Was September's Rainfall of Biblical Proportions?

 Extreme rain events are often referred to as being of biblical proportions. For example, see here, here, and here. Most people will be aware that the expression refers to the Old Testament story of Noah's Ark and the Great Flood. Flood myths predating the bible story are well known and there is no geological evidence for a worldwide (global) flood. Nevertheless, the phrase 'of biblical proportions' is still widely used today for extreme weather events.

As this rainfall chart (courtesy of the UK Met Office) shows, Central and Southern England was extremely damp in September 2024. Many places received double their typical rainfall for September.

Figure 1: Relative UK Rainfall for September 2024

In recent years (2020 - 2024), Hereford has experienced unusually dry Septembers as Figure 2 shows. [Note: HR40DR is the Davis Vantage Pro 2 Weather Station located in my rear garden]

Figure 2: September Rainfall (mm) recorded by my HR40DR Weather Station

There is certainly an argument for saying this September (2024) is unusually wet in Hereford. However, my weather station has only been in operation since November 2019 so it is impossible to say from such limited data what the 'normal' or expected rainfall is for September in Hereford.

Fortunately, the Met Office has a local weather station at Credenhill (Sterling Lines Army Garrison) which is approximately 3.5 miles away (as the crow flies) and hence is a useful proxy for the city of Hereford. Average September rainfall for the last four 30-year climatic periods (1961-1990, 1971-2000, 1981-2010 and 1991-2020) are shown in Figure 3.

Figure 3: Average September Rainfall (Credenhill) for the Last Four 30-Year Climatic Periods

Interestingly, over the last 60 years, there has been a small but noticeable reduction in the September rainfall recorded at the Credenhill weather station.

Figure 4: Average September Rainfall (Credenhill) for the Last Four 30-Year Climatic Periods

In deciding whether this September's rainfall in Hereford (130.8 mm) was 'of biblical proportions', we need to compare it with the expected rainfall (50.62 mm) for the most recent climatic period (1991-2020). Since the September 2024 rainfall was 260% its expected value, I suppose you could make a case for describing it as 'of biblical proportions'.

Since I started recording weather parameters at my home location (59 months and counting), monthly rainfall has exceeded 100 mm on only six occasions (10%). The highest recorded monthly rainfall was 195.4 mm in February 2020 when the UK was hit by three named storms: Ciara, Dennis, and Jorge.