Extratropical Cyclones Christian, Dirk, and Erich

First Posting | Summary
Posting Date: December 27, 2013, 7:15:00 AM

A Cluster of Winter Storms Slams the UK over the Holidays

Over the last several days, a series of three winter storms (named Christian, Dirk, and Erich) cut power to hundreds of thousands of homes across the UK and France and flooded an estimated 1,200 homes across the UK. As of Friday afternoon local time, an estimated 13,000 homes were still without power. Trains, ferries, and aircraft have experienced significant delays and cancellations, particularly at London’s Gatwick airport, where the north terminal temporarily lost power. The storms are being blamed for at least five deaths in the UK.

Meteorological Analysis

Extratropical cyclones, also known as winter storms, form when a warm, tropical air mass interacts with a cold, polar air mass, creating local atmospheric disruptions that can grow into powerful storms. The fronts that created this series of storms were exceptionally warm and cold. Storms in Europe have a tendency to cluster, with several affecting an area in close succession. This can happen with relatively weak storms, as demonstrated by this past week’s storms, or with very powerful ones, like 1999’s Lothar and Martin, which were separated by 36 hours and caused billions of Euros in damage.

This week’s clustered storms also showed a characteristic alternation between wet and dry periods, whereby a warm, humid front dropping significant amounts of precipitation on an area is pushed by  a “dry slot” containing powerful winds. Following that, a cold front arrives and lifts warm, moist air higher into the atmosphere where it cools and falls as precipitation.

Dirk drew a significant amount of warm, moist air from the south as it moved across the Atlantic. North of Scotland it slowed and became associated with a slow-moving cold front. That cold front lifted the warm, moist air higher into the atmosphere, where it cooled and is now falling as rain. This cold front was daisy-chained with the warm front of the next system coming across the Atlantic, Erich.

Systems with a southwest to northeast path, like the storm cluster that affected the UK this week, typically bring more rain to Europe than systems that travel from the northwest to southeast. Systems that originate in colder environments and move southeast into warmer regions (like Xaver earlier this December) generally have less moisture and less precipitation compared to storms that originate in warmer regions.

Reported Damage

There are reports of damage across the UK due to Dirk.  At one point, approximately 300,000 properties in the southeast and east of England lost power, mainly due to falling trees and branches. Some 13,000 properties were still without power as of Friday afternoon local time, most in northwest England. Downed trees are also causing reduced train service across the UK.

While wind damage has been significant, most of the loss from this series of events has been from flooding. The Environment Agency issued more than 350 flood alerts and flood warnings and three severe flood warnings (indicating life-threatening conditions) over the last several days. As of early Friday afternoon local time, more than a hundred alerts and warnings were still in effect across England. About 1,200 homes across the UK have been flooded, most of which are located in South East and South West England.

Power was cut to Gatwick Airport’s north terminal when an electrical substation was flooded, which led to the cancellation of dozens of flights. The airport is now fully operational.

In France, an estimated 200,000 homes in western and northern France were left without power, and most long-haul flights out Paris’ Charles de Gaulle Airport were grounded earlier in the week.

Exposure at Risk

Most of the residential buildings in the United Kingdom are detached, semi-detached, or terraced (row) houses and are primarily of masonry construction.  Commercial exposures use a wider variety of construction types. Smaller buildings are usually masonry and perform similarly to residential buildings under strong winds. Larger buildings are generally reinforced concrete or steel, and wind damage is typically to nonstructural components such as mechanical equipment, roofing, and windows. Large commercial buildings often have a large amount of external glass, which is quite vulnerable to wind damage. Flood vulnerability is usually mitigated by flood defenses, although lower floors and cellars often contain services, fixtures, and electrical and mechanical fittings.

Floods can affect a significant portion of low- and mid-rise buildings, particularly the cellars. Risk is particularly high for finished cellars with furniture and appliances; some may contain entire apartments. Usually, heavily used cellars have better flood defense mechanisms than unfinished ones. Large apartment and condominium buildings often have a higher level of engineering and are more resistant to wind damage. Wind damage ratios for tall buildings are lower because the roofs are a smaller portion of the building as a whole. However, balconies, awnings, and sliding glass doors are susceptible to wind damage.

Forecast

The third storm of this cluster, Erich, is currently impacting the UK and is expected to cause additional damage, mainly flood-related, and impede repair efforts. The Met Office has issued yellow and amber warnings for northern England, Northern Ireland, and parts of Scotland. The Environment Agency has issued well over 100 flood alerts and warnings across all England as of Friday afternoon, local time. Much of England is expected to see gusts of up to 80 mph (129 km/h). While rainfall amounts are not expected to be extreme, the ground is saturated from previous storms, leading to the danger of flooding from rivers and streams that are already at high levels.

AIR’s meteorological team is monitoring this storm system closely and will provide updates as warranted.

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Posting Date: December 27, 2013, 7:15:00 AM