Visibility will be as low as 100m as hazardous patches of fog are forecast to form quickly on Thursday evening.
The fog is expected to clear later on Friday and the rest of the UK is forecast for dry, mild weather for the rest of the year.
Met Office forecaster Simon Partridge told The Independent: “People are quite used to fog developing overnight and seeing it first thing in the morning, but tonight we’re expecting it to form just after sunset.
"It will be dense in some places and the worst thing is when it does start to form, it is initially quite patchy in nature and that’s more dangerous in terms of driving conditions than widespread fog where you get used to the conditions.
"But when it’s patchy people tend to speed up and slow down again and it’s a bit more of an issue.
“And at this time of year there are quite a lot of people on the move as well.”
The severe weather warning is valid from 5pm on Thursday until 11am on Friday.
“It’s partly down to the time of year - long nights means things will cool down quicker, and short days mean things don’t warm up quite as fast either,” Mr Partridge said. “At the moment we’ve got high pressure to the south, bringing in milder air which is fairly moist. With the moisture in the air, as soon as things cool down it will quickly start to condense and that’s how we’re seeing the fog forming.”
“Overnight we should see more cloud heading southwards and with the cloud coming in over the top of the fog we should see it beginning to disperse a little bit. So hopefully it will not be quite as extensive tomorrow.”
The Christmas period has seen higher than average temperatures across the country. Parts of west Scotland recorded temperatures of 13.6C on Christmas day, and more bright sunshine is forecast for Friday.
But across the country averages have been one or two degrees warmer.
“Things have been fairly mild this winter and we haven’t had any significantly cold spells since way back in February-March time when we had all the snow,” Mr Partridge said.
“We’re not looking at it getting much colder any time particularly soon - it’s going to be pretty mild all the way through to the new year. But things may get a bit cooler during the first week of January.”
The mild weather has been caused by the jet stream moving north of the UK allowing higher pressure to arrive from the south.
“Because the jet stream has moved north of the UK, most of the low pressure systems which bring the wind and the rain will be steered north as well, and in addition we get the higher pressure south of the jet, which is what’s keeping the weather so settled at the moment,” Mr Partridge said.
“Unfortunately the air is quite moist, so that’s giving us a fair bit of cloud at the moment. You can’t win ’em all.”
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