Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 1:39 PM GMT on February 03, 2009
Heavy snow in London, England yesterday did something the Nazi Blitz could not--stop the city bus system. The heaviest snow in 18 years hit the city yesterday, when eight inches (20 cm) of snow blanketed the city. All five major airports in the region closed, all London city buses were pulled off the streets, and the subway system was nearly paralyzed by the snows. It was the heaviest snow since February 7, 1991, when 25 cm fell. In that storm, wind-driven snow drifts of up to three meters (ten feet) crippled road and rail networks across the country.
Heavy snows are rare in London, due to the moderating influence of the ocean waters that surround the British Isles. The prevailing winds from the west or northwest pass over these ocean waters, which are heated by the warm Gulf Stream Current. This makes rain the usual form of winter precipitation for London. Monday's snowstorm was made possible by a strong high pressure system over Scandinavia, whose clockwise flow pumped cold air from western Russia westward over the North Sea and into Britain. A trough of low pressure that formed in this cold air mass moved over Britain, bringing heavy snow. As the cold air accompanying the trough passed over the relatively warm waters of the North Sea, it picked up moisture and instability, enhancing the snows over England in a manner similar to Lake Effect snowstorms over the Great Lakes of North America. The instability of the air mass within Monday's snowstorm was so intense that several UK locations reported "thundersnow", a very rare occurrence in the British Isles. Monday's snow storm also caused traffic problems in France, Switzerland, and Spain. The storm spawned a possible EF2 tornado in southern Spain, as well.
This winter has been the coldest winter since 1996-1997 for England and Wales. Minimum temperatures were as much as 2°C below average in December in western portions of the UK. A cold air outbreak the first week of January brought some of the coldest temperatures seen in southern England since 1991.
Figure 1. AVHRR visible satellite image of the United Kingdom on February 2, 2009, at 13:28 GMT. A strong trough of low pressure, propelled by a cold arctic flow of air from Russia, is crossing over London, dumping heavy snow. Image credit: University of Bern, Switzerland.
I'll have a new post Wednesday or Thursday.
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