November 2005 got off to a particularly stormy start with gales and heavy rainfall affecting the south and west of England and Wales on the 3rd. The following week, two intense depressions affected the far Northwest of the British Isles in close succession. Both depressions developed in similar conditions that lead to rapid cyclogenesis.

7th/8th November 2005

On the 6th a broad cyclonic SW’ly flow was affecting the UK, bringing a spell of heavy rain that swept right across the country followed by heavy blustery showers in the northwest. Numerical model output was predicting a secondary low to rapidly deepen throughout the 7th and swing NNE just to the west of Ireland and move up through the Western Isles. This potentially bringing a spell of gales and even severe gales to a swathe of Ireland, Northern Ireland, the Western and Northern Isles later and overnight into the 8th.

At 12Z on the 6th, a shallow depression, centre 1013mbars, had formed at around 47°N, 42°W, and was being driven by the jet westward. This satellite image, (free password needed), taken at 2312Z (thermal infrared) courtesy of Dundee University shows the well-developed cloud structure associated with the developing cyclone.

By 00Z on the 7th, the depression had deepened slightly to circa. 1004mbars and had moved slightly north of west to lie at 48°N, 28°W. The frontal wave that was initially driven west by the powerful jet now lay near to the ‘left-exit’ region of the jet; which along with the ‘right-entrance’ of the jet stream are highly developmental zones. This would become the dominant driving mechanism behind the rapid deepening of the low over the next 24 hours.

Rapid cyclogenesis commenced during the morning of the 7th as the low entered the left-exit region of the jet. The low steered more to the North as it deepened and at 12Z was located at 51°N, 16°W with a central pressure of 986mb. Buoys and coastal stations to the northeast of the depression recorded some large falls in pressure (usually associated with such rapid development) in the order of 12-13mbars in just 3hours during the early part of the afternoon. Mean wind speeds over the sea were recorded as high as 40knots at this stage, though further deepening was predicted by models for the afternoon until the developing cyclone moved out of the left jet exit later on the 7th.

A Satellite image (thermal infrared) taken at 1119Z indicating rapid cyclogenesis occurring: http://www.sat.dundee.ac.uk/abin/piccygridhtml/avhrr/2005/11/7/1119/ch4.jpg

During the afternoon the winds increased dramatically across the west of Ireland, where gusts on the coast at Valentia and Belmullet were recorded above 60mph; the winds also reached gale force strength over much of Ireland, Northern Ireland, the Isle of Man and SW Scotland. The depression had dropped to 975mbars by 18Z on the 7th around 100 miles northwest of the mainland. Between 18Z and 19Z a gust of 80knots (92mph) was recorded at Belmullet (see Table 1 below), thought to have been caused by a sting jet, which usually occurs at the point just before the low completes cyclogenesis (which was between 18Z and 19Z).

Water Vapour imagery from Dundee University showing a pronounced dry slot over NW Ireland at 18Z on the 7th also another indication of a possible ‘sting-jet’ forming over the west of Ireland. http://www.sat.dundee.ac.uk/pdus/BW/200511071800BW1_g.jpg

The low continued to track NNE and deepen further, but being out of the developmental zone this occurred at a much slower rate now, and by 00Z on the 8th the low lay west of the Lewis with a central pressure of 968mb and was starting to fill. The strongest gradient winds occurred throughout the early hours of the morning across the Western and Northern Isles of Scotland before the low began to start filling and move away, the highest of these was recorded at Scalpay (Harris) just after 02Z with a gust to 89 knots (102mph).

Table 1: Highest gusts recorded on the 7th/8th November 2005

The winds caused some structural damage, though the very strong winds not being uncommon to this area and the relatively sparse population meant that there was little in the way of disruption caused. However 5,000 homes across West Highland, Skye, Orkney and Shetland were without power during the evening of the storm, phone and power lines were down due to fallen trees and general tree debris caused disruption on the roads for a time during the 8th. Sadly an elderly man lost his life on the Isle of Skye when his mobile home was severely damaged as a result of the strong winds.

