2 Jan 2013

A new year brings new weather

There is no doubt that the weather of 2012 will be remembered for its remarkable turnaround from drought to flood in a matter of months.

Met Office provisional figures have already shown that England has recorded its wettest year ever, with news expected tomorrow of whether or not the UK as a whole has reached the same milestone.

As I mentioned in my blogs throughout 2012, the cause of continual wet weather has been the jet stream – the fast moving ribbon of air high up in the atmosphere that determines how weather systems develop and where they go.

Since late-spring last year, it spent most of the time sitting to the south of the UK, steering low pressure after low pressure towards us, bringing lots of heavy rain.

Quieter weather to usher in 2013

Thankfully, a new year has brought with it a change in fortunes, with the weather expected to generally be more settled for the first half of January.

This is because the jet stream will move further northwards, taking the rain-bearing weather systems with it.

As a result, areas of low pressure will tend to be steered towards Iceland and Scandinavia, with only the odd one giving us a glancing blow – mainly for Scotland and Northern Ireland.

Therefore, much drier weather is expected during the next 7-10 days with high pressure sitting over or very close to the UK, giving a break from flooding and allowing river levels to fall.

Despite high pressure becoming more dominant, it looks as though it could be a cloudy high, rather than a sunny one – at least initially. It’ll also be quite mild for January, meaning that snow, for now, is unlikely.

Any signs of colder weather?

In recent days, I’ve had quite a few people asking me on Twitter, Facebook and Google+ whether or not there are signs of it turning colder with snow.

The weather computer models are hinting towards a general downward trend in temperature from the middle of January onwards, but at this stage, there is a lot of uncertainty about how cold it is going to get.

Another interesting event that is about to take place in the upper part of the atmosphere (the stratosphere), is a sudden stratospheric warming – SSW for short.

What happens when sudden stratospheric warming occurs?

During the winter months, a polar vortex sits across the north pole in the upper part of the atmosphere. Effectively, this is a huge area of lower pressure around which air flows from west to east in the northern hemisphere – known as a zonal flow.

The presence of this zonal flow keeps the weather generally unsettled, with low pressure systems at the surface affecting mid-latitudes locations, such as the UK and north America.

However, sometimes, a sudden significant warming can occur in the stratosphere, which has the effect of disrupting the polar vortex and the winds that rotate around it.

Initially, the normal west to east flow of air is disrupted 30-50km up in the atmosphere, where it slows down or reverses direction completely. At this point, because it is so high up, it has little effect on the weather we experience at the surface.

But what can happen in certain cases is that this disruption of the west to east flow of air can gradually percolate down to the surface. When this happens, blocking areas of high pressure are more likely to form, potentially bringing a big change in weather patterns.

At this stage, it is too early to tell exactly what the outcome of the imminent SSW will be because the places getting the coldest air will depend on the location and orientation of any blocking high pressures that form.

Nevertheless, the second half of the month could prove interesting – something that I’ll keep you updated on here in my blog and on Twitter – @liamdutton

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