The weather can change suddenly in the mountains. Forecasts are never 100% accurate and are often wrong, particularly with respect to the timing of weather events. We recommend checking the forecast whilst planning your trip and then again as late as possible before heading into the fells. With smartphones it is even possible to check the forecast whilst on the fells (if you have reception). I personally have the MWIS mobile forecast at www.mwis.org.uk/mobile/ld.php bookmarked on my phone (this only works on mobile devices).
Mountain Weather Information Service
The Mountain Weather Information Service produces a very detailed three day forecast available here.
Met Office Mountain Forecast
The Met Office forecast includes a detailed daily fell-top conditions report from Helvelyn during winter months. This can be very useful when planning appropriate days out in winter.