The weather forecast predicted heavy rain for the entire weekend. ______, the skies remained clear and sunny for both days.

The second sentence describes an outcome that contradicts the forecast named in the first sentence, clear skies instead of predicted rain. However signals exactly that kind of contrast. Similarly would suggest agreement between the two sentences, but clear skies and a rain forecast are opposites. For example would frame the second sentence as an illustration of the first, which it isn't. Therefore would incorrectly suggest the clear weather resulted from the forecast, reversing the actual relationship.