I don’t leave my cushions out all the time because they deteriorate faster even when they are fade resistant and water resistant but with some cushions you can if you like. (I live in the UK so rain is more of a problem than sun) Olefin is water resistant so they should be OK in a light shower but it is not necessarily completely waterproof unless the cushions are specifically treated with a waterproofing agent. Check with the label, the store where you bought them or the manufacturer for care instructions. For more info on Olefin see this article at Wisegeek called “What is Olefin”. If cushions are not completely waterproof and they get a chance to dry out after a shower, you shouldn’t have too many problems, even if they are not fully waterproof, but if you get rain for quite a few days and the cushions get no chance to recover then you may get some mold and mildew spores starting to grow.

Hope this helps

Jan