I recall seeing a month long forecast on one of the privately owned services, maybe "accuweather", and wondering just how accurate they were, and thinking probably not very.

OM is right, as good as they are, and they are good, 48 hours is still all I'll trust then for when planning a backpacking trip because I've paid very close attention to the accuracy of those 3-day forecasts over the years. For the most part, the 3-day forecast is pretty accurate here, but when I'm winter camping here I've learned to hike to the top of a ridge where I can get a cell phone signal and check the latest forecast after that 1st 48 hours. That has saved my butt at least once, and I've changed my plans a few more times based on what I learned.