明日晴れるかどうかわからないが、もし晴れたら私たちはピクニックに行く。

、。

I don't know whether it'll be sunny tomorrow, but if it is, we'll go on a picnic.

Literal

Tomorrow clear-up [whether-かどうか] know-not [but-が], if clear-up-if we [topic-は] picnic [destination-に] go.

Chains two clauses with conjunctive が ('but'): first the uncertainty (かどうかわからない), then the conditional plan (もし~たら). もし reinforces the hypothetical nature of the たら conditional — literally 'if by chance.'