Explanation

In a conditional sentence referring to the future, the if-clause takes the simple present tense, not ‘will’ or ‘may’. ‘I shall not be going to the theatre’ expresses a future intention, so the condition must be ‘if it snows’. ‘If it may snow’ wrongly uses a modal in the if-clause, ‘if it snow’ lacks subject-verb agreement, and ‘if snowed’ is ungrammatical. Therefore ‘if it snows’ is correct.