The weather on Ben Nevis is often unpredictable which is not surprising given its latitude and arctic conditions for much of the year and as such walkers and climbers should always be aware that weather conditions on Ben Nevis can change rapidly and often for the worst. A good day at sea level is no guarantee of good weather on the mountain! Be safe. Be prepared.
There are a number of different sites that provide good weather forecasts for either Ben Nevis itself or for the mountain area:
Met Office - Ben Nevis - excellent 5-day forecast of Ben Nevis itself. Provides a wealth of information from temperatures to 'feels like' to wind speeds, visibility, humidity, uv index.
Mountain Weather Information Service (MWIS) - often the hillwalker's first port of call when deciding whether to tackle a hill on any particular day. 3-day forecast gives the weather for particular mountain ranges (linked page is for the West Highlands).
BenNevisWeather.co.uk Rubbish for trying to plan in advance as it doesn't get updated for the following day's weather until the evening before but it does have some bonny graphics. Run by the local authority.