Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They tell us how, when, where, or to what extent something happens.
* In the sentence "It will probably rain today," "probably" modifies the verb "rain," indicating the likelihood of the action.
While "probably" can sometimes appear in a noun phrase ("the probably of rain"), it's acting as an adjective modifying the noun "probability."