As Matt has noted in the social group pages, there is a chance of a wet day under Mt Lofty. Its a question of how much bucketing rain there is and how much you want to wet your gear.
I've PMed the group my mobile number (we need to set up a fast communication system

) and if you text me back your mobiles I can tell you if its damp on the morning.
Given that the Hills are usually damp when the sun shines in town on a cloudy day, a location nearer the plains might be a good backup.
How about Morialta Falls? Not that much further, but lower down. And with all this rain, the falls might be quite impressive.
Cheers