I have a 2007 Prius with Michelin X-ICE winter tires installed (I always prefer Michelins). The regular wheels and tires will return when when the ice and snow are gone. So far this winter combination handles very well - no concerns at all.

The 31.9 mpg is not too bad at all. My first tank got me 42 (already in winter weather) but we were doing cold start p&g, DWB and a few other tricks. I fear the reality is far more cruel and likely to force us into the high 20's even with the use of a block heater.

In any case, if you do not have a block heater I would humbly suggest you get one installed. It is really worth it.
Also, other more experienced Prius owners are likely to offer even better advice. In any case rest assured: the mileage you are getting is not unexpected with the very cold weather.

Cheers;

MSantos