Because its a front with a very breif mild infront of it. As it crosses over and drags very cold air aith a very strong wind wind chill will cause those temps to plumit. Its calm out at the moment but its nto going to stay that way. It snowed pretty much all day in RHondda where I work, on way home temps went from 2oC to 6oC in Llantrisant. Its supposed to happen. I thought I was going to be on the cusp of the rain to snow line, but for some reason I am feeling actually it will snow much further west. I looked at meteo site and all of wales has heavy snow tomorrow even pembroke and personally im betting on meteo being right as its human drawn chart. I think coomputer models have dam line way to far east to be honest.