Trash bags? Are you sure that's a good idea? Personally, I wouldn't want to store water in anything that's not food grade. Trash bags were never intended to store things consumable by humans.

They are not recommended to store food in because they can leach nasty chemicals. Also some trash bags are treated with pesticides. Not things I'd want to add to my water supply.

Maybe you should consider going with water storage tanks for runoff water. Not as cheap as garbage bags, but much more reusable, and made with food grade plastic. Plus if you have it on hand, it will always be full when you need it. You won't need to worry about setting up or when this dry spell will end.