After three of lengthy stops in Xmas/ Rush hour traffic (on a cool drizzley day), the red light on my temperature gage came on. I stopped a convienient Starbucks. When the motor was restearted the temperature light came on but went out after the car was motion for a about a minute.
Then I noticed steam coming out the tail pipe and the motor ran rough. After a short drive home, in which I was enveloped in white smoke at every stop, I let the car sit. After an hour I check the expansion tank and found it very low on water. I topped it off with hot water, and squezzed the cooling hoses to force out any trapped air. I then drove it to my mechanic's shop. Except for a little steam at the start of the trip, the car ran great.
I suspect it may only be a bad head gasket, but I'm interested in hearing about any other potential causes.