As greenkz stated, your car is not reaching operating temperature and is therefore running rich until it does reach operating temperature and can go into closed loop mode. This is a very common problem for any vehicle except pure electric. The only really solution is try out an engine block heater which helps preheat the engine and reduce warm up time. You can also try grille blocking during the colder months. You can also choose to ride a bike or walk during the mild temperature days. This will greatly offset your mpg impact.
There are people on our Prius forum that complain about getting mpg in the low 40s. We then find out that they have a short commute (less than 10 miles). On longer drives they get 50+ mpg. It's just the way cars work.
