

It's been quite a week. Temperatures for the second time this winter in Yorkshire fell to -14C (7F), at Leeming in North Yorkshire on Thursday night. In Scotland -22C was recorded at Altnaharra in the Scottish highlands; and in Wales, the Met Office station in Trawscoed in Ceredigion recorded -14C while ice formed at the marina in Aberystwyth for the first time since the winter of1963.

Across the northern hemisphere extremes of cold have been reported, but It's important to point out that this pattern of weather is also leading to some areas of the world experiencing higher than normal temperatures too.

But what is causing it?

The main culprit is the little known Arctic pressure oscillation. It reversed sharply at the beginning of December and effectively shunted the jet stream much further south than normal, leaving us very much on the cold side of the jet, wide open to the influence of the Arctic and Russia.

So could we have forecast this severe cold spell of weather?

One long range forecaster I spoke to this autumn was convinced that this winter was going to be cold. His name is Joe Bastardi at Accuweather.com. Joe has a common sense approach to long range forecasting, an old fashioned style that has almost gone out of fashion in a meteorological world so dominated by powerful computers. He has an analytical mind second to none, and when I spoke to him he told me he was convinced that the weather patterns that we were having at the time reminded him of those which in the past had been followed by cold winters. He even went on to say that not only could this winter be cold across the USA and Europe, but it could be similar to those we used to experience in the 1970's. And this was way back in September.

Hindsight is a wonderful thing and I would be the first to admit that long range forecasting can be a mug's game. But there have been little clues along the way. Firstly, the North Atlantic oscillation (NAO) went negative during September. This in the past has been a good indicator of a colder, more anticyclonic winter in years gone by. The current El Niño in the Pacific also offers clues. El Niño's in the past have affected Europe's climate late in the winter and into spring. In particular February and March are often colder than average across Europe during Pacific El Niño events. Interestingly the American model has consistently forecast a colder than average February. (And The El Niño could give us a good warm summer but don't quote me on that!)

So why, at the same time that Joe Bastardi at Accuweather.com forecast a cold winter did the Met Office issue a forecast saying that this winter would be mild, with the chances of a cold winter less than 15%?

Clearly there is the rest of January and February to go, but such has been the intensity of the cold spell, which next week will run into its 4th week, then it would take something remarkable during the rest of winter for the Met Office's forecast to be right. It's also worth remembering that this comes off the back of the now infamous barbeque summer forecast, and lets not forget last winter, which the Met Office said again would be mild, but turned out to be the coldest for over 10 years.

The answer may well be quite straight forward. It's likely that the all powerful and dominant Hadley centre supercomputer predicted very little chance of a cold winter, just like it did last winter, and that, as they say, was that.

Which begs other, rather important questions. Could the model, seemingly with an inability to predict colder seasons, have developed a warm bias, after such a long period of milder than average years? Experts I have spoken to tell me that this certainly is possible with such computer models. And if this is the case, what are the implications for the Hadley centre's predictions for future global temperatures? Could they be affected by such a warm bias? If global temperatures were to fall in years to come would the computer model be capable of forecasting this?

If you have time, read again my article called 'Could the sun cast a shadow on global temperatures predictions' that I wrote before Christmas. In particular, read David Archibald's paper, peer reviewed in Energy and Environment journal, where he discusses the prolonged solar minimum we have been in, and what happened to CET temperatures (the longest temperature data set in the world) the last time we experienced such a solar cycle, and the implications for the weather across America and Europe.

Of course the fact that the severe winter has coincided with the very unusual solar cycle could easily be a coincidence, the winter just part of nature's natural variability if you like. Don't forget global warming predictions don't say that cold winters will never happen again, just that they will become less frequent in time. And there's no doubt about it we have been long overdue a prolonged cold winter.

At the very least, though, it's food for thought on this bitterly cold January weekend.

