About 3.5 years ago, I installed a temporary sign at a local mall. it was slightly smaller then 5x10' 1/2" Sintra with a digital print and extruded letters on the face. Since it was supposed to be temporary, we used VHB ( it was supposed to be up only for the Christmas season) The store did so well that it decided to hang out.... for the next couple years. Our installation guys check on it from time to time when they're about installing things at the mall, but it's still up there well.
Granted I'm in Virginia, and we don't get nearly as cold as CA, but it WILL occasionally get down to 0f a couple times a year, plus we have the added benefit of 100f+ summers and 40000% humidity.
I guess it all depends on the circumstances, and whether the great sign gods are smiling on you.
Personally, with a sign that size, being that high in the air, I'd jump up there and drop several screws in it, to insure it stays. Last thing I'd want is that thing coming down on a bus load of nuns, escorting homeless orphans into your shop on a PR campaign, and have your sign go all guillotine on them.
...I'm just saying.