While Britain's recent easterly has brought predominantly dull conditions, attention now turns to a dramatic shift in weather patterns as North America braces for what could be its coldest outbreak of the winter.
We have been experiencing a substantial cold air outbreak over much of Europe during the past week, associated with an impressive area of high pressure to the north, particularly centred around Greenland and Iceland. The really cold air has been limited to Scandinavia and the eastern half of Europe, with Britain remaining in relatively mild air.
The past week has been exceptionally dull and rather hazy, with pollutants being imported from eastern and south-eastern Europe. There have been a few exceptions, notably north-west Scotland, where some places have enjoyed plenty of sunshine.
Reflecting on the 1980s, 1990s and 2000s, it has historically been common for easterlies sourced from south-eastern Europe, like the one we've been experiencing, to produce grey and hazy weather for most of the country with limited snow. When these occurred in January or February, it was rare for them to produce precipitation that fell exclusively as rain in low-lying inland parts of eastern and southern England.
I recall many such easterly days in eastern England that featured thick stratocumulus and small flakes of sleet or snow blowing in the wind—essentially the snow equivalent of drizzle—and sometimes a thin coating of snow on the ground where substantial cold pooling occurred in eastern Europe. When winds were south-easterly, south-east England was often favoured for snowfall during precipitation, even when the air mass wasn't particularly cold, due to the relatively short air track over the North Sea. As Nick Finnis discussed in an earlier blog, the warming of winters in eastern Europe (which have been warming at a faster rate recently than in north-western Europe) may have been a factor in producing temperatures that were not generally cold enough even for sleet at low levels.
However, even if this setup had occurred 30 years ago, we still wouldn't have tapped into much of that very cold air sitting over central and eastern Europe. We consistently received air sourced from south-eastern Europe, with the blocking high to the north aligned in such a way that created a barrier over western Europe, preventing most of that cold air from reaching the British Isles.
Into next week, a shift of regime is forecast with much milder air arriving via southerly and south-westerly winds, as the blocking high to the north moves to become centred over eastern Europe. Conditions look set to remain predominantly cloudy for most, with rain belts sweeping across the country at times, although eastern counties may experience limited rainfall.
In contrast to the mild easterly outbreak we've experienced in Britain, another notable cold air outbreak is forecast to move into North America during the coming week. Parts of the far north of the USA are already affected, with a large anticyclone over Canada keeping very cold air masses locked in over much of central and eastern Canada. As a depression moves into the North Atlantic east of the USA, it will pull a large scoop of that cold air into central and eastern parts of the USA via northerly winds. This is expected to be the most substantial cold outbreak of the winter, with temperatures well below the long-term normal in some areas.
This USA cold air outbreak is associated with a split in the Northern Hemisphere polar vortex, with one part settling over northern Eurasia and another heading southwards through Canada and the eastern USA. Ironically, substantially warmer air will push into the Arctic associated with the Greenland blocking high and will advance into Canada. As a result, for much of next week, a large area of Canada will be considerably milder than most of the central and eastern USA. This pattern has sometimes been referred to as a "warm Arctic, cold continents" scenario.
Considerable scientific debate continues regarding whether the faster warming of the Arctic, relative to other parts of the world, may be making severe cold air outbreaks more common in parts of Eurasia and North America. Despite the Arctic not being as cold as previously, it remains easily cold enough to bring high-impact cold outbreaks to much of continental North America and Eurasia. This is often less apparent in Britain due to the modifying effect of the surrounding seas, which are warmer than in the past, but many other areas of North America and Eurasia can receive cold air masses directly from the Arctic with little or no modification.
Towards the end of next week, the polar vortex is forecast to re-establish in its more usual position centred in the central Arctic. This is likely to bring milder conditions to the USA as well as to north-western Europe, though it may take until next weekend for the cold air to clear the east coast of North America.
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