If you're going out walking it's vital you know what the weather is going to be like as it can change fast up on the fells. There are lots of places you can turn to for this as you can't have too many forecasts ...
The Met Office who provide a detailed Lake District Forecast.
Weatherline offers a five day forecast for the Lake District produced by the Met Office plus, from December to March, their fell top assessors walk up Helvellyn to check conditions, take photos and supply a report to add to the Met Office weather forecast that appears 365 days a year.
MWIS, the Mountain Weather Information Service, provide a three day forecast for the Lake District (and other mountain regions too) aimed specifically at the mountain walker.
Here are the forecast times for the sun to poke its head out today but do remember to tuck a torch in your pack, even if you're sure you will get down off the fells well before dusk: being benighted without a torch is no fun at all.
Cumberland Council have a number of roadside weather stations dotted around the region. as well as various temperature sensors and a wind speed and direction indicator, they have a camera. Whilst not so useful for viewing the condition of the fells they are handy to check on the state of the roads.
The Met Office also have a weather station on Shap, as well as current also gives the previous day's data.
The Environment Agency has a network of monitoring stations throughout the Lake District so if you are at all concerned as to water levels, especially if you are expecting to ford a stream on your walk, then it's worth consulting their site. Start on this page.
Whilst it uses the same data as the EA links above gaugemap has a much nicer interface using google maps.