Stand by! Stand by!
January 17, 2009
Snow and gales to batter Britain
Paul Simons
Severe gales and snow are being forecast for large areas of Britain this weekend.
The Met Office has warned that gusts of wind could reach 100mph in the far North and up to 80mph in the South West. It has forecast possible damage to trees and buildings, and disruption to transport.
Up to 25mm of sleet or hail is expected to fall in many parts of the country overnight while snow is forecast for higher ground.
The gales could be accompanied by a phenomenon known as
thundersnow, when thunder and lightning occur during storms of snow and sleet.
The cause of the stormy weather is a jet streak — a super-fast burst of speed in the jet stream. During the winter the jet stream — a band of wind a few miles high in the atmosphere — usually runs at about 160km/h (100mph). It is generated by a clash of cold air from the Arctic colliding with warmer air from the sub-tropics.
Over the past few days, the jet stream has been invigorated, pushing out the persistent anticyclone that sat over Britain for three weeks and brought such bitterly cold weather.
Now a burst of extra speed has been injected into the jet stream and as it reaches Britain it will hit about 370km/h (230mph).
This powerful surge is detonating an explosive Atlantic storm, which will intensify rapidly as it hits the northwest tip of Scotland before swinging off to Iceland. Because the jet stream is also passing to the south of Britain it is leaving the country exposed to bitterly cold Arctic air streaming down from eastern Canada and Greenland.
As this frigid air mass sweeps over the warm waters of the North Atlantic’s Gulf Stream it makes the atmosphere highly unstable and also scoops up plenty of moisture on the long sea journey — the ideal recipe for heavy rainfall or snow showers. The falls of snow could then turn into blizzards and pile up into drifts in the howling gale-force winds.
Exactly what causes
thundersnow is not understood, but it accompanies heavy falls of snow or hail and is a feature of a highly unstable atmosphere, as cold air rushes over much warmer seas and generates huge cumulonimbus clouds.
With such a volatile atmosphere, forecasters are finding it difficult to predict the weather beyond the weekend. However, there are signs of even worse conditions next week, as another Atlantic depression drives fierce winds towards southern Britain with a blast of more cold air. If that freezing cold air is driven close enough to the ground it could produce several centimetres of snow over much of the country.
http://www.timesonline.co.uk/tol/news/w ... 531920.ece