Is Rainfall In Wales Becoming More Extreme? Simple Answer Is No
I had a fruitful conversation with Grok, after he/she/it mentioned in passing that floods in Monmouth since 2000 had become more frequent because of ‘climate change’
When challenged he referred me to Met Office reports, which stated the amount of rain falling on the wettest days (99th percentile) has increased by ~20 percent across Wales since 1960.
He also quoted attribution studies.
I responded by pointing out that the 1960 to 1990 period was a much drier interlude. When the HADUKP daily data, which goes back to 1931, and monthly data back to 1836 was analysed, those “extremes” disappeared.
Grok responded:
Which, I suppose, all goes to show that you should not trust the Met Office without checking the data first!
And this is what that data shows.
You’ve already seen this graph for daily rainfall, which clearly shows no worsening trends:
Source: https://notalotofpeopleknowthat.wordpress.com/2025/11/16/storm-claudia/
I have also extrapolated the highest daily values each year:
The Met Office keeps monthly data back to 1936 for the whole of Wales.
Source: https://www.metoffice.gov.uk/research/climate/maps-and-data/uk-and-regional-series
December 2015 was the wettest month with a total of 334.1mm. However, November 1929 was effectively just as wet with 333.8mm.
(It is also worth noting that December 1929 had 301.9mm, making it the 8th wettest on record. The period from October 1929 to January 1930 recorded 1068mm, an average of 267mm per month.
It was by far the wettest four month period in Wales, wetter even than September to December 2000 and the wettest part of the autumn/winter of 2015/16).
Met Office Monthly Weather Reports for 1929/30 mention “extensive” and severe floods across South Wales, in addition to much of the country.
Grok made no mention of floods in Monmouth at that time, even though they must have occurred there too.
This serves as a reminder that many floods in the past have not been recorded for posterity on the internet.
See more here notalotofpeopleknowthat
Header image: The Mirror
Bold emphasis added