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 Weather bomb Doris blasts across UK into Western Europe
Extratropical Cyclone Doris smashed its way across the United Kingdom last Thursday, 23 February, with winds gusting to over 150km/h before ploughing on into Europe later the same day. Two people died in Belgium and one in the UK while there were numerous serious injuries all caused by flying debris or falling trees.
The low caused a day of drama in the United Kingdom which the locals, upper lips stiffened, nicknamed "Doris Day". The low bombed as it crossed Northern Ireland and the northern UK close to the Scottish border, its central pressure dropping from 1004 to 974hPa as it moved from the North Atlantic to the North Sea. That 30hPa drop in 24 hours comfortably met the criterion for a weather bomb which is 24hPa in 24 hours. It brought gale to hurricane force winds over sea and land, heavy rain and, in Scotland, heavy snow. Transport disruption and power blackouts were widespread.
Strong winds were the main features of the storm, with gusts of 90 to 105km/h and up to 130 to 145km/h on exposed coasts and hills occurring in a band across North Wales, central England and Norfolk, as far south as London. According to the UK Met Office, the strongest wind measurements included 151km/h at Capel Curig in mountainous far NW Wales and 140km/h at High Bradfield in the hills of South Yorkshire. All the rest of England as well as all Northern Ireland experienced gusts of 70 to 90km/h but up to 140km/h on exposed coasts.
A woman was killed in Wolverhampton, NW of Birmingham, and two others seriously injured. Many flights were cancelled or diverted in London and the Midlands because of high winds, and this image shows that conditions for pilots that chose to land can at best be described as hairy. Rail services were cancelled, such as between London to Manchester and Liverpool, or operated at reduced speed across Wales and central and southern England because of trees and debris blown onto tracks and bringing down wires. All services in and out of Euston were stopped for a time. Irish Sea ferries were cancelled, the port of Liverpool closed and many high-level road bridges closed due to the high winds.
Driving conditions were diabolical due to gale-force side winds and debris on roads, while in Scotland heavy snow and ice closed roads and motorways including the trunk M80 between Glasgow and Stirling. Snow also closed many schools in Scotland. In Cambridgeshire, a double deck bus was blown on its side, shaking but not seriously injuring its 11 occupants. Tens of thousands of properties lost power, mostly in Northern Ireland, Scotland, Suffolk and East Anglia, due to infrastructure damage from wind, fallen trees or heavy snow on powerlines.
In the Republic of Ireland, Mace Head, W of Galway, recorded a gust of 140km/h and about 40,000 properties lost power mostly due to trees falling across powerlines. There were scattered reports of damage, including a commercial building partly unroofed.
By the afternoon of Thursday 23rd February, Doris had crossed the North Sea and moved on to the Continent, where it was given the name "Thomas". Gusts of around 120 to 140km/h occurred on the Dutch, Belgian and northern French coasts, such as 129km/h at Cap-Gris-Nez, near Calais, and 137km/h at Ijmuiden, on the coast WNW of Amsterdam. Thomas then laid a band of snow 100-150km wide across central Denmark and southern Sweden as it continued moving east.
In northern France, heavy rain and a storm surge combined with the strong winds to disrupt traffic, cause 2,000 power blackouts and result in 500 call outs for emergency services. At least two people were injured. In Belgium, similar weather caused two deaths and widespread traffic disruptions.
As Thomas moved farther east on Friday 24 February, there were warnings of strong winds, heavy rain and thunderstorms across Germany spreading east into Poland and south into northern Italy.
Sources include UK Met Office (and blog), netweather.tv* (and blog*), The Guardian, Severe Weather Europe, SBS, and other media and National Meteorological Services.
 What is a weather bomb?
A weather bomb, or low pressure bomb, is the explosive development of a low pressure system. Its correct meteorological name is "explosive cyclogenesis", i.e. it's the explosive birth of a cyclone. We'll just call it a bomb.
Here's a good description of how a weather bomb forms from the UK Met Office, using Doris as an excellent example. The short video is worth watching.
But wait, there's more.
Bombs occur most commonly out at sea, in winter and in four parts of the world: in the NW Atlantic up the coast of the USA/Canada then across towards Iceland or Great Britain; in the NW Pacific around Japan; in the South Atlantic up the coast from Argentina to Brazil; and on the NSW coast of Australia where we call them East Coast Lows (see notes below). You may see some commonalities here if you think of the shapes of the coasts and ocean currents.
And that nice round number for explosive development to occur - 24hPa (or mb) in 24 hours - is roughly correct for the latitudes of Great Britain but it comes down to 12hPa in 24 hours at about the latitude of Brisbane, 18hPa in 24 hours in southern Tasmania and goes up to 28hPa in 24 hours at the poles. That's because of the deflection caused by the spinning of the earth (Coriolis effect) which is least at the equator and greatest at the poles. In short, you don't need as much of a pressure drop to get the same explosive development on the NSW coast as you do in the North Atlantic.
Finally, you need to know about the intriguingly-named sting jet. This is a recently discovered mid-level jet associated with explosively developing lows. It begins 3 to 4km above the surface and descends over three or four hours, to hit the ground with windspeeds of 150km/h or more along a narrow track possibly only 50km wide. These seem to occur every few years in Great Britain, and the UK Met Office has produced this straightforward web page explaining them. The Wikipedia entry gives more detail on how and why they occur, a list of storms which have involved sting jets, and an explanation for their curious name.
East Coast Lows (ECLs), our own version of bombs, are well known in NSW. Here are links to good information about them:
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