I never rely on the weatherman more than the same day or next day if I look in the evening, especially here in the PNW. It changes so much, if you check the weather and screenshot it every day and compare you will see that the temps, if it's supposed to rain, etc. are always changing as the day gets closer.
On days it might rain or might not rain and are questionable, it is extremely hard for them to predict with certainty if it will or not.
Check this study out
Basically it comes out to days where it may or may not, they have a less than a 60% accuracy on those days, just one day out and it drops dramatically after that. A coin flip is more accurate most of the time (once again, especially here in the PNW).
In other areas like Arizona or some places with known dry days or known wet days they have it easier, but when it's iffy, they really have no clue and might as well just be guessing.
We have had days that the weather is set to be nice and right when we're heading to the launch it starts pouring and thunderstorms.
We've had days where we scrapped plans because the night, about 10 hours before, it said it was going to be stormy and it turned out to be a very nice day out.