This is the time of year (in Denver area) that arm warmers are the bee's knees --and again in the Spring. Cool in the morning, warm in the afternoon. I'll wear them on the way in, and go without on the way home. And I keep them (as well as leg warmers) in my commuting backpack all the time, just in case --it is Colorado, after all....

You will need a balaclava. Once December rolls around it's cold enough in the AM that it's worth it.