I always have a rain coat/jacket with me. Either in the car or in my office.

I am rather fond of the Carhartt Waterproof Breathable line. Pricey, but very well made and remarkably waterproof.

I would recommend you go to the store to try them on though, they are a little large cut. I think they are designed so you can wear an insulated parka or something underneath it if you need insulation as well as waterpoofing.