Looking like the CET will probably finish just below 5C now given the very cold end to the month and the likely usual downward adjustment. We had 4.8C back in 2017 and again in 2012. Probably something similar this year. Not had a December colder than this since the infamous 2010.
Despite this December being slightly cool overall, the mean December CET for 2011-2020 will come out at 6.0C which is exceptionally mild. That is 1.4C above the 1981-2010 mean. December is now becoming so mild generally that it is not far off being as warm as March. The March CET mean for 2011-2020 is 6.55C. 4 Decembers in the past decade have been warmer than previous March.
This December has been a month of three distinct weather patterns. Based on my local data the mean temperatures have been as follows:
1-9 Dec 3.3C
10-23 Dec 8.9C
24-30 Dec 2.7C
Overall mean 5.6C
Despite the cold end to December it looks like 2020 will see the the third highest CET mean on record, somewhere around 10.75C.
It is worth noting that since the very cold March of 2013 we have not seen a single month where the CET mean was more than 1.5C below the 1971-2000 mean. In the same period there have been no less than 27 months that came out more than 1.5C above the 1971-2000 mean. The warming trend is so stark now that it is becoming quite frightening.
The new 30 year running CET mean (1991-2020) will come out at 10.22C. This is 0.25C above the 1981-2000 mean and 0.75C above the 1961-1990 mean. The increase in rolling 30 year mean temperatures has been running at about 0.25C per decade for the past three decades now.
1961-1990 9.47C
1971-2000 9.74C (+0.27C)
1981-2010 9.97C (+0.23C)
1991-2020 10.22C (+0.25C)
The trend is continuing although perhaps slowing down a little with the 10 year mean for 2011-2020 coming out at 10.37C. To see another 0.25C increase in the 2001-2030 figures relative to the 1991-2020 figures, the CET mean over the next 10 years would need to be about 10.8C (i.e. similar to the 2020 figure).