Highest temperature in London on September 9?

What this question means

A notably confident distribution for a temperate city: London on 9 September peaks at 20°C with 59%, then 19°C at 21% and 21°C at 14%, with 22°C at 5% and almost nothing beyond.

Fifty-nine percent on one degree is high for this part of the world. Compare with the Wellington question on this board, where the peak takes only 39% — both maritime temperate cities, but with very different spreads. Confidence here suggests settled conditions rather than a system in transit.

The comparison with London two days earlier is the more interesting one, though. That question peaks at 24°C. This one at 20°C. The distributions are both tight, and they disagree by four degrees — which is a forecast of change, made with conviction.

That combination is what good short-range weather forecasting looks like: specific about what happens, specific about when it changes, and honest about the narrow band of remaining doubt.