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Next week is a real chiller for the whole of Europe. Average temperatures are 6°C below normal for this time of year in the UK.  One reason for the continued cold spring weather is exceptionally high pressure over the North Pole compared to that in the mid-latitudes which is measured by an index called the Arctic Oscillation.  The HIGH pressure BLOCKS out milder air and causes Polar air to flood further south than usual, reaching Reigate on several occasions this winter (albeit modified and warmed up along its journey).

Polar air escapes in negative AO

Polar air escapes in negative AO

The Arctic Oscillation index is currently “strongly negative”.  This indicates much higher than normal pressure over the Arctic and a weak jetstream in low pressure further south than usual.  Air flows from HIGH to LOW pressure.  This means that cold Polar air can easily push out from the north unimpeded by weak SW winds: hence the cold weather brought by Polar easterlies reaching the UK.  The jetstream also “holds in” Polar air like a belt, but the belt has slipped down well to the south of the UK causing “pants” weather for the UK.  Usually the jet migrates north of the UK at this time of year bringing in milder SW air at the surface.
The question, of course, is not “if” but “when” exactly Spring weather will arrive. The maps and ensemble graph seem to suggest that next weekend could see initial improvements with milder SW air reaching the UK and a break down of the Polar block… let’s hope this proves to be accurate!