Storm Desmond was the fourth named storm of the 2015/16 winter season. It brought severe gales with gusts of wind up to 81 mph together with record-breaking rainfall leading to flooding across parts of northern England.
It was associated with an exceptionally mild and moist air mass over northwestern parts of the UK. Desmond was named on 4 December 2015 and tracked to the north west of Scotland on the 5th and 6th. A very slow-moving trailing front brought heavy rain to southern Scotland, northwest England and parts of Ireland, with Cumbria and Lancashire receiving the most severe impacts.
While the Cumbrian coast received less than 25 mm of rain, 200–300 mm fell on the Cumbrian fells and Honister Pass recorded 341.4 mm of rainfall in the 24 hours up to 6pm on 5 December 2015: a new UK record. A new 48-hour record (from 0900 to 0900 hrs) was also set with 405 mm rainfall recorded at Thirlmere in just 38 hours.
Figures show it was the wettest and mildest December on record for the UK (dating back to 1910) (the mildest for England, Wales and Ireland, and the fifth mildest for Scotland).
National Flood Resilience Review
Following the flooding events of last winter and as part of the National Flood Resilience Review (NFRR) the Met Office was asked to develop extreme rainfall scenarios that were scientifically valid and plausible.
Our novel and innovative approach was endorsed by the NFRR’s Scientific Advisory Group and corroborated by results from the European Centre for Medium Range Weather Forecasting.
The state-of-the-art Met Office Hadley Centre climate model produced over 11,000 monthly rainfall scenarios for six large regions in England and Wales and for the current climate. These were used to sample many more cases than are available from existing observational records, including several hundred extreme regional rainfall events that are meteorologically possible but lie outside what has been experienced based on our observational records.
The chance of extreme events like these happening was then estimated. The results suggest there is a 1% likelihood every year that winter monthly rainfall totals could plausibly be 20% higher than recent past extremes in some parts of the country and in other areas up to 30% higher than recent past extremes. Over any of the large regions there is also around a 10% chance in any given year of existing monthly rainfall records being matched or broken.
This was used to produce enhanced rainfall data, which was run through the river models of six case studies chosen by the Environment agency as a stress test of their existing flood risk assessments. Using the techniques developed for the flood review, the Met Office will be looking to work with partners on developing a more integrated flood risk modelling approach.
Meanwhile a major upgrade of the UK radar network for meteorology is almost complete providing more accurate, detailed data essential for successful forecasts and crucial for issuing warnings of heavy rainfall events.
Weather radar gives a live picture of precipitation (rain, hail, snow) present in the atmosphere and many may be familiar with radar images of rainfall from TV weather bulletins.
Behind the scenes, radar data are used to continuously update short range “nowcasts” and are used in our numerical weather models to improve forecast accuracy. Richard Bennett, Senior Project Manager said: “Scientific advances mean we can now capture the size and shape of raindrops as well as their composition (ice, water, snow), which will lead to improvements in accuracy of rainfall measurements, particularly during high impact weather events. ”
The changes will help ensure the Met Office continues to play a vital role in providing governments, commercial customers and the public with timely and essential weather forecast and real-time weather information.