@Asopropricuntent said in Difference-from-Average-Temperature Layers:
The only thing I would suggest that average temperature period could be cusotmized: for example, 5 years, 10 years, 20 years...
This would be impossible. As I said above this can be done only at weather stations which record temperature over long period.
The ECMWF provide Anomaly charts (see my link in my previous post) which are based on the comparison of current forecast with their M-climate i.e. a reanalyse over several years.