I live in California too
Winters don't really get too cold where I am.
The lowest I ever go is the high 60s on a windy/cloudy day, maybe high 50s on a really non-windy/sunny day. If your hose has warm water, maybe even a little bit colder.
If its below 65 degrees F, swipe off as much excess water as possible, and then put the horses' blanket on shower.
Another thing you could do is just clean the horse using a damp towel and a bucket of water.
Horses are resistant to heat. Unless it's like snowing outside or something, she should be okay.