Could you maybe put food out for the cat? Make it some kind of shelter (e.g., an upended plastic bin with an entrance cut into the side and bedding under it) it could go into when the weather is bad?

Alley Cat Allies is a nationwide organization dedicated to the care of feral cats. They have lots of information and tips in response to the questions you ask: http://www.alleycat.org/page.aspx?pid=984

Best wishes to you and the cat, and God bless you for taking an interest in him/her.