At Wx Centre, we provide a 7-day forecast. While others go much further into the future, so why don't we?
The answer is quite simple, the Earth's atmosphere is far too complex and unpredictable for anything past a week away.
In fact, the American Meteorological Society states that "forecast of daily or specific weather conditions do not exhibit useful skill beyond 8 days, meaning the accuracy is low."
The most well-known weather models, the GEM & GFS only provide guidance out to 10 days. Those models, along with the Euro model, tend to vary widely as time progresses.
Any forecasts past 8 days when it comes to specific parameters are basically aking to throwing darts at a dartboard.
Seasonal forecasts are different because they speak in general terms over a broad area and do not provide specific parameters (example: sun and cloud, wind west 30 km/h, high 15 degrees 16 days from now). Weather forecasts are not at that point yet, and with how chaotic the atmosphere is, we may bever be at that point.