Not a myth KC. Inner city voters generally walk to the polls. If it's raining and you aren't that enthusiastic about the candidate you are less likely to walk 3 blocks to go vote.
Rain does not have an effect in the burbs and rural areas as most people drive to the polls.
It is a myth. Both 1998 and 2000 were predicted to have skewed results favorable to Republicans because of weather. None of it materialized.