We often use the first one “明天要下雨了。” and some others may use the second one “明天会下雨。”. There's no difference between them.

to some degree,there is no differencebetween them ."要" is used to emphasize mood ,indicate the speaker's attitude toward the factuality or likelihood of the action or condition expressed. it will be rain tomorrow. it's going to rain tomorrow .

其实通常没有特别严格的区别，一样的意思，就像it will rain 和 it's going to rain

i think there is no obvious difference between "明天要下雨"and "明天会下雨" these two sentences are interchangeable .

从语气上来说，前者是表达一种客观事实，非常肯定的语气；后者是表达一种推测，是一种推测的语气。 从意义上来说，没什么差别 从程度上来说，前者要强烈，肯定性更强。