Storm Desmond, 4th - 6th December 2015 

2015 was the first year when the UK Met Office and Met Éireann named British and Irish storms. The 4th named Atlantic storm of the season - Desmond - caused serious devestation in north-west England and southern Scotland and grabbed national headlines. It was not the winds, but the phenominal amounts of rain that caused the real problems in the north of the UK. After a wet end to Autumn, Desmond brought more drenching rain to areas that really did not need it, resulting in over 5,000 homes being flooded, almost 50,000 people without power and unfortunately, 3 deaths. 
Visible satellite image of Storm Desmond on the 4th December 2015. 
From: http://www.sat.dundee.ac.uk/gallery/images/1927m.jpg
As can be interpreted from the visible satellite image above, Desmond remained well to the north of the UK for most of its life. Hovever, it was the trailing front that the storm left over Scotland and northern England that was very problematic. Many parts of western Britain had received significant rainfall during November, especially over the higher ground: one station in the Lake District recorded over 1000 mm during the month. Although lowland and eastern parts of the UK recieved below-average rainfall for the month, the ground in the Cumbrian and Scottish fells was completely saturated before the storm hit. As a result, the excess rain from Desmond simply ran rapidly off the ground and into river systems, which worsened the flooding problems. 
 
Storm Desmond developed in the north Atlantic during the 3rd December, and took a typical north-westerly track between Scotland and Iceland over the following days. What was unusual, was that as the storm advanced westwards, the trailing front stalled over the UK. An upper-level jet stream developed over this surface front, oriented parallel to its direction of movement. This meant that northern Britain was subject to the warm, moist south-westerly winds to the south of this front for almost 48 hours, allowing rainfall totals to consistently build over the mountains. The jet stream also helped to develop several secondary cyclones along this front, which moved north-westwards, along the front and maintained the south-westerly flow all the while. 
UK Met Office forecast surface pressure chart, valid for 1200 UTC, 5th December. Storm Desmond is located to the east of Iceland, with a secondary low developing off the trailing front, south west of the UK. 
Credit: Met Office
 
The south-westerly flow over the UK ultimately resided in the Carribbean, and this was where its source region for the copious rainfall in the Lake District was. Atmospheric soundings from the event show an atmosphere that was virtually saturated with moisture from the ground to the tropopause, resulting in unusually high Precipitatble Water (PW) values for the time of year. (PW refers to the amont of water vapour in a vertical column of the atmosphere. The higher the PW, the more moisture there is to potentially be rained out.) The consistent and long-tracked flow from the south-west resulted in the development of an atmospheric river - a long, narrow band of moisture transport in the atmosphere, analogous to a river on land. This atmospheric river transported a significant quantity of moisture all the way from the Carribbean to the UK - a considerable distance for moisture transport in the atmosphere- indicating just how rare an event of this magnitude it. 
 
Northern Hemispere precipitable water derived from satellite image composites. There is a clear transport of atmospheric moisture from tropical regions of the Atlantic towards the UK, associated with the atmospheric river. 
Credit: NOAA/NWS OPC
This atmospheric sounding from the 5th December, at Castor Bay in Northern Ireland, illustrates the almost complete saturation of the troposphere: as significant rain was falling in the Lake District and southern Scotland. 
Credit: University of Wyoming
This quasi-stationary atmospheric river fed moisutre over the mountains of north-western Britain for almost 48 hours, where it was forced to cool and condense as it rose over the mountains, forming orographic clouds directly over the mountains. This enhanced the rainfall at the surface by the "Seeder-Feeder" effect: when rainfall from pre-existing frontal cloud is enhanced further by collision-coalescence as it falls through the orographic clouds developing directly over the mountains. The fact that the atmospheric river was also oriented perpendicular to the mountain ranges, as well as a strong (50+ kts) low-level jet at below 900 hPa being in place, helped to increase the rainfall rate at the surface. However, due to the nature of the rainfall being enhanced only over the mountain tops, rainfall totals had large variations between high and low ground.
24 hour rainfall totals from 4th - 5th December 2015. 
Credit: Met Office
Courtesy Mark McCarthy and Mike Kendon, Met Office NCIC
The above figure indicates just how large the rainfall variation was: regions on the coast recieved around an inch of rain in this 24 hour period, while rain guages in the fells recorded in excess of 200 mm (7.9 inches)! In fact, one guage, at Honister Pass set a new 24 hour British rainfall record of 341.4 mm (13.44 inches). These tremendous amounts of rainfall, falling on already saturated ground, and in an area where the steep topography funnelled runoff rapidly into rivers and streams, rapidly lead to significant flooding in the towns of Appleby, Keswick and Kendal. 
 
In fact, these towns had advanced flood defences built after a similar rainfall event lead to flooding in 2009: that event was thought to have had a return period of 1 in 100 - 800 years. The flood defences were built to withstand a 1 in 100 year flood. However, just 6 years later, a flood event of even greater magnitude occurred, overtopping the new flood defences and flooding thousands of homes. The fact that these two flood events occurred so close to each other when they were thought to have long recurrence intervals is one of the many obvious signs to climate scientists that our climate is changing: leading to the development of more frequent excessive rainfall events for the UK as the climate warms.