This site scans Twitter for any mention of weather-related words (snow, rain blizzard, etc) in a variety of languages (English, German, Japanese...). This works on the idea that if you have a large enough sample, you'll get accurate results – if a single person were to mention snow, it might not mean much but if 20 people in a city mention snow, there's probably snow there.
If you want more accurate UK snow results, check out Ben Marsh's #uksnow map.