'after storms' is a prepositional phrase. Storms is the object of the preposition. 'I' is the subject of your sentence. The nouns storms and rainbow do not match in the plurality, and in this sentence should match. Since the verb 'is' is singular the nouns should also be. After storms cannot be the subject because of the preposition after, and because you begin the sentence with 'I' followed by the verb believe. A correct fix is, "I believe that after a storm is a rainbow." Because the noun storm is now singular it needs an indefinite article, unless you are describing a specific storm.