I understand your sentences, but here are my suggestions: If the weather is nice tomorrow, can we go on a picnic? According to the weather forecast, it's likely to rain tomorrow. If it's raining tomorrow, maybe we can go to the movies.

Edited, see my answer.

If the weather is nice tomorrow, can we go to the picnic? [correct if 'the picnic' is an event you are attending--otherwise you would say "can we have a picnic?"] According to the weather report, It's likely to rain tomorrow. [correct] If the weather is rainy tomorrow, maybe we [should/could/will] go to the movies.