Winters first appearance
After an almost uninterrupted balmy Autumn of breathtaking mildness, bar a brief spell of 2-3 frosts, the jet stream has become increasingly amplified and has finally tipped under the UK (rather than in summer mode – to its north, as it has been for much of the autumn). This is now delivering a 2-week long spell of mostly cold and showery weather, increasingly wintry on the highest hills. As the week goes on, this wintriness will descend as low as 200m at times, but a mix of hail and sleet could be seen just about anywhere.
There is no real sign of sustained high level blocking patterns developing at this stage that could deliver winter wonderland conditions. The jet stream is too strong for this to happen and there is no forcing from a Stratospheric Warming Event at this stage at least, though one can not be ruled out later in the winter.
The trend therefore, is for the weather to return to a more mobile west to east pattern, oscillating between polar maritime and tropical maritime conditions, mild then cool, but nothing very cold for much of December, after this late autumn cold snap has ended.
The current pattern has been matched with that of 1989, which turned out to be one of the mildest winters on record, ut it is doubtful that this pattern alignment will continue for long, and pattern matching has proven the most unreliable of methods to forecast the weather.
The Easterly QBO and the recent sun-spot minimum and La Niña ENSO, together point to something a little more unusual and colder. There will be more updates in the advent period – to help pin down that big question: Will-it be a white Christmas? So far – so green. Another key question is to contemplate whether the prolonged warmth this autumn is an effect of climate-change? It probably is, in that it mirrors models that show how raised sea temperatures will help to maintain air temperatures for longer, because water retains heat for much longer than air. This would help to delay the onset of cold weather until later in the winter, and serve to shorten winter and reduce ice formation. In turn the late ice formation reduces the albido effect and in turn creates a positive feedback to increasing warming – as less of the suns rays are reflected back by the smaller area of ice.