My RES has been living in our Los Angeles Pond for about 10 years now. She's fine during the winter, though the pond is 2.5 feet deep (which is important).
If you do decide to keep her outside, DO NOT TOUCH HER OR DISTURB HER. Let her sit at the bottom of the pond. Also, be wary of raccoon. They're scoop her up and that'll be it. We have a cage over our pond here in Burbank, and I'm constantly chasing raccoon away that are sitting on top of it trying to reach in. Before the cage I have lost turtles.
The cage can be seen here:
viewtopic.php?f=14&t=35778&p=338301#p338301The water temperature is cold enough here to start brumation. Just know that there is always a chance that something goes wrong.