Here's why:
* "Thunder" is a noun. It refers to the sound of a clap of thunder.
* "Thundering" is an adjective or a participle. It describes something that is making a loud, booming sound.
To make the sentence grammatically correct, you need to use a verb that describes the action of thunder:
* It is thundering outside. (Using the present continuous tense)
* There is thunder outside. (Using the noun "thunder" with the verb "is")
Both options are grammatically correct and convey the same meaning.