I thought, “but baselines shouldn’t change”, and, “informal baselines have been changing with every decade as people grow up with a different normal from their parents”. But, this seems to be talking about something specifically technical for weather forecasters?

[Climate normal has been calculated as 30-yr average] But extreme variations are now occurring in periods of ten years or even shorter. Consequently, multiple increases and decreases can cancel each other out over a 30-year period. This would hide the large changes in statistics of weather variables within that period. [This makes it hard for machine learning to predict local weather]

So maybe just a bad headline