Learn Croatian: Navodno će sutra padati kiša. - Allegedly, it will rain tomorrow.

Navodno means "allegedly" or "supposedly." It shows you are just passing on information (like a weather report) without taking responsibility for its accuracy. In contrast, vjerojatno (probably) is used when you make a logical deduction yourself, and valjda (I guess) expresses your own hesitant assumption or hope.

Why is the future tense helper verb će placed right after the opening adverb?

The word će is an enclitic, meaning it strictly requires the second position in the clause. Because you started the sentence with the adverb navodno, the enclitic naturally locks into that second slot. If you shifted the focus and started with the subject, the order would change to Kiša će navodno sutra padati.

English uses the dummy subject "it" for weather (it will rain). Why isn't there an equivalent for "it" in this sentence?

Croatian handles precipitation differently and does not use a dummy pronoun for weather. Instead, the weather element itself is the grammatical subject. Here, kiša (rain) is the subject in the nominative case, and the verb is padati (to fall). You are literally saying "Allegedly, rain will fall tomorrow."

Could I use the perfective verb pasti instead of the imperfective padati for this forecast?