AFAIK Waze is/has implemented at least partly this idea; if roadspeeds vary by the time of day and by day of the week, one would expect to have the month influence as well. But, where I live, we could well use a calculation based on weather rather then season, as even in winter weeks can go by with nothing worse than a a bit of rain. But WHEN it snows - it should be possible for Waze to detect that from regional driving patterns.
Eg: residential areas are the last to be cleared from snow, the freeways first. So if in a region - say, 50 km in size, ALL rides through simple streets are much slower than usual, something is wrong and it could be the weather. It is different from too much traffic - then, more wazers hit the road, with bad weather it should be fewer... All expected speeds in that area should be significantly slower and ETAs adjusted accordingly. This is different behaviour from expecting different speeds based on time of the year!
AM for some 25.000 km² around Maastricht, the Netherlands
iPhone 3Gs / iOS 6.0.1