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I'm worried it might rain tomorrow.
Literal
Tomorrow rain [to-に] become [isn't-it-that-のではないかと] worry-is.
~のではないかと心配です means 'I'm worried that it might be the case that X.' のではないか is a soft way to suggest a negative possibility — literally 'isn't it that...?' Combined with 心配です, it expresses anxious speculation.