"pluvos" is futur. Maybe the course prefers you to translate this grammatical futur by "will". But "tomorrow / morgaŭ" indicates a logigal sort of futur as well, so both sentences have the same meaning : Morgaŭ pluvos. Morgaŭ pluvas. In fact, if you speak about a period of just a few days, than tomorrow ist future and yesterday is past. But if you speak about a longer (annual) period, then yesterday+today+tomorrow+this week are present, and futur begins only next week or next month. The limits between past-present and present-futur depends on your context.