Another MPG robbing issue in cold weather is in the car trying to keep YOU warm.
Heat to warm the cabin has to come from somewhere, and in the prius, as in most cars, most of that warmth comes from the excess heat from ICE.
Now in the Prius, the ICE will normally shut down a minute or so after the warmup starts 7 seconds from when you turned it on if you are standing still, even though it has not reached optimal running temperature. However, if you are demanding heat, ICE will not shut down so readily.
Thus, some of your gas consumption is not to travel, or even to keep the ICE/CAT/O2 sensor warmed up, but rather to keep you warm, but offering no miles to show for it.