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.
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.
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.
Cumbria Country 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 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, click on the area you are interested in and then, on the next page, click on a river level station to see details of the level at that location.
The Environment Agency also puts out flood warnings. All those for the north west can be found here.
Whilst it uses the same data as the EA links above gaugemap has a much nicer interface using google maps. If you are on twitter you can subscribe to regular updates from particular stations like the one at Keswick Campsite.