Along with the strong winds there was also some heavy rainfall associated with the depression on the cold front, and it was this that went on to cause some problems on the 8th as the front stalled across Wales and NW England for much of the day. Several waves moved NNE along the front bringing continual pulses of rain and caused a thoroughly wet and miserable day, with heavy rain falling continuously for up to 18 hours in some parts.

The heavy rain caused widespread flooding; not surprising considering the past 30 days had been very wet across these areas (Capel Curig had received 525mm in this 30 day period previous to the 8th). A retirement home in Llangadog was evacuated after becoming flooded with 4 feet of water – residents were evacuated but flooded roads in the area hampered the rescue efforts by the emergency services. The entire sidewall of an empty office building in Aberdaron collapsed due to subsidence, four workmen were taken to hospital after a landslide at Llyn Ogwen, nr Snowdonia, crushed their cabin. Schools in Carmarthenshire were closed and many rail services badly disrupted due to the flooding on the 8th. The emergency services were even called out to rescue stranded cows that were submerged to their necks in a field near Llandeilo in South Wales. The highest rainfall totals for the day can be found below in Table 2.

Table 2: High rainfall totals on the 8th November 2005

11th/12th November 2005- further gales

Only three days later and another vigorous secondary depression formed, and followed much the same track as the previous depression that brought up to 90mph gusts for exposed coastal parts of the northwest of the British Isles. The difference in the forecast though, which caused some concern, was that the low was expected to deepen further and also track slightly more southeast and therefore extend the damaging wind field into the more populated central belt of Scotland. This belt includes conurbations such as Glasgow and Edinburgh, and is perhaps more at risk from a westerly gradient wind due to a funnelling effect through the Clyde and Forth Valleys.

The reason for greater deepening associated with this depression comparative to the first, was a greater interaction between the left-exit of the Atlantic jet, and a sharp upper trough located just to the west above a steep baroclinic zone. The consequence of all these conditions coinciding was of explosive cyclogenesis occurring to the west of the UK throughout the 11th.

At 00Z on the 11th the low was centred 55°N, 17°W with a central pressure of 998mbars; 24 hours later it had deepened by 31mbars to 967mbars and was located just 100km off the north coast of Scotland, although at this point it was filling and may have dropped lower prior to this during the early evening to approximately 962mbars!

http://www.wetterzentraleforum.de/archive/2005/brack/bracka20051112.gif

Winds picked up across Ireland during the morning with gusts typically over 50mph and generally squally conditions associated with the passage of the cold front. Belmullet reported a 70mph gust at 08Z. Further north across west Scotland winds strengthened rapidly, to storm force by afternoon across much of the Western Isles, with a gust to 87 knots (100mph) reported from North Rona at 17Z. The winds were not quite as strong as forecast through the central belt of Scotland, however this was a highly developmental situation and the models were clearly showing the potential for some catastrophic winds through this area. Having said that, winds were still strong inland and approaching gale force across a large part of Scotland.

As a result of the storm, ferry crossings were disrupted for the second time in a week. The Erskine bridge was closed to all vehicles and restrictions were in force on the Forth, Tay, Skye and Kessock bridges. Falling trees brought down by the winds blocked roads, 1500 homes in Argyll and the Western Highlands, plus a further 1000 homes in the Western Isles were without power throughout the duration of the storm, though, thankfully there were no major incidents reported.

Heavy rainfall accompanied the storm leading to some high rainfall totals observed during the period. As much as 73.8mm falling at Loch Shin (Sutherland) and 50.8mm at Kinlochewe (Wester Ross) in just 24 hours. Though the rain caused river levels to become dangerously high, this didn’t cause any widespread problems unlike the stationary front on the 8th across Wales.

Table 3: Highest gusts and mean wind speeds on the 11th/12th November 2005