When storing wood in this way, it is very important to allow air to circulate around the planks. Make sure the planks are protected from rain but be careful not to close off the sides and underneath. Periodically, insert spacers so that air can circulate within the stack. As much as possible, the timber should also be stored in a cool place.
Stack your timber this way and see how it looks in the summer when you need to use it. The biggest risk is probably that it warps, making it impossible to use because of that.