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It's hard to predict what tomorrow's weather will be like.
Literal
Tomorrow [possessive-の] weather [subject-が] how-become [question-か] [object-を] predict [nominalizer-の] [topic-は] difficult.
A good example of the nominalizer の converting a whole clause into a noun phrase that can then be the topic. 予想するの = 'the act of predicting.' Note the particle chain を...のは: を marks the internal object (the embedded question) and は marks the nominalized predicting as the topic under discussion. The embedded question どうなるか has no のか here — just か — which is also valid.