Site says it is open rain or shine. Since it is all outside subject to weathering I would not count on a rain refund. Call them to inquire for certainty.
They are really great about accommodating your plans and needs especially when it's not crowded. We bought our tickets the night we went on site, but their web page has a link to purchase as well (that also shows an updated count of how many tickets remain for each showing, too.)
Thanks! Your answer is awaiting moderation.