The reason for the snow is that there's a great big high pressure area over Europe at the moment that is trying to push cold air eastwards into the UK. On the other hand, though, we still have the normal low pressure systems from the Atlantic trying to bring milder air in from the west.
The forecasting problem at the moment is trying to figure out which one of the systems is going to win out over the other - at the moment it seems as though the systems are stuck just over the UK, so the west and south have the milder, cloudy, drizzly weather and the north and east are getting the colder, snowier weather (I'm in Bristol today and it's currently 7 degrees with lots of cloud and a bit of drizzle).
As one of the BBC weather presenters said earlier in the week, this weekend is bit of a forecasting nightmare

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