If the bonus for correctly estimating a month that's 2*C from the LTA is a 0.3*C reduction, then one wrong call could easily cost you a great deal more than that bonus.
I think that's the key here for something that works well; small, yet still appreciable bonuses.
I'd be astounded if there was anyone who could reliably anticipate extremely anomalous months and so 'abuse' this system.
Just my two cents
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As for the current CET situation - seems like there's a 3, maybe even 4*C range in the possible CET returns, depending on whether it's sunny as expected and if so, how much the flow off the North Sea is modified by surface heating.
For this reason I've not bothered doing any rough CET estimates; it'll be what it'll be. Not feeling very positive about the likely final CET though. I imagine that even if Friday was widely mid-high 20s and Sunday high 20s to low 30s, that'd still not be enough to get the CET to the 15.0*C mark (which incidentally would be just over halfway along the range of estimations for this month; midpoint is 14.9*C).
I can't help but feel horribly cheated by this unusually strong ridge formation. From researching, it looks like I can blame the exceptionally high Arctic Heights for that - something that I'd expected to be toned down a bit by now but no, it's raging on! Astounding, really.