Help!!! Warping Plywood

I have not had a chance to cut out my sides and my 1/2" plywood is setting in my garage and we have had a week of continuous rain and cool weather.
Both pieces of plywood are now warped and have a bow in them. What do I need to do to keep them straight?
Thanks
Both pieces of plywood are now warped and have a bow in them. What do I need to do to keep them straight?
Thanks