I feed feral cats in Hartford. In an effort to keep their water supply from freezing, I get a packet of salt from Burger King, McDonalds, etc. and put it in the water. This prevents it from freezing (or freezing too quickly). It will also help the cats retain water in case they don't get any for a day or two. Eventually the water does freeze, but by the time it does, I'm already there replacing it.
Another thing you might want to do is to get a 20 gallon bin with a top on it. Flip the bind over so the cover is on the bottom. Cut out a "U" shaped hole and fill the bin with newspaper, hay and a blanket along with food and water. This will at least give them a place to go out of the cold and wind.
You're a very special person to do that. Working for a rescue organization, I get so angry when people just leave their pets to fend for themselves.