I have had the Prius for more than a year now and based on my experience I can tell you that the fuel consumption normally increases during cold weather due to "more gasoline engine run time" as it tends to keep the engine warm enough to avoid jerky cold starts on hybrid switch overs. secondly, if you use the cabin heating on auto, the engine tends to start more often as well-
During Jan /Feb my average dropped even below 20 KM/L, but now since the temps are rising in Lahore its giving me just under 24 KM/l masha Allah.
hope it helps.