Learn Croatian: Meteorolog najavljuje sutrašnju kišu. - The meteorologist is announcing tomorrow's rain.

The base word is the adverb sutra (tomorrow). To turn temporal adverbs into adjectives, Croatian adds -nji or -šnji. Because sutra ends in a vowel, it takes -šnji to make it easier to say, creating sutrašnji. Here, it ends in -u (sutrašnju) to match the feminine accusative noun kišu.

Would a native speaker ever say kišu od sutra instead of sutrašnju kišu?

No, that would sound very unnatural. English sometimes uses prepositions for time relationships, but Croatian strongly prefers turning the time word into an adjective using -nji or -šnji and placing it directly before the noun.

Why is the imperfective verb najavljuje used here instead of the perfective najavi?

The imperfective najavljivati is used because it describes an action happening right now in the present: the meteorologist is currently announcing the forecast. The perfective najaviti focuses on the completion of the action and is rarely used in standalone present-tense sentences.

Why do sutrašnju and kišu both end in -u?