I live in Sacramento, California. One of my friends has an outdoor goldfish pond in a huge galvanized aluminum tub. Really gigantic. When our night temps stay above 60*, he tosses in a handful of guppies. They seem okay, and by the end of the warm season, he has a lot of them to sell back to the fish store. I told him this year I would supply his guppies, as an expirament to see whether ones raised in a stable environment would do as well in an uncontrolled environment.
I'm thinking it won't be too long before it's warm enough. You can ask me later in the summer how they are doing. Just remind me, because I forget lots of things! (but not feeding my fish. They glare at me if I even think about forgetting to feed them!)