Even before the -25.2°C minimum temperature at Shawbury set a new record for the coldest December night in England , it was always known by forecasters as being a remarkable cold spot due to the local topography with hills all around and its sandy soil. Given the fact that the maximum on the 12th was just -12°C (06-18), and there was 11 cm of lying snow, you don’t need to be a weatherman to realise that the coming night would be very low, especially as the first half of it was going to be clear with little wind.
Here’s a map of 09-09 minimum temperature from the MIDAS data set for Saturday the 12th of December 1981. I display it as happening on the previous day because climatologically minimum temperatures are always thrown back to the previous day and this minimum occurred in the 21-09 period which for the general public means it happened on the 13th. This may not be strictly true because it well have happened before midnight on the 12th (see plotted chart), but I don’t have access to the either thermograph trace or the hourly observations at Shawbury so I can’t say for sure either way.
Interesting to see that their was still a healthy gap of over 3°C between the screen temperature at four feet, and the one recorded by the grass minimum thermometer, which that night was likely to have been just above the top of the snow surface.
Here’s the plotted SYNOP chart for midnight that shows the synoptic situation across England and Wales. As you can see the transient ridge is collapsing quickly from the west as pressure falls, and by 06 UTC on the Sunday morning temperatures at Shawbury had warmed to -12°C. Who knows what the minimum could have been at around 08 UTC if the whole night had been perfectly clear and calm?