AccuWeather notes "the best example of weather potentially affecting an election was the extremely close presidential vote of 1948, when Democrat Harry S. Truman won the presidency over Republican Thomas Dewey. Weather was a factor in two especially close states that were won by Truman, Illinois and California. A slight shift in vote in these states would have awarded the election to Dewey."
Intellicast has another good piece on weather's historical election impact.