Checking the weather forecast is relatively easy, but try to get it as local to the Peaks as possible, rather than using a national forecast. For example the BBC website (at www.bbc.co.uk/weather/ukweather/) allows you to look at a five-day forecast by specifying Fort William for Ben Nevis, Seascale for Scafell Pike, and Caernarfon for Snowdon. If you search the Web you should find other websites which give even more localised info. Here are some current (2006) examples:
Poor weather may slow down your progress, either due to poor visibility making paths difficult to find, or because dampness makes the paths slippery. If poor weather is forecast, take this into account when planning start and finish times.