How were you planning on using the cover? Many people who haven't used them before think of them as a kind of substitute for a garage. They really don't work very well for that.
Their primary benefit is to keep the SUN off of a car and it's interior minimizing UV damage. It's not good at all for the cover or the car to sit in the rain. A car sitting under a cover in the elements (rain/dry cycles) is worse off than just letting it sit exposed.
And I can tell you from personal experience, trying to rush out and get the cover off before it starts raining is a lesson in high maintenance. If the cover does get wet - good luck trying to find a place to store it. They hold an amazing amount of water, and putting them in the trunk is a BIG problem as the water, of course, finds it's way to the bottom of the trunk with no way out - not to mention soaking carpet and pads.
Covers - great for garaged cars at keeping dust/contaminants off; great for keeping the sun off one outside. TERRIBLE for keeping water off.
If you already knew - never mind.