Severe thunderstorm warnings: with reference to the May 9th Otter Valley Flood

Wednesday 11th October 2023 7:30 PM
Location/Venue: Cellar Bar Kennaway House EX10 8NG

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Weather forecasting has made huge strides in the last 50 years, especially in the accurate prediction of winter storms. However, while the conditions for thunderstorms can often be forecast several days in advance, prediction of their precise location and intensity has made limited progress. As a result thunderstorm warnings generally cover large areas within which relatively few people will actually be affected.

In my talk, I will describe why this is so, and look at work that is currently underway, both here and in the USA, to provide very short lead time warnings for the most severe storms. It turns out that the challenges are not just in forecasting the weather. It is also necessary to identify where the impacts will be most dangerous, to construct a warning that tells people what they need to know, and to get the warning to the people who are at risk.

The extreme rainfall in the Otter Valley on May 9th lasted somewhat over 3 hours and is estimated to have amounted to a little over 100mm of rain, an amount with about a 0.1% probability of occurring per year. Somewhat more rain fell in South Somerset on the same day. This was not enough rain to raise the River Otter very much, and only caused minor flooding of the River Sid. On the other hand, it produced torrents of water along tracks and ditches running off Aylesbeare Common into Newton Poppleford and Tipton St John, sufficient to flood houses, block roads and demolish a wall. I will show that several thunderstorms were involved, each of which made a contribution to the impact, but it was the last one that did most of the damage, the rain falling on already sodden ground.

Brian’s interest in the weather dates back to his teenage years and after studying Maths at Leeds, he first joined the Met Office in 1972. During his career he has been associated with three generations of weather forecasting computer models: first as a programmer, then as team leader, and finally as director. His focus has always been on the sharp end of the weather as it affects those who live in the UK. He has contributed widely to the application of forecasts, including in ocean wave forecasting, flood warning, aviation and defence – and in May 2010 he was one of the Met Office television spokespeople during the Ejyafjallajokull eruption. Along the way, Brian has seen a fair bit of the world, including a two year secondment to Australia in the early 1990s. Following partial retirement in 2012, he has been leading an international project to improve weather warnings and recently edited a book called “Towards the perfect weather warning”.