The United Kingdom's wettest day on record – so far – 3 October 2020
Abstract
Saturday, 3 October 2020 was provisionally the wettest day on record UK-wide.
Introduction
Saturday, 3 October 2020 was provisionally the wettest day on record UK-wide, based on a 1km resolution gridded dataset (Hollis et al., 2019). An average of 31.7mm of rain fell across the entire United Kingdom. Communicating science to the general public in a way they will find interesting and engaging can be challenging, and the event was widely reported in the media as ‘enough rainwater to fill Loch Ness’ (7.6km3) – a catchy headline fed from the Met Office.
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We came up with a few different ways to picture this much water. Other alternatives included: 1mm falling across an area the size of Australia; a giant raindrop 2.4km across; or, if one were to leave the kitchen tap running, it would take 2.4 million years to waste this much water!
However, a statistic like this in itself has limited value compared to the full context provided by climate series, such as those maintained by the Met Office National Climate Information Centre. Here we provide a brief overview of this event.
Rainfall of early October 2020
Storm Alex (named by Météo-France) marked an emphatic transition to unsettled autumn conditions across the UK on 2 October, bringing very strong winds to the southern half of the country. The Met Office issued a hurricane force 12 warning for Biscay on the evening of 1 October. Alex then moved southeast into the continent where it brought extreme flooding to parts of southeast France and northwest Italy, with dozens of houses destroyed, roads washed away and several fatalities reported. As much as 500mm of rain fell in the worst affected parts of the Alpes-Maritimes department,
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https://public.wmo.int/en/media/news/mediterranean-episode-causes-unprecedented-rainfall
equivalent to 3 months rainfall.
On 3 and 4 October, fronts associated with a new area of low pressure brought prolonged and widespread heavy rainfall (Figures 1 and 2). Initial heavy rain from storm Alex fell on 2 October across southern England, but the area affected expanded on the 3rd to include much of England, Wales and eastern Scotland in an easterly airflow. Heavy rain continued to affect southern England, Wales, Northern Ireland and eastern Scotland on 4 October but, as the low pressure system gradually began to fill, the fronts moved around the periphery of the UK, with drier weather in the centre.
In the first four days of the month, 50–75mm or more of rain fell widely across southern England and parts of southern and eastern Scotland, with over 100mm across some areas. Much of central southern England and Aberdeenshire recorded more than the whole-month average rainfall in the first four days of the month alone (Figure 3).
October 2020 rainfall in context
By far, the wettest day of this spell was 3 October, when a large number of weather stations recorded their wettest October day on record, including several with 100+ year records, and scattered widely across the UK from London to the West Midlands to Lancashire to Aberdeenshire (Table 1). This was the sixth wettest day at Oxford in an almost 200-year daily record
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For more information on the Oxford series, readers may wish to refer to a new book on the Oxford weather station (Burt and Burt, 2019).
. Rainfall of 30–50mm or more was recorded very widely across the UK and 100mm or more in a few locations. It is highly unusual for rainfall totals as high as this to be recorded on the same day across such a large spatial extent of the country.
| Station | Daily rainfall 3 October 2020 (mm) | Previous wettest October day (mm) | Date | Record length (years) |
|---|---|---|---|---|
| Oxford (Oxfordshire) | 60.0 | 49.3 | 9 October 1875 | 193 |
| Rothamsted (Hertfordshire) | 59.8 | 55.2 | 12 October 1993 | 105 |
| Balmoral (Aberdeenshire) | 66.6 | 53.9 | 15 October 1976 | 105 |
| Leuchars (Fife) | 60.6 | 51.2 | 11 October 2012 | 98 |
| Wellesbourne (Warwickshire) | 48.6 | 47.4 | 15 October 2002 | 65 |
| Benson (Oxfordshire) | 43.6 | 38.8 | 29 October 2000 | 65 |
| Moor Park, Preston (Lancashire) | 51.6 | 50.6 | 27 October 1980 | 63 |
| St James's Park, London | 52.4 | 41.6 | 15 October 1980 | 56 |
On 3 October 2020, the UK recorded a provisional daily rainfall total of 31.7mm as an average across the whole of the UK based on the 1km resolution HadUK-Grid gridded dataset (Hollis et al., 2019). This made this provisionally the UK's wettest day on record in a series from 1891 – over 47 000 days – beating the previous record from 25 August 1986, and the first time that the UK area-average rainfall for a single day has exceeded 30mm (Figure 4). It was also provisionally the wettest day for Scotland with an average of 46.1mm. Remarkably, two of the top-three wettest days on record UK-wide occurred in 2020 – with 15 February (storm Dennis) ranked third, while storm Ciara (8 February 2020) ranked 31st – part of a record wet February for the UK (Davies et al., 2021).
