By Philip Eden
The large-scale weather pattern over Europe and the north Atlantic Ocean has rearranged itself during the last week. Gone is the deep low pressure system, centred between Scotland and Iceland, which fed very mild but very wet weather across the UK last week. In its place a large "blocking" high pressure system has developed, so called because it blocks the normal west to east progress of Atlantic weather systems across Europe.
Westerly weather is typical of the majority of British winters - though not this winter because the westerlies have been exceptionally weak for well over a year now. The middle of January provided a reminder of what a wind from the west can do, with daily rainfall amounts in Snowdonia and the Lake District approaching 100mm (4in), and a ten-day total at Capel Curig, Caernarfon, of 252mm (9.92in). The good news for these rain-soaked areas is that, as long as the present high pressure system lasts, the rain will be held at bay.
The sort of weather we get during a "blocked" episode depends on where the blocking high takes up residence.
If it sits bang slap over the UK it may bring persistent fog and night frosts, although if it remains for long enough somewhat cloudier and less cold Atlantic air will gradually infiltrate and the weather will become grey and drab with temperatures 3 or 4 degrees above freezing. If the high pressure system settles over Scandinavia or the northern North Sea it will feed bitterly cold easterly winds from the continent with temperatures close to zero and a risk of snow in some eastern and southern districts. The most usual location, however, is over the Atlantic to the west of Ireland or Scotland, but to the south of Iceland; the airflow covering Britain is then rather cloudy, damp and chilly, without being exceptionally cold.
If the blocking high is located near to or north of Iceland, as it was for long periods during December, it will open the back door to a northerly airflow plunging across the British Isles directly from the Arctic, and it is at times like this that the weather becomes exceptionally cold and wintry.
Despite our experiences over the last few years, such episodes are historically quite rare; the present relatively bland blocking episode is a much more common occurrence.
© Philip Eden