Weather Blog

Winter with a difference.

The one thing that many will not have failed to notice already is the difference in this winter to last. Winter 2006/7 for instance produced very few air frosts and in the main was exceptionally mild - while winter 2007/8 has already produced numerous air frost taking the total into double digits already.

Another absence from what is now termed the 'modern' winter has been the lack of zonal PM (polar maritime) air. To try to explain this as simple as possible the jet stream that wriggles thousands of feet above our head, at speeds of over 100mph, is the dividing line between air from the Tropical South - and cold air from Polar origins. During the modern winter this jet tended to stick to our north allowing us to stay in the 'mild' zone -and winds to blow from the tropical south. This of course kept us milder than we would expect in winter. These winds are known as TM (Tropical Maritime) air. The result in winter of course were plenty of wind and rain, often dismal mountain tops with fog, and temperatures above average. Very rarely in recent times have we see an intrusion of cold polar air from the Atlantic - and then normally during a passing northerly intrusion caused by a low pressure system tracking to our north - or an easterly derived from a Scandinavian high.

Those reading my forecast for the week commencing January 6 in 'Treorchy Net' will have noticed my mentioning the week will be dominated by PM air origination from Canada - and this is where this blog comes in because this is the air that is so rare from 'modern' winters.

The reason for the cold last summer - a summer that produced one of the coldest, wettest summers in memory - was the jet stream slipping to the south of the UK - not briefly - but for a couple of months - allowing the UK to be on the cool (PM) side of the jet while still being pumped with copious amounts of rain. This is of course a very unusual happening. But for the second time in less than twelve months the jet is taking a break to the South of the UK. Will the jet stream stay south for more than a week? That is hard to say but the consequences will be very different to TM air from the Atlantic - but in the process bring joy to the Scottish Ski Industry.

What is going to happen this next week is for some very cold air indeed to be trapped - like a bubble if you will - in some of the low pressure systems moving across the Atlantic to our shores. At low altitudes - and particularly further south the air will have been moderated by the warmer Atlantic sea enough to make for a cold period - but in the main -not cold enough for snow. However, at higher altitudes, and further north, this is a different story. Hence a week when in some places snow amounts could be high at times. And this could include some of the Welsh mountains and hills too. It is also possible - although an outside chance - that at night when the air is cooler - for snow to fall on the lower ground too.

Looking further ahead towards the second half of January there are the ingredients starting to fit into place that may produce some very severe weather indeed. Russia and many parts of Europe are colder than they have been for some years - all good news for snow lovers should a Scandinavian high form to produce easterly winds. A classic that used to occur quite frequently during late winter and early spring in winters past.