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Why Autumn Flooding Could Lead To More Severe Winters?

As every casual conversation during the office tea break demonstrates, Britain is no stranger to the whims of Mother Nature, and our weather patterns are consistently inconsistent. From soaking wet summer Wimbledon fortnights to hazy sunny Christmas day walks, Britain’s climate more or less has a mind of its own which makes it notoriously difficult to predict.

One emerging trend that has piqued the curiosity of climate scientists is a newfound correlation between autumn flooding and severe winters. Does excess rainfall and autumn flooding actually influence the severity of winter in Britain? In fact, whilst there is not a direct cause-and-effect relationship, the two are still closely related and autumn flooding is usually a sign of a tough winter ahead.

Autumn flooding ahead

Autumn flooding has become a regular sight in Britain, with many rivers and streams overflowing their banks every year due to heavy rainfall. In recent years, there has been a growing concern that autumn flooding is becoming more frequent and severe. This is backed up by the growing body of evidence, including a study by the Environment Agency which found that the number of autumn flood events in Britain has increased by 50% since the 1960s. The study also found that the average severity of autumn flood events has increased by 20% over the same period. The correlation between autumn flooding and severe winters is also well demonstrated, and some of the data that sheds light on this relationship includes:

The Beast from the East, a period of severe cold weather in February 2018, which was caused by a high-pressure system over Russia that pushed a cold air mass to the west. The Beast from the East brought heavy snowfall, strong winds, and sub-zero temperatures to Britain. The previous autumn, 2017, was the wettest on record in Britain, with 404.5mm of rain falling, which is 160% of the average. This excess rainfall contributed to the severity of the Beast from the East, as it saturated the ground and made it more susceptible to flooding. The Beast from the East is estimated to have cost the UK economy £1.2 billion and more than 17 million people were affected by power cuts during the period. It led to the closure of more than 2,000 schools and upwards of 10,000 flights were cancelled during the event.

The autumn of 1946 is also cited in case studies regarding autumn flooding. This was one of the most brutal winters in the UK’s recorded history and followed a period of intense autumn rainfall and flooding. The autumn of 1946 was exceptionally wet, and this excess moisture played a significant role in setting the stage for the cold and snowy winter that followed. The resulting winter is infamous for the heavy and prolonged snowfall it brought to Britain; snow accumulations were particularly significant in eastern and southeastern regions. In some areas, snow depths reached up to 60 cm (approximately 2 feet) or more, disrupting daily life, transportation, and infrastructure. Agriculture and livestock suffered due to the prolonged cold and deep snow cover, leading to food shortages, and the government was forced to implement rationing measures.

During each of these years, there was a clear connection between autumn flooding and the subsequent occurrence of severe winters. Changes in atmospheric circulation, jet stream patterns, and the release of latent heat from excess moisture played a significant role in setting the stage for a cold and snowy winter. While not every instance of autumn flooding results in a severe winter, these cases do highlight the potential influence of early-season weather events on winter weather patterns in Britain.

Winter severity index and other factors

Further meteorological indicators supporting a link between autumn flooding and cold winters include the Winter Severity Index (WSI), used by meteorologists to quantify the severity of winters, taking into account temperature, snowfall, and other factors.

Historical analysis of the index has shown that winters tend to be colder and snowier in Britain following years with significant autumn flooding. In the 2019/20 winter, the WSI was 9.9, which was the highest value since 2010/11. This was preceded by a record-breaking autumn rainfall event in 2019. In the 2020/21 winter, the WSI was 8.5, which was the third highest value since 2010/11. This was preceded by a period of heavy rainfall in autumn 2020.

Overall, there is a correlation between higher WSI values and higher rainfall in the previous autumn, however, there are also exceptions, which suggests that there are other factors at play, including the Jet Stream and the North Atlantic Oscillation.

Excess moisture in the soil and the release of latent heat into the atmosphere can influence the position and strength of the polar Jet Stream, a global atmospheric circulation phenomenon. The interaction between the Jet Stream and the excess moisture from autumn flooding can lead to unusual weather patterns, including cold snaps and persistent low-pressure systems that bring cold air from the Arctic.

The North Atlantic Oscillation (NAO) is a climate pattern that describes the difference in atmospheric pressure between the subtropical North Atlantic and the sub-polar North Atlantic. The NAO has a significant impact on the severity of British winters. When the NAO is in a positive phase, there is a stronger pressure difference between the subtropical and sub-polar North Atlantic. This leads to stronger westerly winds flowing across Britain, which brings milder and wetter weather. When the NAO is in a negative phase, the pressure difference between the subtropical and sub-polar North Atlantic is weaker. This leads to weaker westerly winds flowing across Britain, which brings colder and drier weather.

What does this mean for you?

As a school, a business or a council, clearly it matters to stay ahead of the winter weather and be prepared for tough winters. Whilst the data is abundantly clear that we are facing wetter autumns, whether or not this actually means more snowfall for your location in particular is not as clear. Large scale meteorological trends such as the above certainly shows the direction of travel, however, for accurate weather forecasting for your particular location, meso-scale forecasting which creates a unique forecast for each 6x6m cell in the country and tracks each snowfall front as it progresses across the country with live data is currently a much more useful indicator.

OUTCO uses sophisticated meso-scale forecasting combined with road surface temperatures incorporating heat transfer values for each specific surface type to be sure that whatever the weather throws at us, our teams are as prepared as they possibly can be to make sure that your facility receives the winter gritting that is necessary to keep it safe and secure for traffic, visitors and staff.

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