Figure 1: 24-hour maximum wind gust (m/s) on 5 December based on the ECMWF operational IFS forecasts at 00 UTC on 3 and 5 December (left and middle, respectively; with white contours for the mean sea level pressure in hPa on 12 UTC on 5 December) and from the observations (right).
Storm Desmond caused severe flooding, travel disruption and a power outage across northern England, parts of Scotland and Ireland on 5 December 2015. Cumbria in the northwestern part of England was one of the worst affected regions with more than 200 mm of rain in 24 hours recorded in that area. Storm Desmond broke the United Kingdom's 24-hour rainfall record, with 341.4 mm of rain falling in Honister Pass, Cumbria. On Saturday, 5 December, the UK Met Office issued a red warning of heavy rain for Cumbria. The cyclone also led to flooding in southern Norway.
Orographical enhancement of the precipitation played the major role in the event and the operational model of ECMWF forecast well the highest rainfall amounts over the orographical barriers. However, the forecast underestimated the peak values of about 100 mm in 24 hours in Cumbria and overestimated the precipitation amount in lee of the hills (Figure 2).
Figure 2: 24-hour precipitation amount (mm) between 6 UTC on 5 December and 6 UTC on 6 December, based on the ECMWF operational IFS forecast at 00 UTC on 5 December (left; with cyan contours for the mean sea level pressure in hPa at 12 UTC on 5 December) and from the observations (right).
Hewson, T., Magnusson, L., Breivik, O., Prates, F., Tsonevsky, I., de Vries, H.J.W., 2014: Windstorms in northwest Europe in late 2013. ECMWF Newsletter 139, 22–28. [PDF]