The answer is “due to” because the phrase after the connecting verb “was” is working as an adjective modifying the noun “snowfall.” You can tell that because of the verb “was” is a connecting verb, which means it connects either a noun (or word or phrase functioning as a noun). Because only adjectives can modify nouns, it's “due to,” which, you remember, is adjectival.
      The easiest way to remember it is this way: whenever you see a form of the verb “to be” (is/was, etc.) and that modifying phrase can fit directly after it, it's “due to.” Always.

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