Metcheck is usually pretty good (one of the better ones, I think), but definitely I wouldn't take it as definite more than about 2/3 days ahead, and then only as a guideline. There's too many variables to be THAT accurate that far in advance.
My best friend got married the weekend before last. I started checking the Saturday prior to the wedding. And I checked them all!! Heavy rain. By the tuesday it had changed to light rain. By the Thursday it was cloudy but dry, On Friday they decided sunny.
It turned out to be the hottest day of the year so far! Not a cloud in the sky all day
In short I don't think they're all that reliable for more than a day or so in advance