In the present continuous sentence, the main verb is in the -ing form. 
  
• Present continuous. - - it is raining
• ***
• It (subject)
• Is /am / are
• Raining (first form +ing ) 
• Heavily (rest of the sentence)