Figure 5 compares rainfall totals for the three wettest days on record, UK-wide. 25 August 1986 (perhaps unfortunately a late-summer bank holiday) is all the more remarkable for being the second wettest in this series because this day was largely dry across Scotland, north of the Central Belt, and it was the wettest day on record for England with an average of 39.6mm – a margin of 5mm. The unusually widespread and heavy rainfall on this date was associated with the remnants of Hurricane Charley. In contrast, 15 February and 3 October 2020 were both wet, or very wet, across the vast majority of the UK.
The aggregated UK daily mean rainfall – identifying the widespread nature of rainfall on these days, is, in general, relatively unaffected by network size. So, in contrast to localised extreme rainfall events, a significantly reduced network will still tend to identify these as the wettest days on record (Simpson and McCarthy, 2019). The UK daily rainfall total of 31.7mm for 3 October 2020 is based on near real-time data from around 1000 raingauges, but may subsequently change as further data are included from the full network including manual sites. Nevertheless, once the data are finalized, the two days of 3 October 2020 and 25 August 1986 are likely to remain as stand-out events and the wettest days on record, even when taking into account potential uncertainties in this dataset.
Discussion
The UK's wettest day on record is undoubtedly a newsworthy statistic. The Met Office issued an amber warning for rain for 3 October 2020, but, in the event, impacts in the UK – in contrast to the south of Europe – were relatively modest. Why? The rainfall was sustained in duration but relatively modest in intensity with rain-rates generally less than 8mmh−1 and mostly less than 4mmh−1, and an absence of intense rain-bands embedded in the frontal rain (Figure 2). A further mitigating factor is likely to have been the dry state of ground prior to this event, with most of the UK recording below-average rainfall in September. Fortunately, parts of Norfolk, which recorded 60–80mm of rain (more than the whole-month September average) from a severe but fairly localised rainfall event from 24 to 25 September, were among the relatively drier parts of the UK on 3 October (Figure 5(a)).
Although we might reasonably expect a close relationship between weather extremes and impacts, in reality these impacts are felt at a local scale, which a national statistic cannot fully represent, and flooding is due to a complex integration of numerous factors: duration, spatial extent, antecedent conditions, catchment characteristics, vulnerability and so on. Nevertheless, maintaining a UK daily rainfall series remains a worthwhile exercise. Headlines in the media such as ‘enough rain to fill Loch Ness’ are catchy, but for statistics to be meaningful, they need to provide proper context – and this series allows us to do just that.
The UK mean daily rainfall is a metric that picks up the most widespread heavy rainfall events, but these may not necessarily be associated with local extremes. There are 40 days in the series (from 1891) with UK daily rainfall totals in excess of 20mm, of which 15 occur in the month of October (Figure 6). Only two events have occurred between March and June with the majority of these days through late summer, autumn and winter, while storms Dennis and Ciara account for two of the four days in February. Although we expect this seasonal pattern, the disproportionate count in October is perhaps a little surprising. The highest UK daily rainfall by year suggests some evidence of an increase over time, but the trend is highly sensitive to the start and end points of the series (Figure 7). Further exploration of this particular metric within formal climate attribution studies to evaluate the longer-term trends is certainly warranted.
We note that of the top 100 wettest days in this UK daily series from 1891, 24 have occurred since 2000. the provisional record rainfall of 3 October 2020 was a consequence of the synoptic meteorology, which resulted in such widespread persistent rain. A study to evaluate long-term trends in the annual maximum series (Figure 7), to determine any change in likelihood of an event such as the rainfall of 3 October 2020 under human-induced climate change, has not yet been conducted. However, in general terms, a warmer atmosphere holds more water vapour and, under a warming climate, autumn and winter rainfall over the UK is likely to increase, with individual rainfall events becoming more intense. So, the question as to whether this record will be broken again must remain ‘when’ and not ‘if’.
Acknowledgements
This short article was supported by the Met Office Hadley Centre Climate Programme funded by BEIS and Defra. Thanks to Paul Davies (Met Office) for comments on flood impacts. Thanks to Stephen Burt (University of Reading) for Oxford daily rainfall statistics (series from 1827). Thanks to Dan Hollis and Nick Rayner (Met Office) for helpful comments.
